State and Local
Climate and Energy Program
A Quick Guide to
Greenhouse Gas Inventories
What is a greenhouse gas inventory and why is it important?
A greenhouse gas inventory is an accounting of greenhouse gases emitted to or removed from the
atmosphere over a period of time (e.g., one year). Policy makers use inventories to track emission
trends, develop mitigation strategies and policies, and assess progress. Scientists use them as inputs to
atmospheric and economic models.
Inventory or Registry?
A greenhouse gas inventory is an accounting of
emissions sources and sinks, whereas a greenhouse
gas registry is a collection of inventories. A registry
is used to record emissions and/or emission
reductions by the registry's members, each of which
submits an inventory.
An inventory can help you with any or all of the
following tasks:
Identify the greatest sources of greenhouse gas emissions
within your geographic region.
Understand emission trends.
Quantify the benefits of activities that reduce emissions.
Establish a basis for developing an action plan.
Track progress in reducing emissions.
Set goals and targets for future reductions.
Because it's hard to manage what you don't measure, developing an inventory is usually the first step
taken by states and localities that want to reduce their greenhouse gas emissions.
Key steps and issues in establishing an inventory
Inventories form the basis for decision-making; therefore, they should be transparent, be easily reproducible,
and follow established accounting guidelines. This will also ensure consistency and comparability with other
inventories. The process of designing an inventory entails a number of decisions and procedural steps:
Setting boundaries: It's important to define an inventory's physical, organizational, and operational
boundaries at the outset. Will emissions from imported electricity be counted? What about emissions from
waste that is exported by your jurisdiction? Should product life-cycle emissions be included? Establishing
boundaries will help you focus the design of your inventory.
Scope: You will need to decide which emissions source categories and subcategories should be included
in the inventory (e.g., will you include sources such as agriculture, industrial processes, and land-use
change?) as well as which specific greenhouse gases (e.g., will you focus just on carbon dioxide, methane,
and nitrous oxide or include others as well?).
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Quantification approach: Depending on the data
available and the purposes of the inventory, you may
choose to take a top-down or bottom-up approach to
data collection. State fuel consumption estimates are an
example of top-down data, while end-user utility bills are
an example of bottom-up data. Top-down data are less
detailed than bottom-up data but may be easier to obtain
and have fewer errors. Some states and localities use a
hybrid approach in their inventories.
The 80/20 rule: The scope and detail of an inventory
are driven by the inventory's goals and purpose. Given
limited resources, you have to decide where to draw
the line. In many cases, only 20 percent of the effort is
required to quantify 80 percent of the emissions. Energy
use and transportation typically account for the majority
of emissions in a state or locality.
New Mexico Inventory
YEAR 2000
Industrial
2% Processes
7% Agriculture
Waste
Energy
Source: Draft New Mexico Greenhouse Gas Inventory and
Reference Case Projections, July 2005
• Setting a baseline: Baseline data provide a benchmark
to compare progress going forward. As such, choosing a baseline year for emissions involves careful
research to ensure that (1) data for that year are available, (2) the chosen year is representative (i.e.,
emissions that year were not uncharacteristically high or low), and (3) the baseline is coordinated to
the extent possible with baseline years used in other inventories.
• Engaging stakeholders: Bringing stakeholders into the inventory development process early on
enables them to provide valuable input on establishing a baseline; help build public acceptance of
policies to address climate change; and provide data, information on data resources, and personnel
resources or outreach assistance.
• Certification: A third-party review and certification of the methods and underlying data in
your inventory assures that the inventory is high quality and that it is complete, consistent, and
transparent. Certification is an important option to consider when deciding on the level of rigor you
are trying to achieve, and may be required for participation in some greenhouse gas registries. A
number of standard protocols exist for inventory review and certification.
For more information on greenhouse gas inventories, please visit the following EPA resources online:
State and Local Resources for Inventory Development
http://www.epa.gov/climatechange/emissions/state guidance.html
Inventory Training Opportunities (including webcast files and transcripts)
http://www.epa.gov/climatechange/emissions/state training.html
EPA-430-F-09-003 February 2009
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