U.S. Environmental Protection Agency
Office of Resource Conservation and Recovery
Documentation for Greenhouse Gas Emission and
Energy Factors Used in the Waste Reduction Model
(WARM)
Background Chapters
May 2019
Prepared by ICF
For the U.S. Environmental Protection Agency
Office of Resource Conservation and Recovery
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WARM Version 15
Table of Contents
May 2019
Table of Contents
1 WARM Background and Overview 1-1
2 WARM Definitions and Acronyms 2-1
3 Recent Updates in WARM 3-1
4 Forest Carbon Storage 4-1
5 Transportation Assumptions 5-1
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WARM Version 15
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May 2019
1 WARM BACKGROUND AND OVERVIEW
During the last century, population and economic growth have caused increased consumption
of materials such as minerals, wood products and food. Materials consumption continues to accelerate
while simultaneously shifting away from renewable materials like agriculture and forestry products
toward non-renewable products such as metals and fossil fuel-derived products (EPA, 2009b). Source
reduction, reuse and recycling of materials are ways that we can manage materials more sustainably.
Extracting, harvesting, processing, transporting and disposing of these materials result in
greenhouse gas (GHG) emissions, in part due to the large amounts of energy required for these life-cycle
stages. The U.S. Environmental Protection Agency's (EPA) Waste Reduction Model (WARM), the focus of
this documentation, is a tool designed to help managers and policy-makers understand and compare the
life-cycle GHG, energy, and economic implications of materials management options (recycling, source
reduction, landfilling, combustion with energy recovery, anaerobic digestion, and composting) for
materials commonly found in the waste stream. By comparing a baseline scenario (e.g., landfilling) to an
alternate scenario (e.g., recycling), WARM can assess the economic, energy, and GHG implications that
would occur throughout the material life cycle.
1.1 MATERIALS MANAGEMENT CONTEXT
The United States and the international community are focusing increasingly on a life-cycle
materials management paradigm that considers the environmental impacts of materials at all life-cycle
stages. Recognition is growing that, since traditional environmental policies focus on controlling "end-of-
pipe" emissions, they do not provide a means for systematically addressing environmental impacts
associated with the movement of materials through the economy. While "end-of-pipe" policies are
often effective in controlling direct pollution, they may
result in some environmental impacts being overlooked or
shifted from one area of the life cycle to another (EPA,
2009b).
The EPA Office of Land and Emergency
Management (OLEM) (formerly the Office of Solid Waste
and Emergency Response) found that 42 percent of U.S.
2006 GHG emissions were associated with the
manufacturing, use and disposal of materials and products
(EPA, 2009a). As a result, changing materials management
patterns is an important strategy to help reduce or avoid
GHG emissions. Reducing the amount of materials used to
make products, extending product life spans, and
maximizing recycling rates are examples of possible
materials management strategies that can significantly
reduce GHG emissions (EPA, 2009b).
Private and public entities globally are moving toward life-cycle materials management. For
example, the Organisation for Economic Cooperation and Development (OECD) and the Kobe 3R Action
Plan (a plan issued by the Group of Eight) have recommended that member countries pay increased
attention to life-cycle approaches to material flows. Companies in the metals, cement, agribusiness,
food and retail industries are also formulating approaches to increase efficiency and reduce
environmental impacts by taking a life-cycle view of materials and processes (EPA, 2009b).
Materials management refers to how
we manage material resources as they
flow through the economy, from
extraction or harvest of materials and
food (e.g., mining, forestry, and
agriculture), production and transport of
goods, use and reuse of materials, and, if
necessary, disposal. The EPA 2020 Vision
Workgroup defines materials
management as "an approach to serving
human needs by using/reusing resources
most productively and sustainably
throughout their life cycles, generally
minimizing the amount of materials
involved and all the associated
environmental impacts" (EPA, 2009b).
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1.2 GENESIS AND APPLICATIONS OF WARM
1.2.1 History of WARM Development
Recognizing the potential for source reduction and recycling of municipal solid waste (MSW) to
reduce GHG emissions, EPA included a source reduction and recycling initiative in the original 1994 U.S.
Climate Change Action Plan. EPA set an emission reduction goal based on a preliminary analysis of the
potential benefits of these activities. It was clear that a rigorous analysis would be needed to gauge
more accurately the total GHG emission reductions achievable through source reduction and recycling.
That all of the options for managing MSW should be considered also became clear. By
addressing a broader set of MSW management options, EPA could gain a more comprehensive picture
of the GHG benefits of voluntary actions in the waste sector and assess the relative GHG impacts of
various waste management approaches. To this end, EPA launched a major research effort, which
resulted in the development of life-cycle GHG and energy factors for materials across several categories
(e.g., plastics, metals, wood products), the online GHG and energy calculation tool WARM applying
these factors, and accompanying documentation. The first documentation report, entitled Greenhouse
Gas Emissions from Management of Selected Materials in Municipal Solid Waste, was published in 1998,
the second edition in 2002 (retitled Solid Waste Management and Greenhouse Gases: A Life-Cycle
Assessment of Emissions and Sinks) and the third edition in 2006 (EPA, 1998, 2002, 2006).
In 2010, EPA reorganized the WARM documentation into chapters by material and by process
and included more in-depth descriptions of the WARM emission factors. Whereas the previous
documentation reports were structured only around process chapters (i.e., source reduction, recycling,
composting, combustion, landfilling), this materials-based structure allowed EPA to provide WARM
users with more detailed information about the specific materials analyzed in WARM, which had not
been included to a large extent in previous versions of the report, as well as more detailed information
about of the calculations behind specific material emission factors.
The Recent Updates in WARM chapter describes the revisions made to different model versions
and the documentation. Each year, EPA has updated the model itself to reflect updated statistics on
national average electricity generation fuel mix, transmission and distribution losses, coal weighting for
electricity generation, electricity generation per fuel type, the carbon content of fuels, landfill methane
generation distribution (by type of landfill), landfill gas recovery and flaring rates, and waste generation
and recovery rates. In addition, annual updates have often included new material emission factors and
other improvements to the analysis (Exhibit 1-1 provides the dates when materials were added to
WARM).
In WARM Version 15 (released in May 2019), the updates and improvements include new
economic impact reports, detailed electronic material factors, and revisions to emission factors based
on new reports and databases. Changes to other recent versions include addition of the anaerobic
digestion pathway for managing organic wastes, including food waste, yard trimmings, and mixed
organics; the addition of either updated or new emission factors for food waste, construction and
demolition (C&D) materials, plastics, aluminum cans and ingot, PLA, and carpet; the addition of
component-specific decay rates; and increased specificity in WARM with region-specific electricity grid
factors and an updated method for estimating landfill gas collection efficiency.
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1.2.2 WARM Audience and Related Efforts
The primary application of WARM is to support
materials-related decision-making in the context of climate
change. By quantifying the climate impacts of materials
management decisions, the factors in this report and the
tool enable municipalities, companies and other waste- and
program-management decision-makers to measure the
benefits of their actions. Other EPA decision-support tools
such Individual WARM (iWARM) rely on WARM energy and
emission factors to help users make a wide range of
decisions. For example, the iWARM tool uses life-cycle
information from WARM to quantify energy benefits of
recycling small quantities of common waste materials by
calculating the "run time" of a variety a household
appliances (e.g., clothes washer, hairdryer, etc.) using
electricity savings from recycling materials. Other
applications have included quantifying the GHG reductions
from voluntary programs aimed at source reduction and
recycling, such as E PA's Waste Wise and Pay-As-You-Throw
programs.
The international community has shown
considerable interest in using the emission factors—or
adapted versions—to develop GHG emission estimates for
non-U.S. materials management.1 For example,
Environment Canada and Natural Resources Canada
employed EPA's life-cycle methodology and components of
its analysis to develop a set of Canada-specific GHG
emission factors to support analysis of waste-related mitigation opportunities (Environment Canada,
2005).
1.2.3 Estimating and Comparing Net GHG Emissions
WARM compares the emissions and offsets resulting from a material in a baseline and an
alternative management pathway in order to provide decision-makers with comparative emission
results. For example, WARM could be used to calculate the GHG implications of landfilling 10 tons of
office paper versus recycling the same amount of office paper.
The general formula for net GHG emissions for each scenario modeled in WARM is as follows:
Net GHG emissions = Gross manufacturing GHG emissions - (Increase in carbon stocks + Avoided utility
GHG emissions)
This equation should only be considered in the context of comparing two alternative materials
management scenarios in order to identify the lowest net GHG emissions. The following circumstances
influence the net GHG emissions of a material:
1 Note that waste composition and product life cycles vary significantly among countries. This report may assist
other countries by providing a methodological framework and benchmark data for developing GHG emission
estimates for their solid waste streams.
Global Warming Potentials
CO2, CH4, N2O and perfluorocarbons
(PFCs) are very different gases in terms of
their heat-trapping potential. The
Intergovernmental Panel on Climate
Change (IPCC) has established CO2 as the
reference gas for measurement of heat-
trapping potential (also known as global
warming potential or GWP). By definition,
the GWP of one kilogram (kg) of CO2 is
one. The GWPs of other common GHGs
from materials management activities are
as follows:
• CH4 has a GWP of 25, which means
that one kg of CH4 has the same heat-
trapping potential as 25 kg of CO2.
• l\l20 has a GWP of 298.
• PFCs are the most potent GHG
included in this analysis; GWPs are
7,390 for CF4 and 12,200 for C2F6.
WARM expresses comparative GHG
emissions in metric tons of CO2
equivalents (MTCO2E), which uses the tool
of GWP to allow all emissions to be
compared on equal terms.
WARM uses GWPs from IPCC(2007).
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• Through source reduction (for example, "lightweighting" a beverage can—using less aluminum
for the same function), GHG emissions throughout the life cycle are avoided. In addition, when
paper products are source reduced, additional carbon is sequestered in forests, through
reduced tree harvesting.
• Through recycling, the GHG emissions from making an equivalent amount of material from
virgin inputs are avoided. In most cases, recycling reduces GHG emissions because
manufacturing a product from recycled inputs requires less energy than making the product
from virgin inputs.
• Composting with application of compost to soils results in carbon storage and small amounts of
CH4 and N20 emissions from decomposition.
• The anaerobic digestion captures biogas from the digestion of organic materials. The biogas is
assumed to be combusted to produce energy, offsetting emissions from fossil fuel consumption.
Additionally, the digestate resulting from the digestion process is applied to agricultural lands,
resulting in soil carbon storage, avoided use of synthetic fertilizers, and trace CH4 and N20
emissions during digestate curing and after land application.
• Landfilling results in both CH4 emissions from biodegradation and biogenic carbon storage. If
captured, the CH4 may be flared, which simply reduces CH4 emissions (since the C02 produced
by flaring is biogenic in origin, it is not accounted for in this assessment of anthropogenic
emissions). If captured CH4 is burned to produce energy, it offsets emissions from fossil fuel
consumption.
• Combustion of waste may result in an electricity utility emissions offset if the waste is burned in
a waste-to-energy facility, which displaces fossil-fuel-derived electricity.
1.2.4 Materials Considered in WARM
To measure the GHG impacts of materials management, EPA first decided which materials and
products to analyze. EPA surveyed the universe of materials and products found in the solid waste
stream and identified those that are most likely to have the greatest impact on GHGs. These
determinations were initially based on (1) the quantity generated; (2) the differences in energy use for
manufacturing a product from virgin versus recycled inputs; and (3) the potential contribution of
materials to CH4 generation in landfills. Since the initial assessment, many materials have been added.
Materials that EPA selects for inclusion in WARM are generally selected based on the three principles
above, with the additional criterion that enough data be available to create defensible emission factors.
WARM Version 15, released in May 2019, includes 60 materials, products and mixed categories, as
listed by category type in Exhibit 1-1. Exhibit 1-1 also shows the main sources of virgin and recycled
production energy data for each material, the vintage of those data, the year each material was first
added to WARM, the percentage each material constitutes of total MSW generated in the United States
(to the extent information is available), and whether the recycling process is modeled as open- or
closed-loop in WARM (more information on the recycling process is presented in the Recycling chapter).
EPA is in the process of gathering and reviewing new life-cycle inventory (LCI) data for several material
types to develop updated and new emission factors for WARM. The definitions of the each of the WARM
materials included in Exhibit 1-1 are presented in Exhibit 1-2.
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Exhibit 1-1: Current Materials and Products in WARM, Historical Inclusion, and Source of Data
Year First
Added to
WARM
Open- or
(updated
Source of Main
Approximate
% of MSW
Closed-
year if
Process Energy
Year(s) of Current
Generation
Loop
Material/Product
applicable)
Data
Energy Data11
by Weight1"'
Recycling?'"
Metals and Glass
PE Americas
Aluminum Cans
1998(2012)
(2010)
2006
0.5%
Closed
PE Americas
Aluminum Ingot
2012
(2010)
2006
NE
Closed
Steel Cans
1998
FAL (1998b)
1990
0.7%
Closed
Battelle (1975);
Kusik and
Kenahan (1978);
Copper Wire
2005
FAL (2002b)
1973-2000
NE
Open
Glass
1998
RTI (2004)
Late 1990s
4.4%
Closed
Plastics
HDPE (high-density polyethylene)
1998(2012)
FAL (2011)
2000s
2.3%
Closed
LDPE (low-density polyethylene)
1998(2012)
FAL (2011)
2000s
3.0%
Closed
PET (polyethylene terephthalate)
1998(2012)
FAL (2011)
2000s
1.9%
Closed
LLDPE
2012
FAL (2011)
2000s
NE
Closed
PP
2012
FAL (2011)
2000s
NE
Closed
PS
2012
FAL (2011)
2000s
NE
Closed
PVC
2012
FAL (2011)
2000s
NE
Closed
Paper and Wood
Corrugated Containers
1998
RTI (2004)
Late 1990s
11.9%
Both
Magazines/Third-Class Mail
2001
RTI (2004)
Late 1990s
2.0%
Closed
Newspaper
1998
RTI (2004)
Late 1990s
2.6%
Closed
Office Paper
1998
RTI (2004)
Late 1990s
1.7%
Closed
Phone Books
2001
RTI (2004)
Late 1990s
NE
Closed
Textbooks
2001
RTI (2004)
Late 1990s
NE
Closed
Dimensional Lumber
1998
FAL (1998c)
Mid 1990s
3.8%
Closed
Medium-Density Fiberboard
1998
FAL (1998c)
Mid 1990s
NE
Closed
Organics
Food Waste
2014
NA
NA
15.1%
NA
Food Waste (meat only)
2015
NA
NA
IE
NA
Food Waste (non-meat)
2014
NA
NA
IE
NA
Battagliese et al.
Beef
2015
(2013)
2011
IE
NA
Pelletier (2008,
Poultry
2015
2010)
Late 2000s
IE
NA
LCA Digital
Grains
2014
Commons (2012)
2000s
IE
NA
Espinoza-Orias
Bread
2014
(2011)
2011
IE
NA
Thoma et al.
Dairy Products
2014
(2010)
2008
IE
NA
Luske (2010)
UC Davis
Fruits and Vegetables
2014
(multiple)
Late 2000s
IE
NA
Yard Trimmings
1998
NA
NA
13.3%
NA
Grass
2001
NA
NA
IE
NA
Leaves
2001
NA
NA
IE
NA
Branches
2001
NA
NA
IE
NA
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Year First
Added to
WARM
Open- or
(updated
Source of Main
Approximate
% of MSW
Closed-
year if
Process Energy
Year(s) of Current
Generation
Loop
Material/Product
applicable)
Data
Energy Data3
by Weightb
Recycling?0
NatureWorks, LLC
PLA
2012
(2010)
2009
NE
NA
Mixed Categories
Virgin: FAL
(1998a), RPTA
(2003)
Virgin: 1996;
Recycled: RPTA
Recycled: early
Mixed Paper (general)
1998
(2003)
2000s
NE
Open
Mixed Paper (primarily residential)
1998
FAL (1998a)
1996
NE
Open
Mixed Paper (primarily from
offices)
1998
FAL (1998a)
1996
NE
Open
Mixed Metals
2002
NA
NA
9.1%
NA
Mixed Plastics
2001
NA
NA
13.1%
NA
Mixed Recyclables
1998
NA
NA
NE
NA
Mixed Organics
2001
NA
NA
NE
NA
Mixed MSW
2001
NA
NA
NE
NA
Composite Products
FAL (2002a);
Carpetd
2004 (2012)
Realff (2011)
2000s
1.4%
Open
Desktop CPUsd
2019
Various
2011-2019
NE
Open
Portable Electronic Devicesd
2019
Various
2011-2019
NE
Open
Flat-panel Displays'1
2019
Various
2011-2019
NE
Open
CRT Displays'1
2019
Various
2011-2019
NE
Open
Electronic Peripherals'1
2019
Various
2011-2019
NE
Open
Hard-copy Devicesd
2019
Various
2011-2019
NE
Open
Mixed Electronicsd
2019
NA
2011-2019
1.2%
Open
Construction and Demolition
(C&D)
Athena
Sustainable
Materials Institute
Clay Bricks
2004
(1998)
Mid-late 1990s
NA
NA
U.S. Census
Bureau
(1997),Wilburn
and Goonan
Concreted
2004
(1998)
1997
NA
Open
IPCC (1996), PCA
(2003), Nisbet et
Fly Ash
2004
al. (2000)
Early 2000s
NA
Open
Athena
Sustainable
Materials Institute
(2000), Atech
Group (2001), EIA
(2009), Corti and
Tiresd
2006
Lombardi (2004)
Early 2000s
2.2%
Open
U.S. Census
Bureau (1997),
Athena
Asphalt Concreted
2010
Sustainable
Early 2000s
NA
Closed
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Material/Product
Year First
Added to
WARM
(updated
year if
applicable)
Source of Main
Process Energy
Data
Approximate
Year(s) of Current
Energy Data3
% of MSW
Generation
by Weightb
Open- or
Closed-
Loop
Recycling?0
Materials Institute
(2001), U.S.
