Climate Change Indicators in the United States Technical Documentation U S Greenhouse Gas Emissions

U.S. Greenhouse Gas Emissions

Identification

1. Indicator Description

This indicator describes emissions of greenhouse gases (GHGs) in the United States and its territories
between 1990 and 2014. It reports emissions of GHGs according to their global warming potential, a
measure of how much a given amount of the GHG is estimated to contribute to global warming over a
selected period of time. For the purposes of comparison, global warming potential values are given in
relation to carbon dioxide (C02) and are expressed in terms of C02 equivalents. This indicator is highly
relevant to climate change because greenhouse gases from human activities are the primary driver of
observed climate change since the mid-20th century (IPCC, 2013).

Components of this indicator include:

• U.S. GHG emissions by gas (Figure 1).

• U.S. GHG emissions and sinks by economic sector (Figure 2).

• U.S. GHG emissions per capita and per dollar of GDP (Figure 3).

2. Revision History

April 2010: Indicator published.

December 2012: Updated indicator with data through 2010.

August 2013: Updated indicator on EPA's website with data through 2011.

May 2014: Updated indicator with data through 2012.

June 2015: Updated indicator on EPA's website with data through 2013.

August 2016: Updated indicator with data through 2014.

Data Sources

3. Data Sources

This indicator uses data and analysis from EPA's Inventory of U.S. Greenhouse Gas Emissions and Sinks
(U.S. EPA, 2016), an assessment of the anthropogenic sources and sinks of GHGs for the United States
and its territories for the period from 1990 to 2014.

4. Data Availability

The complete U.S. GHG inventory is published annually, and the version used to prepare this indicator is
publicly available at: www3.epa.gov/climatechange/ghgemissions/usinventoryreport.html (U.S. EPA,
2016). The figures in this indicator are taken from the following figures and tables in the inventory
report:

• Figure 1 (emissions by gas): Figure ES-l/Table ES-2/Table 2-1.

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• Figure 2 (emissions by economic sector): Figure ES-13/Table ES-6/Table 2-10.

• Figure 3 (emissions per capita and per dollar gross domestic product [GDP]): Figure ES-
15/Table ES-8/Table 2-14.

The inventory report does not present data for the years 1991-2004 or 2006-2009 due to space
constraints; however, data for these years can be obtained by downloading the complete supporting
tables or by contacting EPA's Climate Change Division (www3.epa.gov/climatechange/contactus.html).

Figure 3 includes trends in population and real GDP. EPA obtained publicly available population data
from the U.S. Census Bureau's International Data Base at: www.census.gov/population/international/.
EPA obtained GDP data from the U.S. Department of Commerce, Bureau of Economic Analysis. These
data are publicly available from the Bureau of Economic Analysis website at:
www.bea.gov/national/index. htm#gdp.

Methodology

5. Data Collection

This indicator uses data directly from the Inventory of U.S. Greenhouse Gas Emissions and Sinks (U.S.
EPA, 2016). The inventory presents estimates of emissions derived from direct measurements,
aggregated national statistics, and validated models. Specifically, this indicator focuses on the long-lived
greenhouse gases currently covered by agreements under the United Nations Framework Convention
on Climate Change (UNFCCC). These compounds are C02, methane (CH4), nitrous oxide (N20), selected
hydrofluorocarbons (HFCs), selected perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and nitrogen
trifluoride (NF3).

The emissions and source activity data used to derive the emissions estimates are described thoroughly
in EPA's inventory report. The scientifically approved methods can be found in the Intergovernmental
Panel on Climate Change's (IPCC's) GHG inventory guidelines (www.ipcc-

nggip.iges.or.jp/public/2006gl/index.html) (IPCC, 2006) and in IPCC's Good Practice Guidance and
Uncertainty Management in National Greenhouse Gas Inventories (www.ipcc-
nggip.iges.or.jp/public/gp/english) (IPCC, 2000). More discussion of the sampling and data sources
associated with the inventory can be found at: www3.epa.gov/climatechange/ghgemissions.

The U.S. GHG inventory provides a thorough assessment of the anthropogenic emissions by sources and
removals by sinks of GHGs for the United States from 1990 to 2014. Although the inventory is intended
to be comprehensive, certain identified sources and sinks have been excluded from the estimates (e.g.,
C02 from burning in coal deposits and waste piles, C02 from natural gas processing). Sources are
excluded from the inventory for various reasons, including data limitations or an incomplete
understanding of the emissions process. The United States is continually working to improve
understanding of such sources and seeking to find the data required to estimate related emissions. As
such improvements are made, new emissions sources are quantified and included in the inventory. For a
complete list of excluded sources, see Annex 5 of the U.S. GHG inventory report
(www3.epa.gov/climatechange/ghgemissions/usinventoryreport.html).

