Overview

 

Introduction

This report provides a scientific basis and methods for estimating greenhouse gas (GHG) emissions and sinks from management practices at an entity level for a farm, ranch, or forest system.

There are several approaches to entity-scale GHG emissions estimation, each one giving different accuracy and precision. For some agricultural sectors, direct measurement may be the most accurate way to estimate emissions, but it often requires expensive equipment or techniques that are not feasible for a single landowner or manager. Meanwhile, simple lookup tables and estimation equations alone often do not adequately represent local variability or conditions. This report aims to balance straightforward approaches, practical data requirements, and appropriate scientific rigor in a way that is transparent and justified.

The methods in this report have been developed for U.S. conditions and are considered applicable to agricultural and forestry production systems in the United States. The report covers the following land-use sectors: croplands/grazing lands, managed wetlands, animal production systems, and forestry, along with changes in land use. The report does not provide methods for lands categorized as settlements (e.g., residential and commercial buildings), with the exception of urban trees.

The authors evaluated updated sources to reflect current science, including the Intergovernmental Panel on Climate Change (IPCC) 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories. The types of approaches that the authors recommend in this report include multiple levels, or tiers, of complexity and accuracy, based on the best available data and methods. This approach is similar to the methodological tiers developed by IPCC, which are based on the complexity of different approaches for estimating GHG emissions.

The methods described in this report fall into the following categories:

• Basic estimation equations use default equations and emission factors, such as IPCC Tier 1 methods.
• Inference uses geography-, crop-, livestock-, technology-, or practice-specific emission factors to approximate emissions/removal factors. This approach is similar to an IPCC Tier 2 method. It is more accurate and more complex, and requires more data inputs, than the basic estimation.
• Modified IPCC/empirical and/or process-based modeling, comparable to IPCC Tier 2 or IPCC Tier 3 methods. These methods are the most demanding in terms of complexity and data requirements and produce the most accurate estimates.

The figure below depicts GHG fluxes from agriculture and forestry systems included in this report. This includes fluxes from croplands and grazing lands (biomass, litter and soil stock changes, rice cultivation, non-flooded soils, urea and liming, biomass burning), animal production (enteric fermentation, manure, and housing), forestry (silviculture, harvested wood products, forest fires, biomass burning, litter/deadwood, litter clearing, urban forest management), and wetlands.

The report authors used the following selection criteria in confirming or updating management practices and quantification methods:

• The science reflects a mechanistic understanding of the practice's influence on an emission source.
• Published research (including international studies involving management, climate, and soils similar to those in the United States) supports a reasonable level of repeatability and consistency, and the response of emissions to the given practice is understood and quantifiable.
• The exclusion of this method would make the sector incomplete and there is strong enough evidence that the method will hold up for this practice for at least the next 5 years.

Some practices did not fulfill these criteria. These were cited as research gaps, intended to become priority focus areas for agriculture and forestry climate change research by USDA, nongovernmental organizations, universities, and other research institutions.

More information about the report development process, purposes of the report, uses and limitations, and each sector included in the report can be found in chapter 1.

 

Definitions and Considerations

The methods provided in this report depend on standard definitions and common estimation elements for all emission sectors. This standardization ensures that landowners or managers can accurately inventory their direct GHG emissions and removals and make comparisons across years; management practices; or farms, ranches, or forests. The report provides standard definitions, explains the steps in the estimation process, and provides other information to help landowners and managers understand the estimation elements to include and the methods to use.

When using or referencing this report, keep the following considerations in mind:

• The report generally does not provide a range of emission/sequestration accounting options at varying levels of complexity (i.e., tiers) for each source category. However, chapter 5 specifies individual options for entities within source categories where there are significant differences in data and/or user familiarity.
• The methods are not intended to constitute a life cycle assessment (LCA). An LCA evaluates the entire lifespan of a commodity or product to fully quantify its environmental impact. This report focuses on emissions that occur at the entity scale annually. It does not provide the methods required to quantify upstream production (e.g., animal feed production, fertilizer manufacture) or downstream production (e.g., wastewater treatment, pulp and paper manufacture, landfills)—except for harvested wood product treatment, which is discussed in chapter 5.
• The methods are not meant for estimating emissions from stationary source combustion (e.g., burning heating oil or natural gas to heat animal housing) or mobile source combustion (e.g., fuel use in vehicles), with the exception of chapter 5, which includes emission reductions that occur when substituting woody biomass for nonrenewable energy sources. However, the report does qualitatively discuss obvious changes in combustion levels due to a management practice change. For example, a shift from conventional tillage to no-till can significantly reduce fuel consumption, since fewer trips across the field are needed. Methods for quantifying emissions from stationary or mobile combustion sources are available from other Federal agencies (e.g., EPA's Inventory of U.S. Greenhouse Gas Emissions and Sinks), and a calculator that provides emissions reductions associated with changes in on-farm fuel or electricity use is available at the COMET-Energy website.

Information on how to define an entity; information on how to determine physical, temporal, activity, and material boundaries; and basic information on the estimation process, including how to define a project, can be found in chapter 2.