Census Bureau
(2001),
Environment
Canada (2005),
Levis (2008), NREL
(2009)
Asphalt Shinglesd
2010
Athena
Sustainable
Materials Institute
(2000), Cochran
(2006), CMRA
(2007)
Early 1990s
NA
Open
Drywalld
2010
Venta (1997);
recycling data
from WRAP
(2008)
Virgin: 1997;
Recycled: 2008
NA
Both
Fiberglass Insulation
2010
Lippiatt (2007),
Enviros Consulting
(2003) for glass
cullet production
Mid 2000s
NA
NA
Vinyl Flooringd
2010
ECOBILAN (2001),
FAL (2007),
Lippiatt (2007),
Ecoinvent Centre
(2008)
2007
NA
NA
Wood Flooringe
2010
Bergman and
Bowe (2008),
Hubbard and
Bowe (2008),
Bergman (2010)
Late 2000s
NA
NA
NA = Not applicable.
NE = Not estimated.
IE = Included elsewhere.
a Note that years are approximate because each source draws on a variety of data sources from different years.
b Source for percent generation data is EPA (2018).
c Closed-loop recycling indicates a recycling process where end-of-life products are recycled into the same product. Open-loop
recycling indicates that the products of the recycling process (secondary product) are not the same as the inputs (primary
material).
d Indicates composite product.
e Wood flooring also falls under the Paper and Wood category.
The material types listed in Exhibit 1-1 generally fall into two overarching waste categories -
municipal solid waste (MSW) and construction and demolition (C&D). MSW generally includes metals
and glass, plastics, paper and wood, organics, mixed categories and composite products. These materials
are household, commercial, institutional and light industrial waste collected and managed by a
municipality. C&D materials are materials that are produced during construction, renovation or
demolition of structures and include clay bricks, concrete, fly ash, tires, asphalt concrete, asphalt
shingles, drywall, fiberglass insulation, vinyl flooring and wood flooring. EPA's interest in C&D materials
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is the result of a growing interest in environmentally friendly or "green" building practices, including
reusing and recycling the impressive quantities of C&D debris that are generated each year. In 2008,
143.5 million tons of C&D waste were generated (Waste Business Journal, 2009). One major difference
between waste management for C&D materials versus MSW materials is that C&D materials are
typically disposed of in landfills created specifically for C&D waste that do not accept MSW waste. C&D
and MSW landfills differ in several ways, including in the design and operation requirements of the
landfills. From the GHG perspective, the most significant difference between the two landfill types is
that C&D landfills generally do not have the landfill methane capture systems that are common at MSW
landfills. Thus, the methane that is produced in C&D landfills is eventually released directly to the
atmosphere.
As shown in the fifth column of Exhibit 1-1, the listed MSW materials constitute more than 75
percent, by weight, of MSW. Several materials, including most C&D materials, were not included in the
waste characterization report cited here (EPA 2018a), so the utility of this percent estimate is limited.2
Exhibit 1-2: WARM Material Definitions
WARM Material
WARM Data Source Definition
Aluminum Cans
Aluminum cans represent cans produced out of sheet-rolled aluminum ingot.
Aluminum Ingot
Aluminum ingot is processed from molten aluminum in the form of a sheet ingot suitable for
rolling, extruding, or shape casting. Thus, it serves as a pre-cursor to manufacture of aluminum
products such as aluminum cans. It can serve as a proxy for certain aluminum materials such as
electrical transmission and distribution wires, other electrical conductors, some extruded
aluminum products, aluminum product cuttings, joinings and weldings, and consumer durable
products such as home appliances, computers, and electronics.
Steel Cans
Steel cans represent three-piece welded cans produced from sheet steel that is made in a blast
furnace and basic oxygen furnace (for virgin cans) or electric arc furnace (for recycled cans).
Copper Wire
Copper wire is used in various applications, including power transmission and generation lines,
building wiring, telecommunication, and electrical and electronic products.
Glass
Glass represents glass containers (e.g., soft drink bottles and wine bottles).
HDPE
HDPE (high-density polyethylene) is usually labeled plastic code #2 on the bottom of the
container, and refers to a plastic often used to make bottles for milk, juice, water and laundry
products. It is also used to make plastic grocery bags.
LDPE
LDPE (Low-density polyethylene), usually labeled plastic code #4, is often used to manufacture
plastic dry cleaning bags. LDPE is also used to manufacture some flexible lids and bottles.
PET
PET (Polyethylene terephthalate) is typically labeled plastic code #1 on the bottom of the
container. PET is often used for soft drink and disposable water bottles, but can also include
other containers or packaging.
LLDPE
LLDPE (linear low-density polyethylene) is used in high-strength film applications. Compared to
LDPE, LLDPE's chemical structure contains branches that are much straighter and closely aligned,
providing it with a higher tensile strength and making it more resistant to puncturing or shearing
PP
PP (Polypropylene) is used in packaging, automotive parts, or made into synthetic fibres. It can
be extruded for use in pipe, conduit, wire, and cable applications. PP's advantages are a high
impact strength, high softening point, low density, and resistance to scratching and stress
cracking. A drawback is its brittleness at low temperatures
PS
GPPS (General Purpose Polystyrene) has applications in a range of products, primarily domestic
appliances, construction, electronics, toys, and food packaging such as containers, produce
baskets, and fast food containers.
PVC
PVC (Polyvinyl Chloride) is produced as both rigid and flexible resins. Rigid PVC is used for pipe,
conduit, and roofing tiles, whereas flexible PVC has applications in wire and cable coating,
flooring, coated fabrics, and shower curtains
PLA
Polylactic acid or PLA is a thermoplastic biopolymer constructed entirely from annually
renewable agricultural products, e.g., corn, and used in manufacturing fresh food packaging and
food service ware such as rigid packaging, food containers, disposable plastic cups, cutlery, and
2 Note that these data are based on national averages. The composition of solid waste varies locally and regionally.
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WARM Material
WARM Data Source Definition
plates
Corrugated Containers
Corrugated container boxes made from containerboard (liner and corrugating medium) used in
packaging applications.
Magazines/Third-Class Mail
Third Class Mail is now called Standard Mail by the U.S. Postal Service and includes catalogs and
other direct bulk mailings such as magazines, which are made of coated, shiny paper. This
category represents coated paper produced from mechanical pulp.
Newspaper
Newspaper represents uncoated paper made from 70% mechanical pulp and 30% chemical pulp.
For the carbon sequestration portion of the factor, it was assumed that the paper was all
mechanical pulp.
Office Paper
Office paper represents paper made from uncoated bleached chemical pulp.
Phone Books
Phone books represent telephone books that are made from paper produced from mechanical
pulp.
Textbooks
Textbooks represent books made from paper produced from chemical pulp.
Dimensional Lumber
Lumber includes wood used for containers, packaging, and building and includes crates, pallets,
furniture and dimensional lumber like two-by-fours.
Medium-Density
Fiberboard
Fiberboard is a panel product that consists of wood chips pressed and bonded with a resin.
Fiberboard is used primarily to make furniture.
Food Waste
Food waste consists of uneaten food and wasted prepared food from residences, commercial
establishments such as grocery stores and restaurants, institutional sources such as school
cafeterias, and industrial sources such as factory lunchrooms. This emission factor contains a
weighted average of the largest food waste components in the waste stream, including beef,
poultry, grains, dairy products, fruits and vegetables.
Food Waste (meat only)
"Food waste (meat only)" is a weighted average of the two meat food type emission factors in
WARM: beef and poultry. The weighting is based on the relative shares of these two categories
in the U.S. food waste stream
Food Waste (non-meat)
"Food waste (non-meat)" is a weighted average of the three non-meat food type emission
factors developed in WARM: grains, fruits and vegetables, and dairy products. The weighting is
based on the relative shares of these three categories in the U.S. food waste stream
Beef
Beef represents the upstream emissions and energy associated with the production of beef
cattle in the United States, including the upstream energy and emissions associated with feed
production.
Poultry
Poultry describes the upstream emissions and energy associated with the production of broiler
chicken (i.e., domesticated chickens raised specifically for meat production), including the
upstream energy and emissions associated with feed production.
Grains
Grains consists of a weighted average of the relative amounts of grain products in the municipal
waste stream, consisting of wheat flour, corn and rice.
Bread
Bread consists of the upstream emissions and energy associated with wheat flour production, as
well as the additional energy used to bake it into bread.
Dairy Products
Dairy Products consists of a weighted average of the emissions associated with nearly the entire
dairy product waste stream, including milk, cheese, ice cream, and yogurt.
Fruits and Vegetables
Fruits and Vegetables represents the average fresh fruits and vegetable components of food
waste, consists of a weighted average of the six most common fruits and vegetables in the
municipal waste stream, including apples, bananas, melons, oranges, potatoes, and
tomatoes.
Yard Trimmings
Yard trimmings are assumed to be 50% grass, 25% leaves, and 25% tree and brush trimmings
(EPA, 2015, p. 56) from residential, institutional and commercial sources.
Mixed Paper
General
Definition
Mixed paper is assumed to be 24% newspaper, 48% corrugated cardboard, 8% magazines, and
20% office paper (Barlaz, 1998).
Residential
Definition
Residential mixed paper is assumed to be 23% newspaper, 53% corrugated cardboard, 10%
magazines and 14% office paper (Barlaz, 1998).
Office
Definition
Office mixed paper is assumed to be 21% newspaper, 5% corrugated cardboard, 36% magazines
and 38% office paper (Barlaz, 1998).
Mixed Metals
Mixed metals are made up of a weighted average of 35% aluminum cans and 65% steel cans.
Mixed Plastics
Mixed plastics are made up of a weighted average of 40% HDPE and 60% PET plastic.
Mixed Recyclables
Mixed recyclables are made up of a weighted average of approximately 1% aluminum cans, 2%
steel cans, 6% glass, 1% HDPE, 2% PET, 57% corrugated cardboard, 7% magazines/third-class
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WARM Data Source Definition
mail, 10% newspaper, 8% office papers, <1% phonebooks, <1% textbooks, and 5% dimensional
lumber. See those definitions for details.
Mixed Organics
Mixed organics are made up of a weighted average based on 53% food waste and 47% yard
trimmings. See those definitions for details.
Mixed MSW
Mixed MSW (municipal solid waste) comprises the waste materials typically discarded by
households and collected by curbside collection vehicles; it does not include white goods (e.g.,
refrigerators, toasters) or industrial waste.
Carpet
Carpet represents nylon broadloom residential carpet containing face fiber, primary and
secondary backing, and latex used for attaching the backings.
Desktop CPUs
Desktop CPUs include the stand-alone processing unit for a desktop computer and does not
include the monitor or any peripherals (e.g., mice, keyboards).
Portable Electronic Devices
Portable electronic devices include laptops, e-readers, tablets, smart phones, and basic mobile
phones.
Flat-panel Displays
Flat-panel displays include LED and liquid crystal display (LCD) televisions, plasma televisions,
and LED and LCD computer monitors.
CRT Displays
CRT displays include CRT televisions and CRT computer monitors. While CRT displays are no
longer manufactured, many are still entering the waste stream in the U.S.
Electronic Peripherals
Electronic peripherals consist of electronic devices used in conjunction with other products and
include keyboards and mice.
Hard-copy Devices
Hard-copy devices include electronic devices used for preparing hard-copy documents, including
printers and multi-function devices.
Mixed Electronics
Mixed recyclables are made up of a weighted average of approximately 11% desktop CPUs, 5%
portable electronic devices, 23% flat-panel displays, 44% CRT displays, 2% electronic peripherals,
and 15% hard-copy devices. See those definitions for details.
Clay Bricks
Bricks are produced by firing materials such as clay, kaolin, fire clay, bentonite, or common clay
and shale. The majority of the bricks produced in the United States are clay. In WARM, clay brick
source reduction is considered to be the reuse of full bricks rather than the grinding and reusing
of broken or damaged brick.
Concrete
Concrete is a high-volume building material produced by mixing cement, water, and coarse and
fine aggregates. In WARM, concrete is assumed to be recycled into aggregate, so the GHG
benefits are associated with the avoided emissions from mining and processing aggregate.
Fly Ash
Fly ash is a byproduct of coal combustion that is used as a cement replacement in concrete.
Tires
Scrap tires are tires that have been disposed of by consumers and have several end uses in the
U.S. market, including as a fuel, in civil engineering, and in various ground rubber applications
such as running tracks and molded products.
Asphalt Concrete
Asphalt concrete is composed primarily of aggregate, which consists of hard, graduated
fragments of sand, gravel, crushed stone, slag, rock dust or powder.
Asphalt Shingles
Asphalt shingles are typically made of a felt mat saturated with asphalt. Fiberglass shingles are
composed of asphalt cement (22% by weight), a mineral stabilizer like limestone or dolomite
(25%), and sand-sized mineral granules/aggregate (38%), in addition to the fiberglass felt backing
(15%) (CMRA, 2007).
Drywall
Drywall, also known as wallboard, gypsum board or plaster board, is manufactured from gypsum
plaster and a paper covering.
Fiberglass Insulation
Fiberglass insulation is produced from a blend of sand, limestone, soda ash and recycled glass
cullet, which accounts for about 40% of the raw material inputs.
Vinyl Flooring
All vinyl flooring is composed of polyvinyl chloride (PVC) resin, along with additives such as
plasticizers, stabilizers, pigments and fillers.
Wood Flooring
Virgin hardwood flooring is produced from lumber. Coatings and sealants can be applied to
wood flooring in "pre-finishing" that occurs at the manufacturing facility, or onsite.
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1.3 INTRODUCTION TO WARM METHODOLOGY
1.3.1 A Streamlined Life-Cycle Inventory
Source reduction, recycling, composting, anaerobic digestion, combustion and landfilling are all
materials management options that provide opportunities for reducing GHG emissions, depending on
individual circumstances. Although source reduction and recycling are often the most advantageous
practices from a GHG perspective, a material-specific comparison of all available materials management
options clarifies where the greatest GHG benefits can be obtained for particular materials. A material-
specific comparison can help waste managers and policy-makers identify the best options for GHG
reductions through materials management.
EPA determined that the best way to conduct such a comparative analysis is a streamlined
application of a life-cycle assessment (LCA). A full LCA is an analytical framework for understanding the
material inputs, energy inputs and environmental releases associated with manufacturing, using,
transporting and disposing of a given material. A full LCA generally consists of four parts: (1) goal
definition and scoping; (2) an inventory of the materials and energy used during all stages in the life of a
product or process, and an inventory of environmental releases throughout the product life cycle; (3) an
impact assessment that examines potential and actual human health effects related to the use of
resources and environmental releases; and (4) an assessment of the change that is needed to bring
about environmental improvements in the product or processes.
WARM does not provide a full LCA, as EPA wanted the tool to be transparent, easy to access and
use, and focused on providing decision-makers with information on climate change impacts, namely
GHG and energy implications. WARM'S streamlined LCA is limited to an inventory of GHG emissions and
sinks and energy impacts. This study did not assess human health impacts, or air, water or other
environmental impacts that do not have a direct bearing on climate change. WARM also simplifies the
calculation of emissions from points in the life cycle that occur before a material reaches end of life.
1.3.2 Assessing GHG Flux Associated with Material Life-Cycle Stages
The streamlined LCA used in WARM depends on accurately assessing the GHG and energy
implications of relevant life-cycle stages. The GHG implications associated with materials differ
depending on raw material extraction requirements and how the materials are manufactured and
disposed of at end of life. WARM evaluates the GHG emissions associated with materials management
based on analysis of three main factors: (1) GHG emissions throughout the life cycle of the material
(including the chosen end-of-life management option); (2) the extent to which carbon sinks are affected
by manufacturing, recycling and disposing of the material; and (3) the extent to which the management
option recovers energy that can be used to replace electric utility energy, thus reducing electric utility
emissions.
The life cycle of a material or product includes the following primary life-cycle stages: (1)
extraction and processing of raw materials; (2) manufacture of products; (3) transportation of materials
and products to markets; (4) use by consumers; and (5) end-of-life management. GHGs are emitted from
(1) the pre-consumer stages of raw materials acquisition and manufacturing, and (2) the post-consumer
stage of end-of-life management.
WARM does not include emissions from the use phase of a product's life, since use does not
have an effect on the waste management emissions of a product. Since the design and results of WARM
include the difference between the baseline and the alternative waste management scenarios that show
the GHG savings from different treatment options, emissions from the use phase are the same in both
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the baseline and alternative scenarios; therefore, emissions from the use phase are excluded and all
tables and analyses in this report use a "waste generation" reference point.
Materials management decisions can reduce GHGs by affecting one or more of the following:
• Energy consumption (specifically combustion of fossil fuels) and the resulting GHG emissions
associated with material extraction, manufacturing, transporting, using, and end-of-life
management of the material or product ,3
• Non-energy-related manufacturing emissions, such as the carbon dioxide (C02) released when
limestone used in steel manufacturing is converted to lime, or the perfluorocarbons (PFCs)
generated during the aluminum smelting process.
• Methane (CH4) emissions from decomposition of organic materials in landfills.
• C02 and nitrous oxide (N20) emissions from waste combustion.
• Carbon sequestration and storage, which refer to natural or manmade processes that remove
carbon from the atmosphere and store it for long periods or permanently.
The first four mechanisms add GHGs to the atmosphere and contribute to climate change. The
fifth—carbon storage—reduces GHG concentrations. Forest growth is one mechanism for sequestering
carbon; if more biomass is grown than is removed (through harvest or decay), the amount of carbon
stored in trees increases.
Each combination of material or product type and materials management option will have
different implications for energy consumption, GHG emissions and carbon storage. This is because the
upstream (raw materials acquisition, manufacturing and forest carbon sequestration) and downstream
(recycling, composting, combustion, anaerobic digestion, and landfilling) characteristics of each material
and product are different. Section 1.3.2 gives an overview of how WARM analyzes each of the upstream
and downstream stages in the life cycle. The GHG emissions and carbon sinks are described in detail and
quantified for each material in the material-specific chapters.
1.3.2.1 Waste Generation Reference Point
One important difference between WARM and other life-cycle analyses is that WARM calculates
emission impacts from a waste generation reference point, rather than a raw materials extraction
reference point. Raw materials extraction is the point at which production of the material begins, which
is why many life-cycle analyses choose this reference point. However, WARM uses the waste generation
point (the moment that a material is discarded) because in WARM, the GHG benefits measured result
from the choice of one waste management path relative to another. WARM does capture upstream
emissions and sinks, but only when at least one of the practices being compared is recycling or source
reduction, as these are the only instances where the choice of a materials management practice will
affect upstream emissions.