Figure 3 of this indicator compares emissions trends with trends in population and U.S. GDP. Population
data were collected by the U.S. Census Bureau. For this indicator, EPA used midyear estimates of the

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total U.S. population. GDP data were collected by the U.S. Department of Commerce, Bureau of
Economic Analysis. For this indicator, EPA used real GDP in chained 2009 dollars, which means the
numbers have been adjusted for inflation. See: www.census.gov/population/international for the
methods used to determine midyear population estimates for the United States. See:
www.bea.gov/methodologies/index.htm#national meth for the methods used to determine GDP.

6. Indicator Derivation

The U.S. GHG inventory was constructed following scientific methods described in the
Intergovernmental Panel on Climate Change's (IPCC's) Guidelines for National Greenhouse Gas
Inventories (IPCC, 2006) and in IPCC's Good Practice Guidance and Uncertainty Management in National
Greenhouse Gas Inventories (IPCC, 2000). EPA's annual inventory reports and IPCC's inventory
development guidelines have been extensively peer reviewed and are widely viewed as providing
scientifically sound representations of GHG emissions.

U.S. EPA (2016) provides a complete description of methods and data sources that allowed EPA to
calculate GHG emissions for the various industrial sectors and source categories. Further information on
the inventory design can be obtained by contacting EPA's Climate Change Division
(www3.epa.gov/climatechange/contactus.html).

The inventory covers U.S. GHG data for the years 1990 to 2014, and no attempt has been made to
incorporate other locations or to project data forward or backward from this time window. Some
extrapolation and interpolation were needed to develop comprehensive estimates of emissions for a
few sectors and sink categories, but in most cases, observations and estimates from the year in question
were sufficient to generate the necessary data.

This indicator reports trends exactly as they appear in EPA's GHG inventory (U.S. EPA, 2016). The
indicator presents emissions data in units of million metric tons of C02 equivalents, the conventional
unit used in GHG inventories prepared worldwide, because it adjusts for the various global warming
potentials (GWPs) of different gases. The indicator now uses the 100-year GWPs documented in the
IPCC's Fourth Assessment Report (AR4) (IPCC, 2007). EPA is required to use these GWPs for the
development of the GHG inventory to comply with international reporting standards under the UNFCCC.
This requirement ensures that current estimates of aggregate greenhouse gas emissions for 1990 to
2014 are consistent with estimates developed prior to the publication of the IPCC's Fifth Assessment
Report (AR5) in 2013. Annex 6.1 of the U.S. GHG inventory includes extensive information on GWPs and
how they relate to emissions estimates (U.S. EPA, 2016).

Figure 1. U.S. Greenhouse Gas Emissions by Gas, 1990-2014

EPA plotted total emissions for each gas, not including the influence of sinks, which would be difficult to
interpret in a breakdown by gas. EPA combined the emissions of HFCs, PFCs, SF6, and NF3 into a single
category so the magnitude of these emissions would be visible in the graph.

Figure 2. U.S. Greenhouse Gas Emissions and Sinks by Economic Sector, 1990-2014

EPA converted a line graph in the original inventory report (U.S. EPA, 2016) into a stacked area graph
showing emissions by economic sector. U.S. territories are treated as a separate sector in the inventory
report, and because territories are not an economic sector in the truest sense of the word, they have

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been excluded from this part of the indicator. Unlike Figure 1, Figure 2 includes sinks below the x-axis.
The section below the x-axis represents the entire contribution of the land use, land-use change, and
forestry sector, which includes a relatively small amount of emissions that is outweighed by a much
larger sink.

Figure 3. U.S. Greenhouse Gas Emissions per Capita and per Dollar of GDP, 1990-2014

EPA determined emissions per capita and emissions per unit of real GDP using simple division. In order
to show all four trends (population, GDP, emissions per capita, and emissions per unit GDP) on the same
scale, EPA normalized each trend to an index value of 100 for the year 1990.

7. Quality Assurance and Quality Control

Quality assurance and quality control (QA/QC) have always been an integral part of the U.S. national
system for inventory development. EPA and its partner agencies have implemented a systematic
approach to QA/QC for the annual U.S. GHG inventory, following procedures that have been formalized
in accordance with a QA/QC plan and the UNFCCC reporting guidelines. Those interested in
documentation of the various QA/QC procedures should send such queries to EPA's Climate Change
Division (www3.epa.gov/climatechange/contactus.html).

Analysis

8. Comparability Over Time and Space

The U.S. GHG emissions data presented in this indicator are comparable over time and space, and the
purpose of the inventory is to allow tracking of annual emissions over time. The emissions trend is
defined in the inventory as the percentage change in emissions (or removal) estimated for the current
year, relative to the emissions (or removal) estimated for the base year (i.e., 1990) inventory estimates.
In addition to the estimates of uncertainty associated with the current year's emissions estimates,

Annex 7 of the inventory report also presents quantitative estimates of trend uncertainty.

