Inventories of greenhouse gas (GHG) emissions are a critical tool in the fight against climate change. GHG inventories allow entities like countries, states, cities and businesses to measure how much progress they are making toward meeting emissions-reduction targets, such as those set under the Paris Climate Agreement. Climate policies at all levels of government are also informed by data in GHG inventories.

The U.S. Climate Alliance has facilitated ambitious state-level action on climate change since 2017, when the United States government announced its intent to withdraw from the Paris Agreement. To support states’ technical needs in building and implementing these climate action plans — including by developing robust GHG inventories — the U.S. Climate Alliance has convened an “Impact Partnership” of nonprofit organizations with relevant expertise, including WRI. Through that partnership, WRI and the U.S. Climate Alliance have published a guide for states to develop and improve their GHG inventories with an eye toward one particular sector that has often been shortchanged: natural and working lands (NWL). But to understand why a state-level guide specific to land-based GHG inventories is needed, it’s important to first know what a GHG inventory is, why inventories are produced and how they are created.

Inventory Basics: What, Why and How
1. What is a GHG inventory?

An inventory accounts for all human-caused emissions and removals of GHGs associated with a specific entity. The inventory essentially acts as a climate change balance sheet, tracking the total volume of GHG emitted from sources like fossil fuel consumption and agricultural production alongside the volume of GHG removed by sequestration in plants and soils or through technological means. Good inventories transparently report their data sources and methodologies so the calculations and assumptions that underlie GHG estimates are clear. Typically, entities produce GHG inventories annually or on some other regular schedule to monitor changes in their GHG emissions and removals over time.

2. Why produce a GHG inventory?

As the saying goes, “You can’t manage what you can’t measure.” Measuring GHG emissions and removals through GHG inventories is therefore a necessary first step to manage our collective carbon footprint. The United Nations Framework Convention on Climate Change (UNFCCC) has required participating nations, including the United States, to produce and submit annual GHG inventories since 1997 to measure progress toward international climate goals. In more recent years, many U.S. states have voluntarily published their own GHG inventories to inform development of state climate action plans and provide accountability for their emissions reduction goals.

With the U.S. government and the U.S. Climate Alliance’s recent commitment to reduce collective net GHG emissions by 50-52% below 2005 levels by 2030 and achieve overall net-zero GHG emissions no later than 2050, the accuracy and comprehensiveness of these inventories has never been more paramount. Achieving net-zero at both the federal and state levels will require concerted action — not only to reduce emissions throughout the economy, but also to increase carbon removals, including the management of natural and working lands.

NWL, which include forests, croplands, grasslands, wetlands and urban trees and soils, make up the only sector in the U.S. that removes more carbon from the atmosphere than it emits, reducing total U.S. emissions by nearly 800 million tons of carbon dioxide equivalent per year, or about 12% of U.S. gross emissions. This increase in land-based carbon storage, which overwhelmingly comes from forest growth, offsets the 10% of gross U.S. emissions from agricultural production. Emissions from agricultural production, which includes soil fertilization, manure management, enteric fermentation and other sources related to crop and livestock cultivation, are typically considered separately from NWL in GHG inventories.

With additional investment in conservation, restoration and land management, the amount of carbon removed by NWL in the U.S. can grow significantly, offsetting a greater portion of U.S. gross emissions and moving the U.S. closer to meeting its ambitious GHG reduction targets.

3. How are Natural and Working Lands included in GHG inventories?

Unlike GHG emissions from fossil fuel combustion, which are easily tracked through publicly reported energy use data, emissions and removals from NWL are more difficult to measure. These emissions and removals are occurring constantly over millions of acres due to farming and forestry operations alongside natural ecosystem carbon cycles, making universal monitoring very challenging. In many cases, scientists are also still refining our understanding of how land management practices like forest restoration or conservation tillage impact GHG flows in those environments. Therefore, GHG inventories typically rely on sample data to estimate the area of NWL within certain classifications and GHG models or approximate “emission factors” to estimate GHG emissions and removals as a function of area.

These challenges illustrate why estimates of land-based emissions and removals in GHG inventories are typically much more uncertain than energy emissions. Contributors to the uncertainty include:

Timeliness of data inputs (how long ago data were collected).
Spatial and temporal resolution of inventory data (how finely data can be mapped over space and time).
Gaps in inventory coverage (which sources of emissions and removals are omitted).
Error in GHG models and emission factors (how accurately the calculations mirror real-world emissions and removals).
These challenges are compounded at the state level, where most states lack the resources to develop their own inventories and have had to depend on federal data and tools with significant limitations. Many states, for example, use the U.S. Environmental Protection Agency’s (EPA) State Inventory Tool (SIT), which applies the same methods and data sources used for EPA’s National GHG Inventory at the state level. However, much of the data on land-based emissions and removals used in the National Inventory is not available at the state level, so SIT has relied on older and less accurate data to fill gaps. SIT also does not publish measures of uncertainty. For these reasons, many states have opted to leave NWL out of their GHG inventories entirely, while others that do include SIT estimates for that sector have cautioned against relying on them for goal-setting or policymaking purposes.

2. Monitor soil carbon through national field networks.

Carbon sequestration in agricultural soils is currently modeled, not measured, to calculate GHG estimates in the National Inventory and SIT, leading to uncertainty of over 1,000% nationally for some soil carbon removal estimates. Regular, systematic collection of soil carbon field measurements through the federal National Resources Inventory (NRI) could help refine models and reduce this uncertainty dramatically. The National Academies of Sciences has estimated the cost of this endeavor at just $5 million per year.
Read further from the source