To apply the GHG emission factors developed in this report, one must compare a baseline
scenario with an alternate scenario. For example, one could compare a baseline scenario, where 10 tons
of office paper are landfilled, to an alternate scenario, where 10 tons of office paper are recycled.
3 Depending on the material/product type; however, the use phase is not included in WARM, as discussed in the
previous paragraph.
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1.3.3 Emissions Sources and Sinks in WARM
As discussed above, EPA focused on aspects of the life cycle that have the potential to emit
GHGs as materials are converted from raw resources to products and then to waste. Exhibit 1-3
describes the steps in the material life cycle modeled in WARM at which GHGs are emitted, carbon
sequestration is affected, and electric utility energy is displaced. As shown, EPA examined the potential
for these effects at the following points in a material's life cycle:
• Raw material acquisition and manufacturing (fossil fuel energy and other emissions, and
changes in forest carbon sequestration);
• Carbon sinks in forests and soils (forest carbon storage associated with reduced tree harvest
from source reduction and recycling, soil carbon storage associated with application of
compost); and
• End-of-life management (C02, CH4, and N20 emissions associated with composting and
anaerobic digestion, nonbiogenic C02 and N20 emissions from combustion, and CH4 emissions
from landfills); these emissions are offset to some degree by carbon storage in soil and landfills,
as well as by avoided utility emissions from energy recovery at combustors, anaerobic digesters,
and landfills.
• At each point in the material life cycle, EPA also considered transportation-related energy
emissions.
Estimates of GHG emissions associated with electricity used in the raw materials acquisition and
manufacturing steps are based on the nation's current mix of energy sources, including fossil fuels,
hydropower and nuclear power. However, when estimating GHG emission reductions attributable to
electric utility emissions avoided from landfill gas capture, anaerobic digesters, or waste-to-energy at
combustion facilities, the electricity use displaced by waste management practices is assumed to be
from non-baseload power plants to represent the marginal electricity emissions offset. EPA did not
analyze the GHG emissions typically associated with consumer use of products because the purpose of
the analysis is to evaluate one materials management option relative to another. EPA assumed that the
energy consumed during use would be approximately the same whether the product was made from
virgin or recycled inputs. In addition, energy use at this life-cycle stage is small (or zero) for all materials
studied except electronics.
Exhibit 1-3 shows how GHG sources and sinks are affected by each waste management strategy.
For example, the top row of the exhibit shows that source reduction (1) reduces GHG emissions from
raw materials acquisition and manufacturing; (2) results in an increase in forest carbon sequestration for
certain materials; and (3) does not result in GHG emissions from waste management ,4 The sum of
emissions (and sinks) across all steps in the life cycle represents net emissions for each material
management strategy.
4 The source reduction techniques the EPA researchers analyzed involve using less of a given product—e.g., by
making aluminum cans with less aluminum ("lightweighting"); double-sided rather than single-sided photocopying;
or reuse of a product. EPA did not analyze source reduction through material substitution (except in the special
case of fly ash)—e.g., substituting plastic boxes for corrugated paper boxes. For a discussion of source reduction
with material substitution, see the Source Reduction chapter.
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Exhibit 1-3: Components of Net Emissions for Various Materials Management Strategies
Materials
GHG Sources and Sinks Modeled in WARM
Management
Raw Materials Acquisition and
Changes in Forest or Soil
Strategies
Manufacturing
Carbon Storage
End of Life
Source Reduction
Offsets
• Decrease in GHG emissions,
relative to the baseline of
manufacturing with the current
industry average mix of virgin
and recycled inputs
Offsets
• Increase in forest
carbon sequestration
(for paper and wood
products) due to
avoided harvesting
NA
Recycling
Emissions
• Transport of recycled materials
• Recycled manufacture process
energy and non-energy
Offsets
• Transport of raw materials and
products
• Virgin manufacture process
energy and non-energy
Emissions
• Transport to recycling facility and
sorting of recycled materials at
material recovery facility (MRF)
Composting
Emissions3
Offsets
Emissions
• Baseline process and
• Increase in soil carbon
• Transport to compost facility
transportation emissions due to
storage from
• Equipment use at compost facility
manufacture with the current
application of compost
• CH4 and N20 emissions during
mix of virgin and recycled inputs
to soils
composting
Combustion
Emissions
• Baseline process and
transportation emissions due to
manufacture with the current
mix of virgin and recycled inputs
NA
Emissions
• Transport to WTE facility
• Combustion-related non-biogenic
C02 and N20
Offsets
• Avoided electric utility emissions
due to WTE
• Avoided steel manufacture from
steel recovery at WTE for
combusted materials including
steel cans, mixed metals, mixed
recyclables, electronics, tires and
mixed MSW
Landfilling
Emissions
• Baseline process and
transportation emissions due to
manufacture with the current
mix of virgin and recycled inputs
NA
Emissions
• Transport to landfill
• Equipment use at landfill
• Landfill methane
Offsets
• Avoided utility emissions due to
landfill gas to energy
• Landfill carbon storage
Anaerobic Digestion
Emissions3
Offsets
Emissions
• Baseline process and
• Increase in soil carbon
• Transport to anaerobic digester
transportation emissions due to
storage from
• Equipment use and biogas leakage
manufacture with the current
application of digestate
at anaerobic digester
mix of virgin and recycled inputs
to soils
• CH4 and N20 emissions during
digestate curing
• N20 emissions from land
application of digestate
Offsets
• Avoided utility emissions due to
biogas to energy
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Materials
Management
Strategies
GHG Sources and Sinks Modeled in WARM
Raw Materials Acquisition and
Manufacturing
Changes in Forest or Soil
Carbon Storage
End of Life
• Avoided synthetic fertilizer use due
to land application of digestate
NA = Not Applicable.
a Manufacturing and transportation GHG emissions are considered for composting and anaerobic digestion for only food waste
and PLA (composting only) because yard trimmings are not considered to be manufactured.
C02 Emissions from Biogenic Sources
The United States and all other parties to the United Nations Framework Convention on Climate
Change (UNFCCC) agreed to develop inventories of GHGs for purposes of (1) developing mitigation
strategies and (2) monitoring the progress of those strategies. In 2006, the Intergovernmental Panel on
Climate change (IPCC) updated a set of inventory methods that it had first developed in 1996 to be used
as the international standard (IPCC (1996); IPCC (2006)). The methodologies used in this report to
evaluate emissions and sinks of GHGs are consistent with the IPCC guidance.
One of the elements of the IPCC guidance that deserves special mention is the approach used to
address C02 emissions from biogenic sources. For many countries, the treatment of C02 flux from
biogenic sources is most important when addressing releases from energy derived from biomass (e.g.,
burning wood), but this element is also important when evaluating waste management emissions (for
example, the decomposition or combustion of grass clippings or paper). The carbon in paper and grass
trimmings was originally removed from the atmosphere by photosynthesis and, under natural
conditions, it would cycle back to the atmosphere eventually as C02 due to degradation processes. The
quantity of carbon that these natural processes cycle through the Earth's atmosphere, waters, soils and
biota is much greater than the quantity added by anthropogenic GHG sources. But the focus of the
UNFCCC is on anthropogenic emissions—those resulting from human activities and subject to human
control. Those emissions have the potential to alter the climate by disrupting the natural balances in
carbon's biogeochemical cycle and altering the atmosphere's heat-trapping ability.
For processes with C02 emissions, if the emissions are from biogenic materials and the materials
are grown on a sustainable basis, then those emissions are considered simply to close the loop in the
natural carbon cycle. They return to the atmosphere C02 that was originally removed by photosynthesis.
In this case, the C02 emissions are not counted. (For purposes of this analysis, biogenic materials are
paper and wood products, yard trimmings and food discards.) On the other hand, C02 emissions from
burning fossil fuels are counted because these emissions would not enter the cycle were it not for
human activity. Likewise, CH4 emissions from landfills are counted. Even though the source of carbon is
primarily biogenic, CH4 would not be emitted were it not for the human activity of landfilling the waste,
which creates anaerobic conditions conducive to CH4 formation.
Note that this approach does not distinguish between the timing of C02 emissions, provided that
they occur in a reasonably short time scale relative to the speed of the processes that affect global
climate change. In other words, as long as the biogenic carbon would eventually be released as C02,
whether it is released virtually instantaneously (e.g., from combustion) or over a period of a few decades
(e.g., decomposition on the forest floor) is inconsequential.
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1.4 SUMMARY OF THE LIFE CYCLE STAGES MODELED IN WARM
1.4.1 GHG Emissions and Carbon Sinks Associated with Raw Materials Acquisition and
Manufacturing
Raw inputs are needed to make various materials, including ore for manufacturing metal
products, trees for making paper products, and petroleum or natural gas for producing plastic products.
Fuel energy also is required to obtain or extract these material inputs.
The inputs for manufacturing considered in this analysis are (1) energy and (2) either virgin raw
materials or recycled materials.5
When a material is source reduced, GHG emissions associated with raw material acquisition,
producing the material and/or manufacturing the product and managing the post-consumer waste are
avoided. Since many materials are manufactured from a mix of virgin and recycled inputs, the quantity
of virgin material production that is avoided is not always equal to the quantity of material source
reduced. To estimate GHG emissions associated with source reduction, WARM uses a mix of virgin and
recycled inputs (referred to throughout the documentation as "the current mix"), based on the national
average for that material. For example, in source reducing 100 tons of aluminum cans, WARM models
that only 32 tons of virgin aluminum manufacture are avoided, because the current mix for aluminum is
32 percent virgin inputs and 68 percent recycled inputs. WARM also assumes that source reduction of
paper and wood products increases the amount of carbon stored in forests by reducing the amount of
wood harvested. See the Source Reduction process chapter for further information on calculation of
offsets resulting from source reduction.
The GHG emissions associated with raw materials acquisition and manufacturing are (1) GHG
emissions from energy used during the acquisition and manufacturing processes, (2) GHG emissions
from energy used to transport materials, and (3) non-energy GHG emissions resulting from
manufacturing processes.6 Each of these emission sources is described below. Changes in carbon
sequestration in forests also are associated with raw materials acquisition for paper and wood products.
For more information on forest carbon sequestration associated with source reduction of paper and
wood products, see the Forest Carbon Storage chapter.
1.4.1.1 Process Energy GHG Emissions
Process energy GHG emissions consist primarily of C02 emissions from the combustion of fuels
used in raw materials acquisition and manufacturing. C02 emissions from combustion of biomass are not
counted as GHG emissions. (See "C02 Emissions from Biogenic Sources" text box in section 1.3.3.)
The majority of process energy C02 emissions result from the direct combustion of fuels, e.g., to
operate ore mining equipment or to fuel a blast furnace. Fuel also is needed to extract the oil or mine
the coal that is ultimately used to produce energy and transport fuels to the place where they are used.
Thus, indirect C02 emissions from "precombustion energy" are counted in this category as well. When
electricity generated by combustion of fossil fuels is used in manufacturing, the resulting C02 emissions
are also counted.
5 Water is also often a key input to manufacturing processes, but is not considered here because it does not have
direct GHG implications.
6 For some materials (plastics, magazines/third-class mail, office paper, phone books, and textbooks), the
transportation data EPA received were included in the process energy data. For these materials, EPA reports total
GHG emissions associated with process and transportation in the "process energy" estimate. The transportation
energy estimate therefore only includes emissions from transport from the point of manufacture to a retail facility,
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To estimate process energy GHG emissions, EPA first obtained estimates of both the total
amount of process energy used per ton of product (measured in British thermal units or Btu) and the
fuel mix (e.g., diesel oil, natural gas, fuel oil). Next, emission factors for each type of fuel were used to
convert fuel consumption to GHG emissions based on fuel combustion carbon coefficients per fuel type
(EPA, 2018b). As noted earlier, making a material from recycled inputs generally requires less process
energy (and uses a different fuel mix) than making the material from virgin inputs.
The fuel mixes used in these calculations reflect the material-specific industry average U.S. fuel
mixes for each manufacturing process. However, it is worth noting that U.S. consumer products (which
eventually become MSW) increasingly come from overseas, where the fuel mixes may differ. For
example, China relies heavily on coal and generally uses energy less efficiently than does the United
States. Consequently the GHG emissions associated with the manufacture of a material in China may be
higher than they would be for the same material made in this country. In addition, greater energy is
likely to be expended on transportation to China than on transportation associated with domestic
recycling. However, such analysis is beyond the scope of this model, which focuses only on domestic
production, transportation, consumption and disposal.
1.4.1.2 Process Non-Energy GHG Emissions
Some GHG emissions occur during the manufacture of certain materials and are not associated
with energy consumption. In this analysis, these emissions are referred to as process non-energy
emissions. For example, the production of steel or aluminum requires lime (calcium oxide, or CaO),
which is produced from limestone (calcium carbonate, or CaC03), and the manufacture of lime results in
C02 emissions. In some cases, process non-energy GHG emissions are associated only with production
using virgin inputs; in other cases, these emissions result when either virgin or recycled inputs are used.
1.4.1.3 Transportation Energy GHG Emissions
Transportation energy GHG emissions consist of C02 emissions from the combustion of fossil
fuels used to (1) transport raw materials and intermediate products during the manufacturing stage and
(2) transport the finished products from the manufacturing facilities to the retail/distribution point.
The estimates of transportation energy emissions for transportation of raw materials to the
manufacturing or fabrication facility are based on: (1) the amounts of raw material inputs and
intermediate products used in manufacturing one short ton of each material; (2) the average distance
that each raw material input or intermediate product is transported; and (3) the transportation modes
and fuels used. For the amounts of fuel used, the study used data on the average fuel consumption per
ton-mile for each mode of transportation as represented in the industry average life-cycle inventory
data.
The estimates of GHG emissions from transporting manufactured products or materials from the
manufacturing point to the retail/distribution point are calculated using information from the U.S.
Census Bureau, along with the Bureau of Transportation Statistics. These agencies conducted a
Commodity Flow Survey that determined the average distance typical commodities were shipped in the
United States, and the percentage of each of the various transportation modes that was used to ship
these commodities (BTS, 2013). However, there is large variability in the shipping distance and modes
used, and so transportation emission estimates given here are somewhat uncertain.
The final step of the analysis applies fuel combustion carbon coefficients for each fuel type from
the U.S. Inventory in order to convert fuel consumption to GHG emissions (EPA, 2018b).
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1.4.1.4 Carbon Storage, Carbon Sequestration and Carbon Stocks
This analysis includes carbon sequestration and storage when relevant to materials
management practices. Carbon storage is the prevention of the release of carbon to the atmosphere. In
the context of WARM, this storage can occur in living trees, in undecomposed biogenic organic matter
(wood, paper, yard trimmings, food waste) in landfills, or in undecomposed biogenic organic matter in
soils due to compost or digestate amendment.
Carbon sequestration is the transfer of carbon from the atmosphere to a carbon pool, where it
can be stored if it is not rereleased to the atmosphere through decay or burning. Carbon sequestration
occurs when trees or other plants undergo photosynthesis, converting C02 in the atmosphere to carbon
in their biomass. As forests grow, they absorb atmospheric C02 and store it. When the rate of uptake
exceeds the rate of release, carbon is said to be sequestered. In this analysis, EPA considered the impact
of waste management on forest carbon storage. The amount of carbon stored in forest trees is referred
to as a forest's carbon stock. WARM models carbon storage, sequestration and stocks at several points
in the life-cycle analysis, as detailed below:
• Forest carbon storage increases as a result of source reduction or recycling of paper products
because both source reduction and recycling cause annual tree harvests to drop below
otherwise anticipated levels (resulting in additional accumulation of carbon in forests).
Consequently, source reduction and recycling "get credit" for increasing the forest carbon stock,
whereas other waste management options (combustion and landfilling) do not. See the Source
Reduction and Recycling process chapters for more information on this modeling analysis.
• Although source reduction and recycling are associated with forest carbon storage, the
application of compost to degraded soils enhances soil carbon storage. The Composting process
chapter details the modeling approach used to estimate the magnitude of carbon storage
associated with composting.
• Landfill carbon stocks increase overtime because much of the organic matter placed in landfills
does not decompose, especially if the landfill is located in an arid area. See the Landfilling
process chapter for further information on carbon storage in landfills.
1.4.2 GHG Emissions and Carbon Sinks Associated with Materials Management
As shown in Exhibit 1-3, depending on the material, WARM models up to five post-consumer
materials management options, including recycling, composting, combustion, anaerobic digestion, and
landfilling. WARM also models source reduction as an alternative materials management option. This
section describes the GHG emissions and carbon sinks associated with each option.
1.4.2.1 Recycling
When a material is recycled, this analysis assumes that the recycled material replaces the use of
virgin inputs in the manufacturing process. This approach is based on the assumption that demand for
new materials/products and demand for recycled materials remains constant. In other words, increased
recycling does not cause more (or less) material to be manufactured than would have otherwise been
produced. In WARM, each ton of recycled material would displace the virgin material that would have
been produced in the absence of recycling. EPA recognized that, in reality, there may be a relationship
between recycling and demand for products with recycled content since these products may become
cheaper as the supply of recycled materials increases. However, for the purpose of simplicity in WARM,
EPA assumed that increased recycling does not change overall demand for products.
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The avoided GHG emissions from remanufacture using recycled inputs is calculated as the
difference between (1) the GHG emissions from manufacturing a material with 100 percent recycled
inputs, and (2) the GHG emissions from manufacturing an equivalent amount of the material
(accounting for loss rates associated with curbside collection losses and remanufacturing losses) with
100 percent virgin inputs. The GHG emissions associated with manufacturing a material with 100
percent recycled inputs includes the process of collecting and transporting the recyclables used in
remanufacture. EPA did not consider GHG emissions at the MSW management stage because the
recycled material is diverted from waste management facilities (i.e., landfills or combustion facilities).7 If
the product made from the recycled material is later composted, combusted or landfilled, the GHG
emissions at that point would be attributed to the product that was made from the recycled material.