9. Data Limitations

Factors that may impact the confidence, application, or conclusions drawn from this indicator are as
follows:

1. This indicator does not yet include emissions of GHGs or other radiatively important substances
that are not explicitly covered by the UNFCCC and its subsidiary protocol. Thus, it excludes gases
such as those controlled by the Montreal Protocol and its Amendments, including
chlorofluorocarbons and hydrochlorofluorocarbons. Although the United States reports the
emissions of these substances as part of the U.S. GHG inventory (see Annex 6.2 of U.S. EPA
[2016]), the origin of the estimates is fundamentally different from those of the other GHGs, and
therefore these emissions cannot be compared directly with the other emissions discussed in
this indicator.

2. This indicator does not include aerosols and other emissions that affect radiative forcing and
that are not well-mixed in the atmosphere, such as sulfate, ammonia, black carbon, and organic

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carbon. Emissions of these compounds are highly uncertain and have qualitatively different
effects from the six types of emissions in this indicator.

3. This indicator does not include emissions of other compounds—such as carbon monoxide,
nitrogen oxides, non-methane volatile organic compounds, and substances that deplete the
stratospheric ozone layer—that indirectly affect the Earth's radiative balance (for example, by
altering GHG concentrations, changing the reflectivity of clouds, or changing the distribution of
heat fluxes).

4. The U.S. GHG inventory does not account for "natural" emissions of GHGs from sources such as
wetlands, tundra soils, termites, and volcanoes. These excluded sources are discussed in Annex
5 of the U.S. GHG inventory (U.S. EPA, 2016). The "land use," "land-use change," and "forestry"
categories in U.S. EPA (2016) do include emissions from changes in the forest inventory due to
fires, harvesting, and other activities, as well as emissions from agricultural soils.

10. Sources of Uncertainty

Some estimates, such as those for C02 emissions from energy-related activities and cement processing,
are considered to have low uncertainties. For some other categories of emissions, however, lack of data
or an incomplete understanding of how emissions are generated increases the uncertainty of the
estimates presented.

Recognizing the benefit of conducting an uncertainty analysis, the UNFCCC reporting guidelines follow
the recommendations of IPCC (2000) and require that countries provide single point uncertainty
estimates for many sources and sink categories. The U.S. GHG inventory (U.S. EPA, 2016) provides a
qualitative discussion of uncertainty for all sources and sink categories, including specific factors
affecting the uncertainty surrounding the estimates. Most sources also have a quantitative uncertainty
assessment in accordance with the new UNFCCC reporting guidelines. Thorough discussion of these
points can be found in U.S. EPA (2016). Annex 7 of the inventory publication is devoted entirely to
uncertainty in the inventory estimates.

For a general idea of the degree of uncertainty in emissions estimates, WRI (2015) provides the
following information: "According to the most recent IPCC report (IPCC, 2014), estimated uncertainty for
global C02 emissions from fossil fuels is relatively low, about 8%. For non-C02 GHG emissions, CH4 and
F-gases have relatively 'intermediate' uncertainties of around 20%, while N20 has a higher uncertainty
of around 60%. C02 emissions from land-use change and forestry have very large uncertainties of 50-
75%. In total, when combining these uncertainties, estimates of global total GHG emissions have an
uncertainty of around 10%."

Overall, these sources of uncertainty are not expected to have a considerable impact on this indicator's
conclusions. Even considering the uncertainties of omitted sources and lack of precision in known and
estimated sources, this indicator provides a generally accurate picture of aggregate trends in GHG
emissions over time, and hence the overall conclusions inferred from the data are solid. The U.S. GHG
inventory represents the most comprehensive and reliable data set available to characterize GHG
emissions in the United States.

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11. Sources of Variability

Within each sector (e.g., electricity generation), GHG emissions can vary considerably across the
individual point sources, and many factors contribute to this variability (e.g., different production levels,
fuel type, air pollution controls). EPA's inventory methods account for this variability among individual
emissions sources.

12. Statistical/Trend Analysis

This indicator presents a time series of national emissions estimates. No special statistical techniques or
analyses were used to characterize the long-term trends or their statistical significance.

References

IPCC (Intergovernmental Panel on Climate Change). 2000. Good practice guidance and uncertainty
management in national greenhouse gas inventories, www.ipcc-nggip.iges.or.jp/public/gp/english.

IPCC (Intergovernmental Panel on Climate Change). 2006. IPCC guidelines for national greenhouse gas
inventories, www.ipcc-nggip.iges.or.jp/public/2006gl/index.html.

IPCC (Intergovernmental Panel on Climate Change). 2007. Climate change 2007: The physical science
basis. Working Group I contribution to the IPCC Fourth Assessment Report. Cambridge, United Kingdom:
Cambridge University Press, www.ipcc.ch/report/ar4/wgl/.

IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science
basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom:
Cambridge University Press, www.ipcc.ch/report/ar5/wgl.

U.S. EPA. 2016. Inventory of U.S. greenhouse gas emissions and sinks: 1990-2014. EPA 430-R-16-002.
www3.epa.gov/climatechange/ghgemissions/usinventorvreport.html.

WRI (World Resources Institute). 2015. CAIT Country greenhouse gas emissions: sources and methods.
http://cait.wri.Org/docs/CAIT2.0 CountrvGHG Methods.pdf.

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