The Recycling chapter discusses the process in further detail.
Recycling processes can be broadly classified into two different categories: open-loop and
closed-loop recycling. Most of the materials in WARM are modeled in a closed-loop recycling process
where end-of-life products are recycled back into the same product (e.g., a recycled aluminum can
becomes a new aluminum can). Decisions about whether to model materials in an open-loop or closed-
loop process are based on how the material is most often recycled and the availability of data. For
materials recycled in an open loop, the products of the recycling process differ from the inputs. In open-
loop emission factors, the GHG benefits of material recycling result from the avoided emissions
associated with the virgin manufacture of the secondary products into which the material is recycled.
The materials modeled as open-loop recycling processes in WARM are: mixed paper, corrugated
containers (partial open-loop) copper wire, carpet, electronics, concrete, tires, fly ash, asphalt shingles
and drywall (partial open-loop).8,9 For more detail on the recycling pathways for particular materials or
products, see the material-specific chapter. For more information on recycling, see the Recycling process
chapter.
1.4.2.2 Source Reduction
In this analysis, source reduction is measured by the amount of material that would otherwise
be produced but is not generated due to a program promoting waste minimization or source reduction.
Source Reduction refers to any change in the design, manufacture, purchase or use of materials or
products (including packaging) that reduces the amount of material entering the waste collection and
disposal system. Source reduction conserves resources and reduces GHG emissions. The avoided GHG
emissions are based on raw material acquisition and manufacturing processes for the industry average
current mix of virgin and recycled inputs for materials in the marketplace.10 There are no emissions from
end-of-life management because it is assumed that a certain amount of material or product was never
produced in the first place.
7 The EPA researchers did not include GHG emissions from managing residues (e.g., wastewater treatment sludges)
from the manufacturing process for either virgin or recycled inputs.
8 Note that corrugated is modeled using a partial open-loop recycling process. Roughly 70 percent of the recycled
corrugated is closed-loop (i.e., replaces virgin corrugated) and 30 percent is open-loop (i.e., replaces boxboard).
9 Most recycled drywall is used for a variety of agricultural purposes, but can also be recycled back into new
drywall. Approximately 20 percent of recycled drywall is closed-loop (i.e., replaces virgin drywall) and 80 percent is
open-loop (i.e., used for agricultural purposes).
10 Changes in the mix of production (i.e., higher proportions of either virgin or recycled inputs) result in
incremental emissions (or reductions) with respect to this reference point.
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1.4.2.3 Composting
WARM models composting as resulting in both carbon storage and minimal C02 emissions from
transportation and mechanical turning of the compost piles. Composting also results in C02 emissions
from the decomposition of source materials, which include leaves, brush, grass, food waste and
newspaper. However, as described in the text box on "C02 Emissions from Biogenic Sources," the
biogenic C02 emitted from these materials during composting is not counted toward GHG emissions.
Composting also produces small amounts of CH4 and N20 (due to anaerobic decomposition during
composting), which vary depending on the carbon and nitrogen ratios of the waste being composted.
Because recent literature indicated that these fugitive emissions occurred even in well-managed
compost piles, these emissions were added into WARM version 13. Composting does result in increased
soil carbon storage due to the effects of compost application on soil carbon restoration and humus
formation. For more information on GHG flux resulting from composting, see the Composting process
chapter.
1.4.2.4 Combustion
When materials are combusted at waste-to-energy facilities, GHGs in the form of C02 and N20
are emitted. Nonbiogenic C02 emitted during combustion (i.e., C02 from plastics) is counted toward the
GHG emissions associated with combustion, but biogenic C02 (i.e., C02 from paper products) is not.
WARM assumes that the combustion pathway involves only waste-to-energy facilities that produce
electricity. This electricity substitutes for utility-generated electricity and therefore the net GHG
emissions are calculated by subtracting the electric utility GHG emissions avoided from the gross GHG
emissions. GHG emissions from combustion are described further in the Combustion chapter.
1.4.2.5 Anaerobic Digestion
During anaerobic digestion, degradable materials, such as yard trimmings and food waste, are
digested in a reactor in the absence of oxygen to produce biogas that is between 50-70% CH4. This
biogas is then typically burned on-site for electricity generation. WARM includes anaerobic digestion as
a materials management option for yard trimmings, food waste, and mixed organics. As modeled in
WARM, anaerobic digestion results in C02 emissions from transportation, preprocessing and digester
operations, carbon storage (associated with application of digestate to agricultural soils), nitrogen and
phosphorous fertilizer offsets, net electricity offsets, and where applicable, digestate curing. Emissions
estimates also include fugitive emissions of CH4 and N20 produced during digestate decomposition.
1.4.2.6 Landfiiling
When organic matter is landfilled, some of this matter decomposes anaerobically and releases
CH4. Some of the organic matter never decomposes at all; instead, the carbon becomes stored in the
landfill. Landfiiling of metals and plastics does not result in CH4 emissions or carbon storage.
At some landfills, virtually all of the CH4 produced is released to the atmosphere. At others, CH4
is captured for flaring or combustion with energy recovery (e.g., electricity production). Almost all of the
captured CH4 is converted to C02, but is not counted in this study as a GHG because it is biogenic. With
combustion of CH4 for energy recovery, emission factors reflect the electric utility GHG emissions
avoided. Regardless of the fate of the CH4, the landfill carbon storage associated with landfiiling of some
organic materials is accounted for. GHG emissions and carbon sinks from landfiiling are described in the
Landfiiling chapter.
1.4.2.7 Forest Carbon Storage
See section 1.4.1.4 for discussion.
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1.4.2.8 Avoided Electric Utility GHG Emissions Related to Waste
Waste that is used to generate electricity (either through waste combustion, biogas capture at
an anaerobic digester, or recovery and burning of CH4 from landfills) displaces fossil fuels that utilities
would otherwise use to produce electricity. Fossil fuel combustion is the single largest source of GHG
emissions in the United States. When waste is substituted for fossil fuels to generate electricity, the GHG
emissions from burning the waste are offset by the avoided electric utility GHG emissions. When gas
generated from decomposing waste at a landfill is combusted for energy, GHG emissions are reduced
from the landfill itself, and from avoided fossil fuel use for energy.
1.4.3 Temporal Aspects of Emission Factors in WARM
The emission factors used by WARM represent the full life-cycle changes in GHG emissions
resulting from an alternative end-of-life management practice relative to the current, or baseline
practice. Certain components of these life-cycle GHG emission factors, however, do not occur
immediately following end-of-life management of a material, but over a longer period of time. For
example, for paper, yard waste and food waste materials, not all of the GHG reductions occur within the
same year of recycling: a portion of the reduction in GHG emissions results from avoided methane
emissions from landfills and increased carbon storage in soils and forests. These emission reductions,
resulting from the avoided degradation of organic materials into methane in landfills and the
accumulation of carbon in forests, can occur over a timeframe of years to decades.
Consequently, WARM correctly accounts for the full range of GHG emission benefits from
alternative waste management practices, but it does not explicitly model the timing of GHG reductions
from these practices. Therefore, since WARM is a tool that describes the full life-cycle benefits of
alternative waste management pathways, it is not appropriate to directly compare the benefits of
alternative waste management as modeled through WARM with traditional GHG Inventory reports,
which quantify GHG emissions from different sectors on an annual basis. This section explains the
temporal components of WARM'S emission factors, and explains how WARM considers these timing
issues.
1.4.3.1 Temporal Components of WARM
The GHG emissions that occur throughout a materials management pathway can be released
instantaneously or over a period of time. For example, while combustion instantaneously releases GHGs,
the energy used to transport materials releases GHGs over the course of the trip, and materials
decomposing in landfills may release methane for decades. Four main parts of the life-cycle GHG
emissions and sinks calculated by WARM occur over time: (1) landfill methane emissions, (2) landfill
carbon storage, (3) forest carbon sequestration and storage, and (4) soil carbon storage from compost.
All four temporal components are relevant to management of organic materials such as paper and other
wood products, food waste and yard trimmings.
• Landfill Methane Emissions: When placed into a landfill, a fraction of the carbon within organic
materials degrades into methane emissions. The quantity and timing of methane emissions
released from the landfill depends upon at least four factors: (1) how much of the original
material decays into methane (varies from material to material), (2) how readily the material
decays, (3) landfill moisture conditions (wetter leading to faster decay), and (4) landfill gas
collection practices. Food waste and yard trimmings degrade within 20 to 30 years; materials
with slower decay rates, such as paper and wood products, release a sizable fraction of their
ultimate methane emissions after 30 years.
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• Landfill Carbon Storage: The fraction of carbon in organic materials that does not degrade into
landfill gas is permanently stored in the landfill. Consequently, the amount of carbon stored in
the landfill overtime is affected by how much of the original material decays into landfill gas,
and the speed (or rate) at which the material decays.
• Forest Carbon Sequestration and Storage: Recycling or "source reducing" wood products offsets
the demand for virgin wood. Trees that would otherwise be harvested are left standing in
forests. In the short term, this reduction in harvest increases carbon storage in forests; over the
longer-term, some of this additional carbon storage decreases as forest managers adjust by
planting fewer new trees in managed forests. Results from USDA Forest Service models suggest
that the forest carbon storage benefit is long-term, lasting at least for several decades (EPA,
2006, p. 41). WARM's life-cycle perspective includes several timing issues involving complex
economic relationships that affect the market for wood products (e.g., change in demand for
virgin wood, adjustment in harvest practices and change in forest management in response to
tree harvesting) relevant to carbon storage and release.
• Soil Carbon Storage: The stock of carbon in soils is the result of a balance between inputs
(usually plant matter) and outputs (primarily C02 flux during decomposition of organic matter).
When compost or digestate is applied to soils, a portion of the carbon in the compost remains
un-decomposed for many years and acts as a carbon sink. While research into the mechanisms
and magnitude of carbon storage is ongoing by EPA, WARM currently assumes that carbon from
compost and digestate remains stored in the soil through two main mechanisms: direct storage
of carbon in depleted soils and carbon stored in non-reactive humus compounds. Although the
carbon storage rate declines with time after initial application, the life-cycle perspective in
WARM assumes that the carbon stored in compost and digestate after a 10-year period is stable
in the long term.
Evaluating the timing of GHG emissions from waste management practices involves a high level
of uncertainty. For example, the timing of methane emissions from and carbon storage in landfills
depends upon uncertain and variable parameters such as the ultimate methane yield and rate of decay
in landfills; evaluating forest carbon storage involves complex economic relationships that affect the
market for wood products and the management of sustainably harvested forests. In addition to the four
components described above, timing issues may also apply to process energy and non-energy emissions
from raw material acquisition and manufacturing, transportation and other activities. Timing issues for
these components could depend upon factors such as how quickly markets respond to changes in
demand for virgin materials given increases in recycling.
EPA designed WARM as a tool for waste managers to use to compare the full, life-cycle GHG
benefits of alternative waste management pathways. Its strength as a tool is due to the relatively simple
framework that distills complicated analyses of the life-cycle energy and GHG emissions implications of
managing materials into a user-friendly spreadsheet model. The purpose of WARM, therefore, is to
capture the full life-cycle benefits of alternative waste management practices rather than model the
timing of GHG emissions or reductions.
This is fundamentally different from GHG inventories that quantify GHG emissions from
different sectors on an annual basis. GHG inventories, in contrast, are used to establish baselines, track
GHG emissions and measure reductions over time. The annual perspective of inventories, however,
changes depending upon the timeframe used to evaluate GHG emissions, offering a narrow—and
sometimes incomplete—picture of the full life-cycle benefits of materials management options. In
contrast, the life-cycle view is exactly the perspective that WARM is designed to communicate. As a
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result, WARM's emission factors cannot be applied to evaluate reductions from annual GHG inventories
because they do not necessarily represent annual reductions in emissions (i.e., emission reductions that
occur within the same calendar year).
1.5 LIMITATIONS
When conducting this analysis, EPA used a number of analytical approaches and numerous data
sources, each with its own limitations. In addition, EPA made and applied assumptions throughout the
analysis. Although these limitations would be troublesome if used in the context of a regulatory
framework, EPA believes that the results are sufficiently accurate to support their use in decision-
making and voluntary programs. Some of the major limitations include the following:
• The manufacturing GHG analysis is based on estimated industry averages for energy usage, and
in some cases the estimates are based on limited data. In addition, EPA used values for the
average GHG emissions per ton of material produced, not the marginal emission rates per
incremental ton produced. In some cases, the marginal emission rates may be significantly
different.
• The forest carbon sequestration analysis deals with a very complicated set of interrelated
ecological and economic processes. Although the models used represent the state-of-the-art in
forest resource planning, their geographic scope is limited. Because of the global market for
forest products, the actual effects of paper recycling would occur not only in the U.S. but in
Canada and other countries. Other important limitations include: (1) the model assumed that no
forested lands will be converted to non-forest uses as a result of increased paper recycling; and
(2) EPA used a point estimate for forest carbon sequestration, whereas the system of models
predicts changing net sequestration over time. Forest carbon sequestration is discussed further
in the Forest Carbon Storage chapter.
• The composting analysis considered a small sampling of feedstocks and a single compost
application (i.e., agricultural soil). The analysis did not consider the full range of soil
conservation and management practices that could be used in combination with compost and
their impacts on carbon storage.
• The combustion analysis used national average values for several parameters; variability from
site to site is not reflected in the estimate.
• The landfill analysis: (1) incorporated some uncertainty on CH4 generation and carbon
sequestration for each material type, due to limited data availability; and (2) used estimated CH4
recovery levels for the year 2013 as a baseline.
• Every effort has been made to tailor WARM to the conditions found in the U.S., including, where
possible, production processes, fuel mixes and other underlying factors. Therefore, the results
can only be considered applicable to the U.S., and caution should be used in applying or
extrapolating them to other countries.
EPA cautions that the emission factors in WARM should be evaluated and applied with an
appreciation for the limitations in the data and methods, as described further at the end of each
chapter.
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1.6 REFERENCES
Atech Group. (2001). A National Approach to Waste Tyres. Prepared for Environment Australia, June,
Table 6.10, p. 34.
Athena. (2001). A Life Cycle Inventory for Road and Roofing Asphalt. Ottawa: Athena Sustainable
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Athena Sustainable Materials Institute. (1998). Life Cycle Analysis of Brick and Mortar Products.
Merrickville, ON.
Barlaz, M.A. (1998) Carbon storage during biodegradation of municipal solid waste components in
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Battelle. (1975). Energy Use Patterns in Metallurgical and Nonmetallic Mineral Processing (Phase 4—
Energy Data and Flowsheets, High-priority Commodities). In U.S. National Technical Information
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Columbus Laboratories.
Bergman, R. (2010). Personal communication between Richard Bergman, USDA Forest Service, and
Robert Renz and Christopher Evans, ICF International, March 15, 2010.
Bergman, R., & Bowe, S. A. (2008). Environmental impact of producing hardwood lumber using life-cycle
inventory. Wood and Fiber Science, 40 (3), 448-458. Retrieved from
http://www.treesearch.fs.fed.us/pubs/31113.
BTS. (2013). Commodity Flow Survey Preliminary Tables. Table 1: Shipment Characteristics by Mode of
Transportation forthe United States: 2012. Washington, DC: U.S. Bureau of Transportation
Statistics, Research and Innovative Technology Administration. Retrieved from
http://www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/publications/commodity flow survey/2
012/united states/tablel.html.
CMRA. (2007). Environmental Issues Associated with Asphalt Shingle Recycling. Gainesville, FL:
Innovative Waste Consulting Services, LLC, prepared for the Construction Materials Recycling
Association, October. Retrieved from http://www.p2pavs.org/ref/42/41583.pdf.
Cochran, K. (2006). Construction and Demolition Debris Recycling: Methods, Markets, and Policy.
Gainesville: University of Florida, Environmental Engineering Department.
Corti, A., & Lombardi, L. (2004). End life tyres: Alternative final disposal processes compared by LCA.
Energy, 29 (12-15), 2089-2108. doi: 10.1016/j.energy.2004.03.014.
ECOBILAN. (2001). Eco-profile of high volume commodity phthalate esters (DEHP/DINP/DIDP). La
Defense, France: ECOBILAN, prepared for The European Council of Plasticisers and
Intermediates (ECPI)
Ecoinvent Centre. (2008). ecoinvent Database v2.1. Swiss Centre for Life Cycle Inventories. Retrieved
February 13, 2009, from http://www.ecoinvent.ch/.
EIA. (2009). 2006 Manufacturing Energy Consumption Survey, Table 3.2: Fuel Consumption, 2006 for
Synthetic Rubber. (NAICS 325212.) Energy Information Administration. Retrieved from
http://www.eia.doe.gov/emeu/mecs/mecs2006/2006tables.html.
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Environment Canada. (2005). Determination of the Impact of Waste Management Activities on
Greenhouse Gas Emissions: 2005 Update, Final Report. Report prepared for Environment
Canada and Natural Resources Canada by ICF Consulting.
Enviros Consulting. (2003). Glass Recycling - Life Cycle Carbon Dioxide Emissions. Prepared by Enviros
Consulting, Limited for the British Glass Manufacturers Confederation, Public Affairs Committee.
November.
EPA. (2018a). Advancing Sustainable Materials Management: Facts and Figures 2015. (EPA530-F-18-
004).Washington, DC: U.S. Government Printing Office. Retrieved from
https://www.epa.gov/facts-and-figures-about-materials-waste-and-recvcling/advancing-
sustainable-materials-management.
EPA. (2018b). Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 - 2016. (EPA 430-R-18-004).
Washington, DC: U.S. Government Printing Office. Retrieved from
https://www.epa.gov/ghgemissions/us-greenhouse-gas-inventorv-report-archive.
EPA. (2015). Advancing Sustainable Materials Management: Facts and Figures 2013. (EPA530-R-15-002).
Washington, DC: U.S. Government Printing Office. Retrieved from
http://www.cta.tech/CorporateSite/media/environment/eCycle/2013 advncng smm rpt.pdf.
EPA. (2015b). Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 - 2013. (EPA 430-R-15-004).
Washington, DC: U.S. Government Printing Office. Retrieved from:
http://epa.gov/climatechange/emissions/usinventoryreport.html.
EPA. (2011). Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2009. Annex 2. April. (EPA
publication no. EPA 430-R-11-005.) U.S. Environmental Protection Agency, Office of Atmospheric
Programs. April. Retrieved from:
http://epa.gov/climatechange/emissions/usinventoryreport.html.
EPA. (2009a). Opportunities to Reduce Greenhouse Gas Emissions through Materials and Land
Management Practices. (EPA publication no. 530-R-09-017.) Washington, DC: U.S.
Environmental Protection Agency, Office of Solid Waste and Emergency Response.
EPA. (2009b). Sustainable Materials Management: The Road Ahead. (EPA publication no. EPA 530-R-09-
009.) Washington, DC.: U.S. Environmental Protection Agency.
EPA. (2008). Municipal Solid Waste Generation, Recycling, and Disposal in the United States: Facts and
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EPA. (2006). Solid Waste Management and Greenhouse Gases: A Life-Cycle Assessment of Emissions and
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Agency, Office of Solid Waste.
EPA. (2002). Solid Waste Management and Greenhouse Gases: A Life-Cycle Assessment of Emissions and
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EPA. (1998). Greenhouse Gas Emissions From Management of Selected Materials in Municipal Solid
Waste. Background Document A: A Life Cycle of Process and Transportation Energy for Eight
Different Materials. Washington, DC: U.S. Environmental Protection Agency.
FAL. (2011). Cradle-to-Gate Life Cycle Inventory of Nine Plastic Resins and Two Polyurethane Precursors.
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FAL. (2002a). Energy and Greenhouse Gas Factors for Nylon Broadloom Residential Carpet. Prairie
Village, KS: Franklin Associates, Ltd, prepared for U.S. Environmental Protection Agency, Office
of Solid Waste.
FAL. (2002b). Energy and Greenhouse Gas Factors for Personal Computers. Prairie Village, KS: Franklin
Associates, Ltd, prepared for U.S. Environmental Protection Agency, Office of Solid Waste.
FAL. (1998a). Background Document A. Attachment 1: A Partial Life Cycle Inventory of Process and
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FAL. (1998b). A Life Cycle Inventory of Process and Transportation Energy for Eight Different Materials-
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fiberboard.
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United States. Consortium for Research on Renewable Industrial Materials.
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Realff, M. (2011b). "WARM_information_FINAL.xls". Excel spreadsheet with life-cycle data provided to
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2 WARM DEFINITIONS AND ACRONYMS
2.1 DEFINITIONS
Aerobic
Anaerobic
Anthropogenic
Baseload electricity
Biogas
Biogenic
C&D landfill
Carbon offset
Carbon sequestration
Carbon storage
Cellulose
Closed-loop recycling
Combustion
Occurring in the presence of free oxygen.
Occurring in the absence of free oxygen.
Derived from human activities.
An estimate of the electricity produced from plants that are devoted to the
production of baseload electricity supply. Baseload plants are the production
facilities used to meet continuous energy demand, and produce energy at a
constant rate. Plants that run at over 80% capacity are considered
"baseload" generation; a share of generation from plants that run between
80% and 20% capacity is also included based on a "linear relationship."
A gas produced during the breakdown of organic matter in the absence of
oxygen and comprised of a mixture of different gases.
Of non-fossil, biological origin.
A landfill designed for and accepting only construction and demolition
materials.
Emission savings or storage that can be considered to cancel out emissions
that would otherwise have occurred. For example, electricity produced from
burning landfill gas is considered to replace electricity from the grid, leading
to a carbon offset because landfill gas production and combustion results in
lower GHG emissions than grid electricity production from fossil fuels.
The removal of carbon (usually in the form of carbon dioxide) from the
atmosphere, by plants or by technological means.
Prevention of the release of carbon to the atmosphere by its storage in living
plants (e.g., trees) and undecayed and unburned dead plant material (e.g.,
wood products, biogenic materials in landfills).
A polysaccharide that is the chief constituent of all plant tissues and fibers.
A recycling process in which the primary product type is remanufactured into
the same product type, (e.g., Aluminum cans recycled into aluminum cans.)
A waste management strategy in which the waste material is burned. Waste-
to-energy combustion facilities are set up to produce useful heat and/or
electricity.
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Combustion
emissions
Composting
Curing
Demanufacturing
Digestate
Downstream
emissions
Dry Digestion
Embedded energy
Emission factor
End-of-life pathways
Emissions from combustion adjusted based on regional avoided utility
emission factors.
A waste management strategy in which aerobic microbial decomposition
transforms biogenic material such as food scraps and yard trimmings into a
stable, humus-like material (compost).
The aerobic drying of digestate after it has been dewatered.
Disassembly and recycling of obsolete consumer products such as
computers, electronic appliances, and carpet into their constituents in order
to recover the metal, glass, plastic, other materials, and reusable parts.
The material remaining after anaerobically digesting biogenic matter.
Digestate can be in liquid or solid form and can either be cured before land
application or directly applied.
Emissions that occur at life-cycle stages after use: e.g., waste management.
The process of breaking down organic waste into useful biogas and compost
in an environment with little or no oxygen. This process accepts all organic
matter and operates at high total solids levels (20->40% total solids).
The energy contained within the raw materials used to manufacture a
product. For example, the embedded energy of plastics is due to their being
made from petroleum. Because petroleum has an inherent energy value, the
amount of energy that is saved through plastic recycling and source
reduction is directly related to the energy that could have been produced if
the petroleum had been used as an energy source rather than as a raw
material input.
Greenhouse gas emission in metric tons of carbon dioxide equivalent per
short ton of material managed.
The end-of-life management strategies available in WARM: recycling,
composting, combustion, and landfilling. Sometimes source reduction is
included in this phrase, although source reduction does not occur at end of
life.
Energy content
Fertilizer offset
The inherent energy of a material. For example, the amount of energy in a
plastic potentially available for release during combustion.
WARM calculates fertilizer offsets by assuming that the application of
compost or digestate avoids the GHG emissions associated with the
production and application of some portion of the fertilizer required for
arable land.
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Forest carbon
sequestration
Fugitive Emissions
Hemicellulose
Inorganic
Landfill carbon
storage
Landfilling
Leachate
Life-cycle assessment
Loss rate
As forests grow, they absorb atmospheric C02 and store it. When the rate of
uptake exceeds the rate of release, carbon is said to be sequestered. See also
carbon sequestration and carbon storage.
During the composting process, microbial activity decomposes waste into a
variety of compounds, whose composition depends on many factors,
including the original nutrient balance and composition of the waste, the
temperature and moisture conditions of the compost, and the amount of
oxygen present in the pile. In WARM, this process is refers to the generation
of small amounts of CH4 and N20.
Constituent of plant materials that is a polysaccharide, easily hydrated, and
easily decomposed by microbes.
1. Not referring to or derived from living organisms. 2. In chemistry, any
compound not containing carbon (with a few exceptions).
Biogenic materials in a landfill are not completely decomposed by anaerobic
bacteria, and some of the carbon in these materials is stored. Because this
carbon storage would not normally occur under natural conditions (virtually
all of the organic material would degrade to C02, completing the
photosynthesis/respiration cycle), this is counted as an anthropogenic sink.
However, carbon in plastic that remains in the landfill is not counted as
stored carbon, because it is of fossil origin.
A waste management strategy involving the anaerobic decomposition of
organic substrates producing CH4 and C02.
Liquid that percolates through waste material in a landfill picking up
contaminants from the waste material. Landfill leachate must be collected
and properly disposed of to avoid transferring the contaminants to
groundwater
An accounting method that evaluates and reports the full life-cycle inputs
and outputs (including GHG emissions) associated with the raw materials
extraction, manufacturing or processing, transportation, use, and end-of-life
management of a good or service.
The amount of recovered material that is lost during the recycling process,
relative to the total amount of collected material. The inverse of the
retention rate.
Materials (or waste) One of the five strategies in WARM: source reduction, recycling, composting,
management strategy combustion, and landfilling.
Methanogenic
MSW landfill
Biologically producing methane.
A landfill designed for and accepting only municipal solid waste.
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Non-baseload
electricity
An estimate of the marginal electricity produced from plants that are more
likely to respond to incremental changes in electricity supply and demand
based on their capacity factor. All power plants with capacity factors below
20% are considered "non-baseload". Plants that run at over 80% capacity are
considered "baseload" generation and not considered the "non-baseload"; a
share of generation from plants that run between 80% and 20% capacity is
included based on a "linear relationship".
Open-loop recycling
A recycling process in which the primary product is remanufactured into
other products that are different from the original primary product, (e.g.,
carpet recycled into molded auto parts).
Organic
1. Referring to or derived from living organisms. 2. In chemistry, any
compound containing carbon (with a few exceptions).
Partial-open-loop
recycling
A recycling process in which a portion of the primary product type is
remanufactured into the same product type, while the remaining portion is
recycled into other product types, e.g., corrugated containers are recycled
into both corrugated containers and paperboard.
Post-consumer
emissions
Emissions that occur after a consumer has used a product or material:
generally, waste management emissions.
Post-consumer
recycling
Materials or finished products that have served their intended use and have
been diverted or recovered from waste destined for disposal, having
completed their lives as consumer items. In contrast, pre-consumer recycling
is material (e.g., from within the manufacturing process) that is recycled
before it reaches the consumer.
Pre-combustion
emissions
The GHG emissions that are produced by extracting, transporting, and
processing fuels that are in turn consumed in the manufacture of products
and materials.
Process energy
emissions
Emissions from energy consumption during the acquisition and
manufacturing processes
Process non-energy
emissions
Emissions occurring during manufacture that are not associated with energy
consumption, e.g., perfluorocarbons (PFCs) are emitted during the
production of aluminum.
Recovery
The collection of used materials for recycling. Generally recovered materials
are taken from the point of use to a materials recovery facility (MRF).
Recycled input credit
WARM calculates the recycled input credit by assuming that the recycled
material avoids—or offsets—the GHG emissions associated with producing
the same amount of material from virgin inputs.
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Recycling
Recovering and reprocessing usable products that might otherwise become
waste.
Retail transport
emissions
Retention rate
The typical emissions from truck, rail, water, and other-modes of
transportation required to transport materials or products from the
manufacturing facility to the retail/distribution point.
The amount of recovered material that is transformed into a recycled
product, relative to the total amount of collected material. The inverse of the
loss rate.
Source reduction
Transportation
emissions
Upstream emissions
Any change in the design, manufacture, purchase, or use of materials or
products that reduces or delays the amount or toxicity of material entering
waste collection and disposal. These practices include lightweighting, double-
sided copying, and material reuse. It is also possible to source reduce one
type of material by substituting another material.
Emissions from energy used to transport materials, including transport of
manufactured product to retail/distribution point.
Emissions that occur at life-cycle stages prior to use: e.g., raw materials
acquisition, manufacturing, and transportation.
Waste-to-energy
facility
Wet digestion
Municipal solid waste incinerator that converts heat from combustion into
steam or electricity
The process of breaking down organic waste into useful biogas and compost
in an environment with little or no oxygen. This process accepts only food
waste and operates at low total solids levels (<10-20% total solids). Water is
added during the digestion process.
2.2 ACRONYMS
AF&PA American Forest and Paper Association
BBP benzyl butyl phthalate
Btu British thermal unit
C carbon
C2F6 hexafluoroethane
CaC03 limestone
CaO lime
CF4 tetrafluoromethane
CH4 Methane
CO2 carbon dioxide
CPU central processing unit
CRT cathode ray tube
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DINP diisononyl phthalate
EF emission factor
eGRID U.S. EPA's Emissions & Generation Resource Integrated Database
EPA U.S. Environmental Protection Agency
FAL Franklin Associates, Ltd.
FC forest carbon
FRA Forest Resources Association
GHG greenhouse gas
GWP global warming potential
HDPE high-density polyethylene
IPCC Intergovernmental Panel on Climate Change
kg kilogram
kWh kilowatt-hour
lb pound
LCA life cycle assessment
LCD liquid crystal display
LCI life cycle inventory
LDPE low-density polyethylene
LED light-emitting diode
LFG landfill gas
MDF medium-density fiberboard
MRT mean residence time
MSW municipal solid waste
MTCE metric tons carbon equivalent
MTC02E metric tons carbon dioxide equivalent
N nitrogen
N20 nitrous oxide
NAPAP North American Pulp and Paper
NREL National Renewable Energy Laboratory
PCB printed circuit board
PET polyethylene terephthalate
PRC paper recovery
PVC polyvinyl chloride
PWH pulpwood harvest
RDF refuse-derived fuel
RMAM raw materials acquisition and manufacturing
TS total solids
USDA U.S. Department of Agriculture
USDA-FS U.S. Department of Agriculture, Forest Service
VCT vinyl composition tile
VOC volatile organic compound
WARM Waste Reduction Model
WTE waste-to-energy
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3 RECENT UPDATES IN WARM
Since the release in 2006 of the 3rd edition of the Solid Waste Management and Greenhouse
Gases: A Life-Cycle Assessment of Emissions and Sinks Report, EPA has restructured the life-cycle
emission factor documentation previously published as a single report. As of 2010, the resulting WARM
documentation consists of individual chapters for each material type and waste management practice
that EPA has analyzed. This approach is more suited to the model structure and allows for easier
updating in the future than the previous hard-copy report structure. This Recent Updates document is
designed to communicate the structure of updates to recent versions of WARM.
With each new version of WARM, the model documentation is updated to reflect the regular
annual updates made to WARM, as well as other changes and improvements made to the model, as
described below. It should be noted that changes listed in "Annual Changes" and "Changes Made for
WARM Version 15" have not been implemented in other EPA tools including iWARM.
3.1 ANNUAL CHANGES
Certain updates to underlying WARM data are made annually, and have been implemented in
WARM Version 15. These include:
• Assumptions about landfill methane generation are updated based on the Inventory of U.S.
Greenhouse Gas Emissions and Sinks.
• MSW generation and recovery rates are updated based on the latest Advancing Sustainable
Materials Management: Facts and Figures 2015. Assessing Trends in Material Generation,
Recycling and Disposal in the United States report.
• The composition of yard trimmings is updated based on the Inventory of U.S. Greenhouse Gas
Emissions and Sinks.
• Various aspects of the U.S. average electricity mix are updated based on ElA's Annual Energy
Review and the Inventory of U.S. Greenhouse Gas Emissions and Sinks.
• State electricity grid emission factors are updated based on the eGRID database.
• GHG equivalencies are updated to match EPA's GHG Equivalency Calculator.
3.2 CHANGES MADE FOR WARM VERSION 15
In addition to the Annual Changes listed above, other updates made to WARM since Version 14
include:
• Electronics - EPA has replaced the previous life-cycle emission factors for Personal Computers
with seven more detailed electronic materials. EPA developed separate materials management
factors for Desktop CPUs, Portable Electronic Devices, Flat-panel Displays, CRT Displays,
Electronic Peripherals, Hard-copy Devices, and Mixed Electronics.
• Economic Impacts - EPA expanded the impact assessment reports in WARM to include
economic impacts from employment (labor hours), wages, and taxes. The economic impacts
include direct impacts associated with the actual transformation of recyclable materials into the
marketable products and indirect impacts including the collection, sorting and transportation of
a material.
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3.3 CHANGES MADE FOR WARM VERSION 14
In addition to the Annual Changes listed above, other updates made to WARM since Version 13
include adding Anaerobic Digestion as a new management pathway:
• Anaerobic Digestion - WARM now includes anaerobic digestion as a materials management
option for yard trimmings, food waste, and mixed organics.
o EPA developed separate estimates of emissions for wet anaerobic digestion and dry
anaerobic digestion. Wet digestion and dry digestion are possible pathways for food
waste while yard trimmings are only able to be processed in a dry digester.
o EPA included two scenarios for handling digestate: the direct application of digestate to
land and the curing of digestate before land application.
• Transportation - EPA updated the emission factor used for post-consumer transportation for
the combustion, composting, landfilling and anaerobic digestion. The updated emission factor
was also included in the user-defined transportation distances.
• Landfilling - EPA updated the material properties of organic matter, including the initial carbon
content, proportion of carbon stored and the methane yield. These updates affect the amount
of carbon stored during landfilling.
3.4 CHANGES MADE FOR WARM VERSION 13
Updates made for WARM Version 13 include the following:
• Food Waste - EPA added new emission factors to characterize the energy and GHG emissions
associated with the source reduction of food waste.
o These new emission factors include three separate weighted averages of food wastes
available in the online version of WARM: Food Waste, Food Waste (meat only), and
Food Waste (non-meat). EPA also added individual emission factors for beef, poultry,
grains, bread, fruits and vegetables, and dairy products available in the Excel tool.
o The scope of the new emission factors encompasses farm-to-retail and are informed by
a variety of food production life-cycle inventories and peer-reviewed studies.
• Landfilling - EPA revised the landfill gas methodology in WARM to improve the estimates of gas
collection system operating efficiency and align it with more recent scientific literature.
o This analysis improves upon the landfill gas collection efficiency modeling in WARM and
updates the methane oxidation rates.
o EPA used a Monte Carlo analysis model developed by James Levis and Morton Barlaz to
more accurately estimate the fraction of total produced landfill gas that is used
beneficially, flared, and vented to the atmosphere at landfills that manage landfill gas.
o The Excel version of WARM now allows users the option of selecting and reviewing
results based on California regulatory gas collection scenario as one of four landfill gas
collection scenarios, developed using a Monte Carlo analysis and informed by recent,
peer-reviewed scientific literature.
• Composting - EPA updated the composting waste management pathway to include fugitive
emissions of CH4 and N20 during composting.
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o These estimates were derived from a literature review of recent studies on composting.
o "Green", or predominantly nitrogenous organic wastes such as yard trimmings have
differing fugitive emissions than "brown", or predominantly carbon waste such as food
waste. The Mixed Organics material type uses a weighted average of both types of
waste.
• Source Reduction—EPA updated the source reduction management pathway to include source
reduction emissions for several different mixed material categories, including: Mixed Paper,
Mixed Metals, and Mixed Plastics.
3.5 CHANGES MADE FOR WARM VERSION 12
Updates made to WARM for Version 12 include the following:
• The Excel macro programming in WARM has been removed. The removal of macros does not
affect the results or functionality of the tool. All of the energy and emissions (both MTC02E and
MTCE) results are displayed automatically (previously, the user could choose which to display).
• The emission factor for the broadloom carpet recycling pathway was updated to include two
new plastic resin components. These were based on input and data from Dr. Matthew Realff of
the Georgia Institute of Technology, which were informed by the 2009 Carpet America Recovery
Effort (CARE) 2009 annual report.
• The energy content of broadloom carpet was updated to incorporate more recent data provided
by Dr. Matthew Realff of Georgia Institute of Technology, which were informed by the 2009
Carpet America Recovery Effort (CARE) 2009 annual report.
• Revised the emission factors for three plastics: high-density polyethylene (HDPE), low-density
polyethylene (LDPE), and polyethylene terephthalate (PET).
• Developed emission and energy factors for four new plastics to add to the model: Linear low-
density polyethylene (LLDPE), polypropylene (PP), polystyrene (PS) and polyvinyl chloride (PVC).
• The Mixed Recyclables and Mixed Plastics emission and energy factors were updated to remove
the inclusion of LDPE as a recycled plastic type. Previously, these factor incorporated LDPE, but
updated data for recycling LDPE plastic were unavailable.
• The Mixed Recyclables and Mixed Plastics emission and energy factors were updated to reflect
revisions to the underlying numbers in the virgin and recycled HDPE and PET emission factors.
• The emission and energy factors for aluminum cans were updated based on life-cycle data from
the Aluminum Association. In addition, new emission and energy factors for aluminum ingot
were developed.
• The emission and energy factors for polylactide (PLA), a biopolymer, were developed using life-
cycle data provided by NatureWorks.
3.6 CHANGES MADE BETWEEN THE 3RD EDITION OF THE REPORT AND WARM VERSION 11
The primary changes and improvements to the life-cycle analysis since the 3rd edition of the
report include the following:
• Overarching Changes
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o New GHG equivalencies were added to show the change in emissions calculated by the
user in terms of gallons of gasoline, cylinders of propane, railway cars of coal, as a
percentage of the annual C02 emissions from the U.S. transportation sector, and as a
percentage of the annual C02 emissions from the U.S. electricity sector. All the GHG
equivalencies were updated to match EPA's GHG Equivalency Calculator.
o EPA modified the interface to display results in metric tons of carbon dioxide equivalent
(MTC02E) as the default unit for GHG emissions, but results are still available in units of
metric tons of carbon equivalent (MTCE).
o The 1605(b) functionality in the Excel version of WARM was removed because 1605(b)
no longer supports the reporting of savings from waste reduction.
Changes affecting Material Types
o New emission factors were added for six construction and demolition (C&D) materials:
asphalt concrete, asphalt shingles, drywall, fiberglass insulation, vinyl flooring, and wood
flooring.
o Emission factors for tires were updated: the tire recycling pathway now encompasses
ground and shredded rubber applications and no longer includes retreading as a
recycling application. This change has decreased the overall net benefit of recycling
scrap tires.
o The material type "corrugated cardboard" was renamed to "corrugated containers" to
eliminate redundancy of the former naming convention.
Changes affecting Waste Management Options
o The Excel version of WARM now incorporates region-specific electricity grid factors to
more accurately model emissions associated with avoided generation of electricity due
to landfill gas recovery in the landfilling pathway and waste-to-energy in the combustion
pathway. This change increases the flexibility of WARM and allows the user to generate
more precise results for their scenario. This functionality is not available in the online
version of WARM where the default national average electricity grid mix (i.e., national
average) is implicit.
o The Excel version of WARM includes an updated method for estimating the landfill gas
collection efficiency, allowing the user to select between three landfill gas collection
efficiency scenarios based on specific landfill recovery characteristics: typical operation,
worst-case collection, and aggressive gas collection. This change increases the flexibility
of WARM and allows the user to generate more precise results for their scenario. This
functionality is not available in the online version of WARM where the default national
average landfill gas collection scenario (i.e., typical operation) is implicit.
o Component-specific decay rates were added to the Excel version of WARM for all
organic materials to more accurately model the rate at which each material decays
within a landfill under given landfill moisture conditions: dry, average, wet, or
bioreactor. This change increases the flexibility of WARM and allows the user to
generate more precise results for their scenario. This functionality is not available in the
online version of WARM where the default national average landfill moisture conditions
(i.e., average) scenario is implicit.
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o The waste-to-energy combustion pathway energy values (MMBTU) incorporate a
revised methodology that considers the ratio of mass burn combustion facilities (17.8%)
and the national average electric utility grid combustion efficiency (32%).
o The recycling emission factors for the Mixed Paper material types were modified to
include updated recycled boxboard data.
3.7 FUTURE UPDATES TO WARM
WARM is regularly updated to expand its coverage of materials and waste management pathways, to
keep its methodology consistent with current research and literature, and to maintain the accuracy of its
background data. Updates to WARM that may be implemented in the near future include the following:
• Donations: EPA is currently assessing the viability of adding donations as an alternative pathway
for waste management, building off of the source reduction pathway and the reuse alternative
described in the memo "Modeling Reuse in EPA's Waste Reduction Model" available through
the WARM Documentation page.
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4 FOREST CARBON STORAGE
This chapter describes the development of material-specific estimates of changes in forest
carbon storage in WARM. It summarizes the approach used to estimate changes in forest carbon storage
in managed forests resulting from source reduction and recycling of wood and paper products.
4.1 A SUMMARY OF THE GREENHOUSE GAS IMPLICATIONS OF FOREST CARBON STORAGE
Forests absorb (i.e., sequester) atmospheric carbon dioxide (C02) and store it in the form of
cellulose and other materials. In the early stages of growth, trees store carbon rapidly; consequently, as
tree growth slows, so does carbon sequestration. Trees naturally release carbon throughout their life
cycle as they shed leaves, branches, nuts, fruit, and other materials, which then decay; carbon is also
released when trees are cleared and processed or burned.
When paper and wood products are recycled or the production of these materials is avoided
through source reduction, trees that otherwise would be harvested are left standing in forests. In the
short term, this reduction in harvesting results in more carbon storage than would occur in the absence
of the recycling or source reduction. Over the long term, when forest managers find they have more
trees standing resulting from reduced harvesting, they will respond by planting fewer trees; therefore,
while the carbon storage effect of source reduction and recycling is high in the short term, it is less
pronounced in the long term.
WARM evaluates forest carbon storage implications for all wood and paper products, which
include all of the paper types in WARM,11 dimensional lumber, medium-density fiberboard (MDF), and
hardwood flooring. Paper products are primarily nondurable goods, or goods that generally have a
lifetime of less than three years (EPA, 2008, p. 76). Wood products such as dimensional lumber, MDF,
and wood flooring are considered durable goods because they typically have a lifetime of much longer
than three years (Skog, 2008). Because of the differences in harvesting practices, use, and service life of
paper and wood products, EPA analyzed the forest carbon storage implications for paper products
separately from wood products.
In the United States, uptake by forests has long exceeded release, a result of forest
management activities and the reforestation of previously cleared areas. EPA estimated that the 2013
annual net carbon flux (i.e., the excess of uptake minus release) in U.S. forests was about 765.5 million
metric tons of carbon dioxide equivalent (MMTC02E), which offset about 14 percent of U.S. energy-
related C02 emissions. In addition, about 2,520 MMTC02E was stored in wood products currently in use
(e.g., wood in building structures and furniture, paper in books and periodicals) (EPA 2015). Considering
the effect of forest carbon sequestration on U.S. net GHG emissions, the data clearly showed that a
thorough examination was warranted for use in WARM.
This chapter summarizes the methodology, approach, and results of EPA's analysis of forest
carbon storage. The next section outlines the overall methodology, including the key components in the
assessment of changes in forest carbon storage. Sections 4.3 and 4.4 summarize forest carbon storage
estimates for source reduction and recycling for paper and wood products. Section 4.5 outlines the
limitations associated with EPA's analysis of forest carbon storage.
11 Corrugated containers, magazines/third-class mail, newspapers, office paper, phonebooks and textbooks.
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4.2 FOREST CARBON STORAGE METHODOLOGY
EPA estimated the net change in
forest carbon storage from source reduction
or recycling of forest products by evaluating
three components:
1. Changes in timber harvest (i.e.,
trees that have been cut from
the forest) as a result of
changes in demand for virgin
wood.
2. Changes in forest stocks as a
result of changes in harvest.
3. Changes in carbon storage in
the in-use product pool (for
durable wood products).
These three components taken
together provide the net change in carbon
storage resulting from recycling or source
reduction of forest products. Exhibit 4-1 is a
flow chart explaining the approach. First, for
a forest product that is recycled or source
reduced instead of being put in a landfill or
combusted, WARM assumes that—if
demand for forest products remains
constant—recycling or reuse results in a
reduction in the demand for virgin timber
from forests. Second, this reduction in
timber harvest results in a small increase in
the stock of carbon that remains in U.S.
forests. Third, durable wood products
remain in use for many years,12 and are
themselves a significant source of carbon
storage that is tracked in the U.S. GHG
Inventory13 (EPA, 2015). Since source
reduction reduces the amount of virgin
wood products that enter the market, and
remanufacturing wood products into
recycled products results in some loss of
WARM'S Approach to Forest Carbon Storage
WARM adopts a waste management perspective that
assumes life-cycle boundaries start at the point of waste
generation (i.e., the moment a product such as paper or
dimensional lumber reaches its end-of-life stage), and the
methodology examines the resulting life-cycle GHG
implications of alternative material management pathways
relative to a baseline waste management scenario.
To evaluate forest carbon storage, WARM first assesses the
amount of wood that would have been harvested from the
forest with no efforts to increase source reduction or
recycling. This establishes a "business-as-usual" baseline of
wood harvests. Next, WARM examines how increased
source reduction or recycling reduces the demand for
wood harvests from the forest by avoiding the use of wood
or by conserving paper and wood products relative to this
business-as-usual baseline. The forest carbon storage is
equal to the amount of carbon contained in wood that is
not harvested as a result of increased recycling or source
reduction.
In other words, rather than evaluating the entire stock and
flows of carbon into and out of forests in the United States,
WARM evaluates the difference, or marginal change, in
forest carbon storage resulting from efforts to increase
source reduction or recycling beyond the business-as-usual
baseline. This approach is consistent with WARM'S purpose
of evaluating the benefits of alternative management
practices relative to baseline activities.
On average in the United States, timber harvests are more
than compensated by replanting; therefore, baseline forest
carbon withdrawals need to be considered as part of the
overall carbon stocks-and-flows cycle for forest and
harvested wood products. This methodology is consistent
with and supported by the Intergovernmental Panel on
Climate Change (IPCC) Inventory Guidelines (IPCC, 2006)
that distinguish between biogenic carbon that is harvested
on a sustainable basis versus non-sustainable harvest, and
the fact that land use change and forestry provide a large
net sink for GHG emissions in EPA's U.S. GHG Inventory
(2015).
12 For example, Skog (2008) estimates that the half-life of wood (i.e., the amount of time it takes for half of an
initial amount of wood to reach the end-of-life stage) is 100 years in single-family housing and 30 years in other
end uses.
13 Durable wood products (also known as harvested wood products) accounted for 70.8 million metric tons of CO2
of net carbon flux (equivalent to 19.3 million metric tons of carbon) in 2013. See Chapter 6 of the U.S. GHG
Inventory (EPA, 2015).
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material, increasing source reduction or recycling decreases the amount of carbon stored in in-use
products.
Consequently, for durable wood products, recycling and source reduction increase the amount
of carbon that is stored in U.S. forests, but simultaneously they decrease the amount of carbon from
virgin products that would have been stored in durable wood products. Together, these two factors
equal the net change in carbon storage resulting from increased source reduction or recycling. Note that
the decrease in carbon storage in in-use products applies only to durable (wood) products; WARM does
not consider changes in the in-use product carbon pool for nondurable (paper) goods because these
products have shorter lifetimes, typically less than three years, and the carbon in these goods cycles out
of the in-use pool over a relatively short period.
Exhibit 4-1: Forest Carbon Storage Methodology
Source reduction
or recycling occurs
r ^
Reduction in
timber harvests
relative to
"Business As
Usual" (BAU)
baseline
Increased storage
of carbon in forest
stocks relative to
BAU baseline
Net change in
carbon storage
Decrease in
carbon storage in
in-use products
relative to BAU
baseline (wood
products only)
4.3 FOREST CARBON STORAGE AND PAPER PRODUCTS
Paper products in WARM include corrugated containers, magazines/third-class mail,
newspapers, office paper, phonebooks, and textbooks. These products are short-lived, nondurable
goods that are harvested primarily from forests that are grown for making wood pulp for paper
production. This section describes the methodology used to evaluate the two relevant components of
forest carbon storage, outlined in Section 4.2, for paper products: changes in timber harvest and
changes in forest stock.
Paper types fall into two broad categories, mechanical- and chemical-pulp papers. Mechanical
pulping involves grinding logs into wood fibers and mixing with hot water to form a pulp suspension.
Chemical pulping, also known as kraft pulping, involves removing the surrounding lignin in the wood raw
material during a cooking process. (Verband Deutscher Papierfabrikin e.V., 2008) Of the paper types
modeled in WARM, mechanical pulp papers include newspaper and textbooks. Office paper, corrugated
containers, textbooks, and magazines/third-class mail are considered chemical-pulp paper types.14
4.3.1 Effect of Source Reduction and Recycling on Timber Harvests
Several U.S. Department of Agriculture Forest Service (USDA FS) efforts have analyzed the
relationship between paper recovery (i.e., recycling) rates and pulpwood harvests (i.e., wood harvested
14 In general, shipping and packaging containers, paper bags, and printing and writing papers are manufactured
from chemical pulp, while newspaper, specialty papers, tissue, toweling, paperboard, and wallboard are produced
from mechanical pulp (AF&PA, 2010a).
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for paper production) based on data compiled by the American Forest and Paper Association (AF&PA)
and the Forest Resources Association (FRA). AF&PA collects information on the mass of recovered paper
and wood pulp consumed (AF&PA, 2005) and paper and paperboard production (AF&PA, 2004). FRA
publishes information on the annual amount of pulpwood received at pulp mills (FRA, 2004). Based on
this information, along with assumptions about moisture content,15 Dr. Peter Ince of USDA FS developed
the following equation to relate paper recovery to pulpwood harvests (Ince and McKeever, 1995):
PWH=X x {PP - [PR x (1— EX) x Y]} (Eqn. 1)
Where,
PWH = Pulpwood harvests at 0 percent moisture content, i.e., ovendry (short tons)
PP = Paper production at 3 percent moisture content (short tons)
PR = Paper recovery at 15 percent moisture content (short tons)
EX = Percentage of recovered paper that is exported
X = Process efficiency of converting ovendry pulpwood to paper and paperboard at 3
percent moisture content, which is the ratio of finished paper to pulp, and accounts for
the portion of paper and paperboard that is water and fillers
Y = Process efficiency of converting recovered paper at 15 percent moisture to paper and
paperboard at 3 percent moisture, which is the ratio of recovered paper to finished
paper, and accounts for the water in recovered paper
The values of X and V are based on process efficiency estimates provided by John Klungness
(Research Chemical Engineer, USDA FS) and Ken Skog (Project Leader, Timber Demand and Technology
Assessment Research, USDA FS). The value for EX, the export rate, is based on AF&PA statistics on U.S.
recovered paper exports. In 2008, approximately 40 percent of recovered paper was exported from the
United States (AF&PA, 2010b).16
EPA used the relationship developed in Equation 1 to describe how a change in paper recovery
affects pulpwood harvests. For example, if paper recovery increases by one short ton, by how much
would pulpwood harvests be reduced to meet the same level of paper production in the United States?
Exhibit 4-2 column (f) shows that increasing paper recovery by one short ton would reduce (i.e.,
avoid) pulpwood harvests by 0.58 short tons for mechanical pulp papers and by 0.89 short tons for
chemical pulp papers. This difference results from the lower ratio of pulp to finished paper for chemical-
pulp papers because the chemical pulping process in paper manufacturing removes lignin from the raw
wood material.
15 The moisture contents are pulpwood as harvested, 50 percent; paper and paperboard, 3 percent; wood pulp
consumed, 10 percent; and recovered paper consumed, 15 percent. Knowing the moisture content is important to
accurately gauge carbon contents of these materials.
16 EPA included the export rate in the calculation of avoided pulpwood harvest per ton of paper recovered because
the WARM analysis focuses on the United States; therefore, EPA assumed the avoided pulpwood harvest was
affected only by recovered paper that stays in the United States. Recovered paper that is exported will produce a
different offset for pulpwood harvests in other countries because forest management practices outside of the
United States are likely to be different. The inclusion of the exported recovered paper as a factor in calculating
avoided pulpwood harvest per ton of paper recovered is a conservative assumption because it results in a smaller
reduction in pulpwood harvests from increased paper recovery.
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Exhibit 4-2: Relationship Between Paper Recovery (i.e., Recycling) and Pulpwood Harvest (Values of Eqn. 1
Parameters)
(a)
(b)
(c)
(d)
(e)
(f)
Avoided Short Tons PWH
X= Process
/ = Ratio of
per Short Ton Paper
Ratio of Pulp to
Efficiency
Recovered Paper to
EX
Recovered
Finished Paper
(c = 1/b)
Finished Paper
(%)
(f = c x d x [1 - e])
Mechanical Pulp
0.900
1.11
0.875
40
0.58
Chemical Pulp
0.475
2.11
0.700
40
0.89
For source reduction, the change in pulpwood harvests from source reducing paper can be
calculated directly from the process efficiency (X) of mechanical and chemical pulp production. This is
because source reduction, by reducing consumption of paper, directly reduces paper production (PP in
Equation 1) and, consequently, the amount of pulpwood harvested. Based on the process efficiency
estimates in
Exhibit 4-2, WARM estimates that one short ton of source reduction avoids 1.1 short tons of
pulpwood harvests for mechanical pulp, and 2.11 short tons of chemical pulp.
4.3.2 Effect of Changes in Timber Harvests on Forest Carbon Stocks
EPA based its analysis of carbon storage on model results provided by the USDA FS using its
FORCARB II model of the U.S. forest sector. USDA FS models and data sets are the most thoroughly
documented and peer-reviewed models available for characterizing and simulating the species
composition, inventory, and growth of forests, and the Forest Service has used them to analyze GHG
mitigation in support of a variety of policy analyses. FORCARB II is a USDA FS model that simulates the
complex, dynamic nature of forest systems, including the interaction of various forest carbon pools, how
carbon stocks in those pools change over time, and whether the response of forest carbon is linearly
proportional to harvests. To explore these questions, USDA FS ran two enhanced recycling/source
reduction pulpwood harvest scenarios in FORCARB II.
The base assumptions on pulpwood harvests are derived from the North American Pulp and
Paper (NAPAP) model baseline projections developed for the Forest Service 2001 Resource Planning Act
Timber Assessment. To investigate the effect of small and large changes in pulpwood harvests, the
Forest Service modeled two reduced harvest scenarios, which involved decreasing pulpwood harvest by
6.7 million metric tons and 20.2 million metric tons for the period 2005 to 2009.17 The Forest Service
selected the values of 6.7 million and 20.2 million metric tons as representative low- and high-end
reductions in pulpwood harvests based on the 50-percent paper recycling rate in 2005 (Freed et al.,
2006). Harvests in all other periods were the same as the baseline.
The relative change in forest carbon storage per unit of reduced pulpwood harvest across the
two decreased harvest scenarios is virtually identical (i.e., less than 1 percent), which suggests that the
relationship between forest carbon storage and reduced pulpwood harvests is not affected by the size
of the reduction in pulpwood harvests over the range investigated by the two scenarios.
17 EPA selected this timeframe because, at the time the EPA did the analysis, that period represented a short-term
future time horizon over which reduced forest withdrawals could be evaluated against baseline projections.
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For each scenario, the Forest Service calculated the change in carbon stocks compared with the base case; the
change represents the carbon benefit of reduced harvests associated with recycling or source reduction. The
change in metric tons of carbon equivalents (MTCE) is divided by the incremental metric tons of pulpwood
harvested and multiplied by the weight ratio of CO2 to carbon (44/12, or approximately 3.667) to yield results in
units of MTCO2E per metric ton of pulpwood not harvested (i.e., the carbon storage rate). For more details,
please refer to the conversions provided in Exhibit 4-4 and
Exhibit 4-5.
As shown in Exhibit 4-3, the cumulative carbon storage rate starts at about 0.99 MTCE per
metric ton pulpwood in 2010, increases to about 1.08 MTCE per metric ton pulpwood in 2030, and
declines with time to about 0.81 MTCE per metric ton pulpwood in 2050. According to EPA's detailed
analysis of the FORCARB II results, the primary effect of reduced pulpwood harvests is to increase
carbon stored in live trees that otherwise would have been harvested (shown by the sharp increase in
carbon storage in 2010). This effect is offset to a small degree by a decrease in carbon storage in the
amount of downed wood in the forest. Carbon storage in dead trees, the forest floor, and forest
understory increases slightly; carbon stored in forest soils has no effect. Most of the changes in each of
these pools of forest carbon peak in 2010 and moderate somewhat over the next 40 years, although the
increase in carbon storage in the forest floor peaks over a longer time period in 2030. After 2030, the
amount of carbon stored in live trees begins to decline, causing a reduction in forest carbon storage.
This decline likely reflects the effect of market forces, which result in less planting of new managed
forests in response to a lower level of demand for pulpwood harvests.
Exhibit 4-3: Change in Forest Carbon Storage Per Unit of Reduced Pulpwood Harvest for (a) Incremental Change
in Forest Carbon Storage and (b) Cumulative Change in Forest Carbon Storage Per Unit of Reduced Pulpwood
Harvest
1.2 T-"
2010 2015 2020 2025 2030
2045 2050
2010 2015 2020 2025 2030
2045 2050
(b)
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Note: Colored bar for 2020 represents the value EPA selected to estimate the forest carbon storage benefit in WARM'S GHG
emission factors. EPA calculated the results by dividing the change in forest carbon storage in each year by 6.7 million metric
tons of pulpwood harvests reduced over the period 2005 to 2009.
the major driver of the net carbon storage estimate appears to be the time it takes for the
increase in carbon storage in live trees and the decrease in carbon storage in downed wood to begin to
decline back toward baseline levels. Because the decrease in carbon storage in downed wood returns to
baseline levels more quickly than the increase in carbon storage in live trees, the net change in carbon
storage actually increases through 2030.
The FORCARB II results indicated that the effect of paper recycling or source reduction on
carbon storage appears to be persistent (i.e., lasting at least for several decades). EPA chose to use the
value for 2020 in the emission factors, or 1.04 MTCE per metric ton of pulpwood. The choice of 2020
represents a delay of about 5 to 15 years for the onset of incremental recycling, long enough to reflect
the effects of the recycling program, but at a rate lower than the peak effect in 2030. As shown in
Exhibit 4-3, the effect is relatively stable over time, so the choice of year does not have a significant
effect.
For additional details on this methodology and a comparison of the FORCARB II results to those
from other analyses, please see the Revised Estimates of Effect of Paper Recycling on Forest Carbon
(Freed et al., 2006).
4.3.3 Changes in In-Use Product Carbon Pool
WARM does not consider changes in the in-use product carbon pool for nondurable goods
because these products have shorter lifetimes, typically less than three years, and the carbon contained
in these goods cycles out of the in-use pool over a relatively short period.
4.3.4 Net Change in Carbon Storage
To estimate the rate of forest carbon change per metric ton of paper recovery, multiply the rate of pulpwood
of pulpwood harvest (PWH) per metric ton of paper recovery (PRC) (from Section 4.3.1) by the rate of forest
forest carbon (FC) change per metric ton of pulpwood harvest (from Section 4.3.2), as shown in Exhibit 4-4.
4-4. Exhibit 4-4 shows the net change in carbon storage per unit of increased paper product recycling, while
Exhibit 4-5 shows the net change in carbon storage per unit of increased paper source
reduction. The various paper grades fall into mechanical or chemical pulp categories as follows:
• Mechanical pulp papers: newspaper, telephone books.
• Chemical pulp papers: office paper, corrugated containers, textbooks, magazines/third class
mail.
Note that the net change in carbon storage for recycling and source reduction of wood products
(compared with paper products) is different, as discussed in Section 4.4.
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Exhibit 4-4: Net Change in Carbon Storage per Unit of Increased Paper Product Recycling
(a)
(b)
(c)
(d)
(e)
(f)
Paper
Change in Forest
Product
Reduction in
Carbon Storage per
Change in Forest
Change in Carbon
Net Change in
Recycled
Timber Harvest per
Unit of Reduced
Carbon Storage per
Storage in In-use
Carbon Storage per
Unit of Increased
Timber Harvest
Unit of Reduced
Products per Unit
Unit of Increased
Recycling (Short
(Metric Tons Forest
Timber Harvest
of Increased Paper
Paper Product
Tons Timber/Short
Carbon/Metric Ton
(MTC02e/ Short Ton
Product Recycling
Recycling
Ton of Wood)
Timber)
Timber)
(MTCOzE/Short
(MTC02E/Short Ton)
(from Section 4.3.1)
(from Section 4.3.2)
(d = c x 0.907 x 3.667)
Ton)
(e = b x d + e)
Mechanical
pulp
0.58
1.04
3.46
NA
2.02
Chemical
pulp
0.89
1.04
3.46
NA
3.06
NA = Not applicable.
Exhibit 4-5: Forest Carbon Storage from Source Reduction of Paper Products
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Material
Mechanical
or Chemical
Pulp
Reduction in
Timber
Harvest per
Unit of
Increased
Source
Reduction
(Short Tons
Timber/Short
Ton of Wood)
(from Section
4.3.1)
Change in
Forest Carbon
Storage per
Unit of
Reduced
Timber
Harvest
(Metric Tons
Forest
Carbon/Metric
Ton Timber)
(from Section
4.3.2)
Change in
Forest
Carbon
Storage per
Unit of
Reduced
Timber
Harvest
(MTC02e/
Short Ton
Timber)
(e = d x 0.907
x 3.667)
Net Change
in Carbon
Storage per
Unit of
Increased
Source
Reduction,
100% Virgin
Inputs
(MTCOzE
/Short Ton)
(f = c x e)
Virgin
Inputs in
the
Current
Mix of
Inputs3
(%)
Net Change
in Carbon
Storage per
Unit of
Increased
Source
Reduction,
Current Mix
(MTCOzE
/Short Ton)
(h = f x g)
Corrugated
Containers
Chemical
2.11
1.04
3.46
7.26
65.1
4.73
Magazines/
Third-class
Mail
Chemical
2.11
1.04
3.46
7.26
95.9
6.96
Newspapers
Mechanical
1.11
1.04
3.46
3.83
77.0
2.95
Office Paper
Chemical
2.11
1.04
3.46
7.26
95.9
6.96
Phonebooks
Mechanical
1.11
1.04
3.46
3.83
100.0
3.83
Textbooks
Chemical
2.11
1.04
3.46
7.26
95.9
6.96
a Source: FAL (2003).
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The net forest carbon storage for source reduction of paper products is shown in
Exhibit 4-5. The reduction in timber harvest per unit of increased source reduction (
Exhibit 4-5, column (c)) is the process efficiency of converting pulpwood to finished paper (i.e., 1/ratio of pulp to
finished paper), as described in Section 4.3.1. The net change in forest carbon storage depends on whether the
source reduction of paper products is assumed to displace paper that would have been produced from 100-
percent virgin inputs or the current industry-average mix of virgin and recycled inputs (FAL, 2003). For source
source reduction that offsets paper produced from 100-percent virgin pulp, the net change in forest carbon
carbon storage is shown in
Exhibit 4-5, column (e). For the case where source reduction offsets paper produced from the current mix of
virgin and recycled inputs, however, WARM assumes that the net forest carbon effect is attributable only to the
proportion of inputs that are virgin pulp, as shown in
Exhibit 4-5, column (g). WARM makes this assumption because displacing recycled inputs, which
have already been harvested from the forest, are unlikely to have a direct effect on forest carbon
storage.
4.4 FOREST CARBON STORAGE AND WOOD PRODUCTS
Wood products in WARM include dimensional lumber, MDF, and wood flooring. These products
are long-lived, durable goods that are harvested from sustainably managed soft- and hardwood forests.
This section describes the methodology EPA used to evaluate the three components of forest carbon
storage, outlined in Section 4.2, for softwood products (i.e., dimensional lumber and MDF). The
approach for evaluating forest carbon storage for hardwood flooring is similar and is provided in further
detail in the Wood Flooring chapter.
4.4.1 Effect of Source Reduction and Recycling on Timber Harvests
To estimate the change in timber harvests that result from increased recycling and source
reduction of softwood products, EPA used estimates provided by Dr. Skog for the system efficiencies (on
a weight basis) of producing wood products from virgin inputs or recycled inputs. Assuming that overall
demand for softwood products is constant, increases in recycling will reduce timber harvests according
to the following ratio:18
TH=X/Y (Eqn. 2)
Where,
TH = Change in timber harvests resulting from increased recycling of wood products
X = Process efficiency of converting virgin roundwood into finished wood product
Y = Process efficiency of converting recycled wood into finished wood product
Based on the estimates provided by Dr. Skog, EPA assumed that one short ton of finished wood
product requires 1.1 short tons of virgin roundwood19 (i.e., harvested logs, with or without bark), on
average, or 1.25 short tons of recycled wood. According to this relationship, each additional short ton of
wood products recycled will reduce the demand for virgin roundwood from timber forests by a ratio of
1.1/1.25 = 0.88 short tons.
18 Unlike EPA's consideration of paper products, WARM does not consider exports of recycled wood outside of the
United States. In contrast with recovered paper, which is exported to other countries for recycling, recovered
wood typically is not directly exported for recycling. Instead, finished wood products or wood packaging materials
(such as pallets, skids, containers, crates, boxes, cases, bins, reels, and drums) may be manufactured from recycled
materials in the United States for export (Ince 1995; FAO 2005).
19 Harvested logs, with or without bark; roundwood may be round, spilt, or roughly squared (FAO, 1997).
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The effect of source reduction on timber harvests can be calculated from the process efficiency
(X) of wood products production, assuming that one short ton of source reduction completely offsets
virgin roundwood harvests that otherwise would be harvested to produce one short ton of wood
products. Section 4.5 discusses the sensitivity of the forest carbon storage results to this assumption.
Consequently, WARM estimates that one short ton of source reduction avoids 1.1 short tons of
roundwood harvests for dimensional lumber and MDF wood products.
These values describe the change in timber harvests resulting from increased recycling and
source reduction of softwood products. Together with the effects that changes in timber harvests have
on forest carbon stocks (developed in Section 4.4.2), these two parameters describe how forest carbon
storage changes as a result of increases in recycling and source reduction. The values developed in this
section are also used to determine how source reduction and recycling affect carbon storage in in-use
wood products, which is discussed in Section 4.4.3. The net changes in carbon storage from recycling
and source reduction are calculated in Section 4.4.4, taking into account both changes in forest carbon
storage and in-use product carbon storage.
4.4.2 Effect of Changes in Timber Harvests on Forest Carbon Stocks
To investigate the change in forest carbon resulting from increased recycling and source
reduction of wood products, EPA used estimates developed from the USDA FS's FORCARB II model. The
method for wood products is similar to the approach for paper described in Section 4.3.2. First, EPA
applied a harvest scenario developed in consultation with Dr. Skog and Dr. Linda Heath at USDA FS. EPA
determined that the majority of wood products are derived from softwood and evaluated an increased
wood recycling/source reduction scenario corresponding to a 1.7-percent reduction in softwood
harvest. The 1.7-percent reduction is a representative estimate of the reduction in softwood harvests
that could be achieved with a national increase in wood product recycling above current levels.
This reduction is distributed throughout the USDA FS regions in proportion to baseline harvest
for the period 1998 to 2007. The cumulative reduction in softwood harvest from the 1.7-percent
reduced harvest scenario is 26.4 million short tons over this period.
The effect of this reduction in harvest is to increase carbon sequestration in forests. To be
consistent with the approach for paper recycling and source reduction, EPA analyzed effects only for
tree and understory components (and excluded forest floor and soils). Exhibit 4-6 displays the results of
the analysis for wood products. The results show that every metric ton of avoided timber harvest results
in 0.96 to 0.99 metric tons of forest carbon storage. For consistency with the paper recycling/source
reduction analysis, EPA selected the forest carbon storage benefit in 2010, representing a delay of 5 to
15 years from the onset of the simulated period of incremental recycling. This period is consistent with
the 5 to 15 year timeframe used in the paper forest carbon analysis in Section 4.3. Consequently, EPA
estimated that a one-metric-ton reduction in timber harvests increases forest carbon storage by 0.99
metric tons.
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Exhibit 4-6: Cumulative Change in Forest Carbon Storage per Unit of Reduced Timber Harvest
"D
O _C
.y -o
% §
E J
g. s
3
E
D
U
1.2
1.0
0.8
0.6
0.4
0.2
2000
2010
2020
2030
2040
Note: Colored bar for 2010 represents the value EPA selected to estimate the forest carbon storage benefit in WARM'S GHG
emission factors. EPA calculated the results by dividing the change in forest carbon storage in each year by 24 million metric
tons of pulpwood harvests reduced over the period 1998 to 2007.
4.4.3 Changes in In-Use Product Carbon Pool
The final step involves estimating the effects of increased wood product recycling on carbon
storage in in-use wood products.
For recycling, based on the estimates developed in Section 4.4.1, EPA assumed that 1.25 short
tons of recycled wood are required to produce one short ton of finished wood product; in other words,
every short ton of wood recycled yields 0.8 short tons of finished wood product (i.e., 1/1.25 = 0.8), and
0.2 short tons of wood are lost from in-use products. For wood products, EPA assumed a carbon density
of 0.48 MTCE per short ton of wood, corresponding to softwoods in Southeast and South Central pine
forests (Birdsey, 1992). Consequently, the carbon loss from the product pool is given by:
(1 short ton recycled - 0.8 short tons retained) x 0.48 MTCE/short ton x 44/12 MTC02E/MTCE = 0.35
MTC02E/short ton
For source reduction of wood products, a short ton of wood offset by source reduction results in
a decline in carbon that otherwise would have been stored in the in-use wood product.20 This essentially
represents a one-to-one relationship, where source reducing one short ton of wood avoids one short
ton of wood that otherwise would have been manufactured into in-use products. Consequently, the
change in the in-use product carbon pool from source reduction of one short ton of wood product is
equal to the carbon density of the wood product, given by:
1 short ton source reduced x 0.48 MTCE/short ton x 44/12 MTC02E/MTCE = 1.77 MTC02E/short ton
20 Because dimensional lumber and MDF are not commonly manufactured from recycled inputs in the United
States, WARM assumes that source reduction of wood products avoids virgin wood inputs only. This is a different
approach than for source reduction for paper products, where the net change in forest carbon storage depends on
whether the source reduction of paper products is assumed to displace paper that would have been produced
from 100-percent virgin inputs, or the current industry-average mix of virgin and recycled inputs.
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Both source reduction and recycling decrease the amount of carbon stored in in-use products;
this decrease offsets some of the benefit of increasing storage in forests; see Section 2 for more details.
4.4.4 Net Change in Carbon Storage
Based on the estimates developed in the previous sections, Exhibit 4-7 shows the net change in
forest carbon storage for recycling and source reduction of wood products. These results show that
recycling and source reduction of one short ton of wood products corresponds to an increase in net
carbon storage. In both cases, the increase in forest carbon storage is offset by a reduction in carbon
storage in in-use products as a result of recycling or source reduction.
Exhibit 4-7: Net Change in Carbon Storage per Unit of Increased Wood Product Recycling
(a)
(b)
Reduction in Timber
Harvest per Unit of
Increased Recycling
or Source Reduction
(Short Tons
Timber/Short Ton of
Wood)
(from Section 4.4.1)
(c)
Change in Forest
Carbon Storage per
Unit of Reduced
Timber Harvest
(Metric Tons Forest
Carbon/Metric Ton
Timber)
(from Section 4.4.2)
(d)
Change in Forest
Carbon Storage per
Unit of Reduced
Timber Harvest
(MTC02e/ Short
Ton Timber)
(d = c x 0.907 x
3.667)
(e)
Change in Carbon
Storage in In-use
Products per Unit of
Increased Wood
Product Recycling
(MTCOzE/Short Ton)
(from Section 4.4.3)
(f)
Net Change in
Carbon Storage
per Unit of
Increased Wood
Product Recycling
(MTCOzE/Short
Ton)
(e = b x d + e)
Recycling
0.88
0.99
3.29
-0.35
2.53
Source
Reduction
1.1
0.99
3.29
-1.77
1.84
Note: Positive values denote an increase in carbon storage; negative values denote a decrease in carbon storage.
4.5 LIMITATIONS
Several limitations are associated with the analysis. The forest product market is very complex,
and EPA's simulation of some of the underlying economic relationships that affect the market simplifies
some important interactions.
A general limitation of the analysis is that it does not account for any potential long-term
changes in land use caused by a reduction in pulpwood or softwood demand, and landowners' choices
to change land use from silviculture to other uses. If overall forest area is reduced, this would result in
significant loss of carbon stocks. Hardie and Parks (1997) developed an area base model for use in
Resource Planning Act assessments to help determine factors that influence land area change. They
derived a model that estimated the elasticity of (a) forest land area change with respect to (b) pulpwood
price change. They estimated the elasticity to be -0.10, but this was not significant at the 10-percent
confidence level. This suggests that forest area change would be limited with a modest price change in
pulpwood demand.
The following limitations relate to the estimate of forest carbon storage for paper products:
• Results are very sensitive to the assumption on paper exports (i.e., that paper exports
comprise a constant proportion of total paper recovery). If all of the recovered paper is
exported, none of the incremental recovery results in a corresponding reduction in U.S.
pulpwood harvest. At the other extreme, if all of the incremental recovery results in a
corresponding reduction in U.S. pulpwood harvest, the storage factor would be higher. The
results are also sensitive to assumptions on the moisture content and the carbon content of
pulpwood, pulp, and paper.
• This analysis does not consider the effect that decreases in pulpwood harvest may have on
the supply curve for sawtimber, which could result in a potential increase in harvests of
other wood products. This could result in a smaller reduction in harvest, offsetting some of
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the carbon storage benefit estimated here. Prestamon and Wear (2000) investigated how
pulpwood and sawtimber supply would change with changes in prices for each. They
estimated that non-industrial private forest and industry may increase sawtimber supply
when the price for pulpwood increases—and the change is perceived as temporary—
although the estimate was not statistically significant. The sawtimber supply, however, may
decrease when the pulpwood price increases—and the change is perceived as permanent—
but, once again, the estimate was not statistically significant. Given that the relationship
between the price change for pulpwood and supply of sawtimber was not consistent and
was often statistically insignificant, there is not compelling evidence to indicate that the
omission of this effect is a significant limitation to the analysis.
• A related issue is that if the domestic harvest of pulpwood decreases, it could result in a
decrease in the cost of domestic production, which could shift the balance between
domestic paper production and imports to meet demand.
The following limitations relate to the estimate of forest carbon storage for wood products:
• The estimated changes in timber harvests resulting from increased recycling and source
reduction are based on process efficiency estimates that assume overall demand for
softwood products remains constant. Increased recycling or source reduction of wood
products could increase or decrease demand for new wood products to the extent that
these changes influence factors such as virgin wood-product prices. EPA has not explicitly
modeled this effect because of the complexity of virgin wood-product markets and the fact
that the current assumption provides a first-order estimate of the change in timber harvests
from recycling and source reduction.
• Similarly, in-use product carbon storage is modeled based on first-order reductions in
carbon storage associated with losses from recycling wood products and avoided in-use
product carbon storage from source reduction of wood products. This analysis provides an
estimate of the direct, first-order effects on the in-use carbon pool associated with recycling
or source reduction of wood products.
As shown in Exhibit 4-3 and Exhibit 4-6, estimates of forest carbon storage resulting from increased
paper recycling vary over time. As noted earlier, WARM applies a single point estimate reflecting a time
period that best balances the competing criteria of (1) capturing the long-term forest carbon
sequestration effects, and (2) limiting the uncertainty inherent in projections made well into the future.
The variation in forest carbon storage estimates over time and the limitations of the analysis discussed
earlier indicate considerable uncertainty in the point estimate selected. In comparison to the estimates
of other types of GHG emissions and sinks developed in other parts of WARM, the magnitude of forest
carbon sequestration is relatively high. Based on these forest carbon storage estimates, source
reduction and recycling of paper are found to have substantial net GHG reductions. Because paper
products make up the largest share of municipal waste generation (and the largest volumes of waste
managed through recycling, landfill use, and combustion), it is important to bear in mind the uncertainty
in the forest carbon sequestration values when evaluating the results of this analysis.
4.6 REFERENCES
AF&PA. (2010a). Paper Products Glossary. Washington, DC: American Forest & Paper Association.
AF&PA. (2010b). Where Recovered Paper Goes. Annual AF&PA Fiber Survey. Washington, DC: American
Forest and Paper Association.
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AF&PA. (2005). Wood pulp, recovered paper, pulpwood 25th Annual survey, 2004-2007. Washington,
DC: American Forest and Paper Association.
AF&PA. (2004). 2004 Statistics—Paper, paperboard and wood pulp. Washington, DC: American Forest
and Paper Association.
Birdsey, R. A. (1992). Carbon Storage and Accumulation in the United States Forest Ecosystems.
Washington, DC: U.S. Department of Agriculture Forest Service. Retrieved October 18, 2009,
from http://nrs.fs.fed.us/pubs/gtr/gtr wo059.pdf.
EPA (2015). Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2012. (EPA publication no. EPA
430-R-14-003.) Washington, DC: U.S. Environmental Protection Agency, Office of Atmospheric
Programs, April. Retrieved from:
http://epa.gov/climatechange/emissions/usinventorvreport.html.
EPA. (2008). Municipal Solid Waste Generation, Recycling, and Disposal in the United States: Facts and
Figures for 2007. United States Environmental Protection Agency, Office of Solid Waste. EPA530-
R-08-010.
FAL. (2003). Personal communication between Randy Freed, ICF International and William E. Franklin,
Franklin Associates Limited regarding recycled contents for use in the ReCon Tool. December 10,
2003.
FAO. (2005). Trends in Wood Products. Rome: United Nations Food and Agricultural Organization.
Retrieved from http://www.fao.org/docrep/008/a0142m/a0142m00.htm.
FAO. (1997). FAO model code of forest harvesting practice. Rome: United Nations Food and Agriculture
Organization. Available at http://www.fao.org/docrep/v6530e/v6530el2.htm
FRA. (2004). Annual Pulpwood Statistics Summary Report, 1999-2003. Rockville, MD: Forest Resources
Association.
Freed, R., Choate, A., & Shapiro, S. (2006). Revised Estimates of Effect of Paper Recycling on Forest
Carbon. U.S. Environmental Protection Agency (EPA). Retrieved from
http://www.epa.gOv/climatechange/wvcd/waste/SWMGHGreport.html#background.
Hardie, I.W., & P.J. Parks. (1997). Land Use with Heterogeneous Land Quality: An Application of an Area
Base Model. American Journal of Agricultural Economics, 79, 299-310.
Ince, P. J., McKeever, D. B., & Forest Products Laboratory (U. S.). (1995). Recovery of paper and wood for
recycling: actual and potential. General technical report FPL-GTR-88.: U.S. Department of
Agriculture, Forest Service, Forest Products Laboratory. Madison, Wl.
Ince, P.J. (1995). Recycling of Wood and Paper Products in the United States. General technical report
FPL-GTR-89. U.S. Department of Agriculture Forest Service, Forest Products Laboratory.
Madison, Wl. Retrieved from http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr89.pdf.
IPCC. (2006). 2006IPCC Guidelines for National Greenhouse Gas Inventories, Volume 5: Waste, Chapter
3: Solid Waste Disposal. Intergovernmental Panel on Climate Change. Retrieved from
http://www.ipcc-nggip.iges.or.jp/public/2006gl/vol5.html
Prestamon, J.P., & Wear, D.N. (2000). Linking Harvest Choices to Timber Supply. Forest Science 46, 3,
377-389.
Skog, K.E. (2008). Sequestration of carbon in harvested wood products for the United States. Forest
Products Journal, 58 (6), 56-72.
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VDP. (2008). Papermaking: Information on Raw Materials and Papermaking. Bonn: Verband Deutscher
Papierfabrikin e.V.
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5 TRANSPORTATION ASSUMPTIONS
This chapter describes the development of waste and material transportation assumptions in
WARM. It summarizes the approach used to estimate the greenhouse gas (GHG) and energy impacts
from transportation stages in the materials management practices modeled in WARM.
5.1 SOURCE REDUCTION
When a material is source reduced, GHG emissions associated with producing the material
and/or manufacturing the product and managing the post-consumer waste are avoided. The raw
material acquisition and manufacturing (RMAM) calculation in WARM incorporates GHG emissions from
energy used to transport materials, including "retail transportation/' which consists of the average
truck, rail, water and other-modes transportation emissions required to get raw materials from the
manufacturing facility to the retail/distribution point. Transportation emissions from the retail point to
the consumer are not included. The transportation assumptions and data sources for source reduction
modeling in WARM vary by material. For more information, reference individual material chapters of the
WARM documentation.
5.2 RECYCLING
When a material is recycled, it is used in place of virgin inputs in the manufacturing process,
rather than being disposed of and managed as waste. Transportation-related impacts from recycling
include collection and transportation to recycling center, transportation of recycled materials to
remanufacturing, and avoided impacts from transport of raw materials and products for virgin material
production.
5.2.1 Collection and Transportation to Recycling Center
The default distances and modes for collection and transportation of recycled materials to
recycling center are included as part of the transportation of recycled materials to remanufacturing
impacts, described under Section 5.2.2 and modeled on a material-specific basis. WARM assumes a
default transportation distance to a recycling center of 20 miles but allows users the option of providing
the distance needed for transportation to a recycling center for their operations. When modeling user-
provided information on transportation to recycling center, EPA assumed that transportation by a
diesel-powered short-haul truck using data from the National Renewable Energy Laboratory (NREL) U.S.
Life Cycle Inventory Database (USLCI) (NREL, 2015) to quantify energy and emissions from
transportation for each short ton of materials transported each mile (i.e., on a short ton-mile basis). EPA
also used a pre-combustion scale-up factor for diesel fuel to account for fuel needed for crude oil
extraction, refining, and transportation; the diesel heating value (FAL, 2011); and a diesel carbon
coefficient from the U.S. EPA (2017). These assumptions and calculations are presented in Exhibit 5-1.
Exhibit 5-1: Emissions Associated with Transporting Waste to Recycling Centers
(a)
(b)
(c)
(d)
(e)
Combination
Diesel Fuel Pre-
Diesel Fuel
Diesel Fuel
Recycling Center Transport
Truck Diesel
Combustion Scale-
Heating Value
Emission Factor
Emission Factor
Fuel Use
up Factor
(Btu/Gallon)
(MTC02E/Million
(MTCOzE/Short Ton-Mile)
(Gallons/Short
Btu)
(e=axbxcxd-r 106)
Ton-Mile)
Recycling
0.01
1.19
140,000
0.07
0.00016
Sources: NREL, 2015; FAL, 2011; EPA, 2017
Note: Totals in table may not sum due to independent rounding
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5.2.2 Transportation of Recycled Materials to Remanufacturing
The assumptions and data sources for transport of recycled materials to remanufacturing in
WARM vary by material. For more information, reference individual material chapters of the WARM
documentation.
5.2.3 Avoided Impacts from Transportation of Raw Materials and Products for Virgin Material
Production
The assumptions and data sources for avoided impacts from transportation of raw materials and
products for virgin material production in WARM vary by material. For more information, reference
individual material chapters of the WARM documentation.
5.3 ANAEROBIC DIGESTION
WARM accounts for the GHG emissions resulting from fossil fuels used in vehicles collecting and
transporting waste to the anaerobic digestion facility. Exhibit 5-2 shows the diesel used for transporting
the feedstock and solids to the anaerobic digester and the post-consumer transportation. To calculate
the emissions, WARM relies on assumptions from the NREL USLCI (NREL, 2015). The NREL emission
factor assumes a diesel, short-haul truck. WARM assumes a default transportation distance to an
anaerobic digester of 20 miles but allows users the option of providing the distance needed for
transportation to an anaerobic digestion for their operations using the transportation factor presented
in Exhibit 5-1.
Exhibit 5-2: Diesel Use by Process and by Material Type for Dry Digestion
Material
Transportation
and Spreading
(Million Btu)
Post-Consumer
Transportation
(Million Btu)
Total Energy Required for
Dry Anaerobic Digestion
(Million Btu)
Total C02 Emissions from
Dry Anaerobic Digestion
(MTCOzE)
Food Waste
0.25
0.04
0.33
0.02
Yard Trimmings
0.30
0.04
0.34
0.02
Grass
0.28
0.04
0.33
0.02
Leaves
0.31
0.04
0.36
0.02
Branches
0.32
0.04
0.37
0.02
Mixed Organics
0.29
0.04
0.34
0.02
5.4 COMPOSTING
WARM includes emissions associated with transporting and processing of the compost in
aerated windrow piles. Transportation energy emissions occur when fossil fuels are combusted to
collect and transport yard trimmings and food waste to the composting facility and then to operate
composting equipment that turns the compost. EPA did not count transportation emissions from
delivery of finished compost from the composting facility to its final destination. To calculate the
emissions, WARM relies on assumptions from FAL (1994) for the equipment emissions and the NREL
USLCI (NREL, 2015). The NREL emission factor assumes a diesel, short-haul truck. Exhibit 5-3 provides
the transportation emission factor calculation. WARM assumes a default transportation distance to a
composting facility of 20 miles but allows users the option of providing the distance needed for
transportation to a composting facility for their operations using the transportation factor presented in
Exhibit 5-1.
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Exhibit 5-3: Emissions Associated with Transporting and Turning Compost
Material Type
Diesel Fuel Required to
Collect and Transport One
Short Ton (Million Btu)a
Diesel Fuel Required to
Turn the Compost Piles
(Million Btu)b
Total Energy Required
for Composting
(Million Btu)
Total C02 Emissions
from Composting
(MTCOzE)
Organics
0.04
0.22
0.26
0.02
a Based on estimates from NREL (2015)
b Based on estimates in Table 1-17 in FAL (1994), p.132.
5.5 COMBUSTION
WARM includes emissions associated with transporting of waste and the subsequent
transportation of the residual waste ash to the landfill. Transportation energy emissions occur when
fossil fuels are combusted to collect and transport material to the combustion facility and then to
operate on-site equipment. Transportation of any individual material in MSW is assumed to use the
same amount of energy as transportation of mixed MSW. To calculate the emissions, WARM relies on
assumptions from FAL (1994) for the equipment emissions and NREL USLIC (NREL, 2015). The NREL
emission factor assumes a diesel, short-haul truck. Exhibit 5-4 provides the transportation emission
factor calculation. WARM assumes a default transportation distance to a combustion facility of 20 miles
but allows users the option of providing the distance needed for transportation to a combustion facility
for their operations using the transportation factor presented in Exhibit 5-1.
Exhibit 5-4: Emissions Associated with Transporting Waste to Combustion Facilities and Ash Transportation
Material Type
Diesel Fuel Required to
Collect and Transport
One Short Ton of Waste
(Million Btu)a
Diesel Fuel Required to
Collect and Transport
One Short Ton of
Waste (MTC02E)a
Diesel Fuel Required
for Ash Landfill
Disposal from One
Short Ton of Waste
(MTC02E)b
Total C02 Emissions
from Combustion
(MTCOzE)
Combustion
0.04
0.00
0.01
0.01
a Based on estimates from NREL (2015)
b Based on estimates in Table 1-24 in FAL (1994)
5.6 LANDFILLING
WARM includes emissions associated with transportation and landfilling the material.
Transportation energy emissions occur when fossil fuels are combusted to collect and transport material
to the landfill facility and then to operate landfill operational equipment. To calculate the emissions,
WARM relies on assumptions from FAL (1994) for the equipment emissions and NREL USLCI (NREL,
2015). The NREL emission factor assumes a diesel, short-haul truck. Exhibit 5-5 provides the
transportation emission factor calculation. WARM assumes a default transportation distance to a
combustion facility of 20 miles but allows users the option of providing the distance needed for
transportation to a combustion facility for their operations using the transportation factor presented in
Exhibit 5-1.
Exhibit 5-5: Landfilling Transportation and Equipment Energy and Emissions Assumptions and Calculation
Material Type
Diesel Fuel Required to
Collect and Transport One
Short Ton (Million Btu)a
Diesel Fuel Required for
Landfilling Equipment
(Million Btu)b
Total Energy Required
for Composting
(Million Btu)
Total C02 Emissions
from Composting
(MTCOzE)
Landfilling
0.04
0.23
0.27
0.02
a Based on estimates from NREL (2015)
b Based on estimates in Table 1-5 in FAL (1994)
5.7 REFERENCES
EPA. (2017). Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2015. (EPA 430-R-15-004).
Washington, DC: U.S. Government Printing Office. Retrieved from
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http://www3.epa.gov/climatechange/Downloads/ghgemissions/US-GHG-lnventory-2015-Main-
Text.pdf.
FAL. (2011). Cradle-to-Gate Life Cycle Inventory of Nine Plastic Resins and Two Polyurethane Precursors.
Revised Final Report. Prairie Village, KS: Franklin Associates, Ltd.
FAL. (1994). The Role of Recycling in Integrated Solid Waste Management for the Year 2000. Franklin
Associates, Ltd. (Stamford, CT: Keep America Beautiful, Inc.), September, pp. 1-27, 30, and 31.
National Renewable Energy Laboratory (2015). "U.S. Life Cycle Inventory Database." Retrieved from
https://www.lcacommons.gov/nrel/search
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