finding 5.1 : key-message-5-1

Agricultural greenhouse gas (GHG) emissions in 2015 totaled 567 teragrams (Tg) (1) of carbon dioxide equivalent (CO2e) (2) in the United States and 60 Tg CO2e in Canada, not including land-use change; for Mexico, total agricultural GHG emissions were 80 Tg CO2e in 2014 (not including land-use change) (high confidence). The major agricultural non-CO2 emission sources were nitrous oxide (N2O) from cropped and grazed soils and enteric methane (CH4) from livestock (very high confidence, very likely).(3)

1 Excludes emissions related to land use, land-use change, and forestry activities.
2 Carbon dioxide equivalent (CO;2e): Amount of CO;2 that would produce the same effect on the radiative balance of Earth’s climate system as another greenhouse gas, such as methane (CH4) or nitrous oxide (N;2O), on a 100-year timescale. For comparison to units of carbon, each kg CO;2e is equivalent to 0.273 kg C (0.273 = 13.67). See Box P.2 in the Preface for more details.
3 Estimated 95% confidence interval lower and upper uncertainty bounds for agricultural greenhouse gas emissions: –11% and +18% (CH4 emissions from enteric fermentation) and –18% and +20% and –16% and +24% (CH4 and N;2O emissions from manure management, respectively; U.S. EPA 2018).

This finding is from chapter 5 of Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report.

Description of evidence base: Bottom-up estimates of GHG emissions are from U.S. EPA (2018), ECCC (2017), and FAOSTAT (2017) data for the United States, Canada, and Mexico, respectively. These estimates include rice cultivation, field burning of agricultural residues, fertilization and liming, enteric fermentation, and manure management, but they do not include land-use change. The major components of agricultural non-CO2 emissions have been consistent in numerous reports including those listed above for the emissions estimates part of this Key Finding.

New information and remaining uncertainties: Uncertainty exists in any measurement or projection of GHG emissions. Emissions from enteric fermentation are relatively well studied and predictable, but there is larger uncertainty regarding manure CH4 and N2O emissions. Considerable uncertainty exists in soil carbon accumulation and quantities as well as in terms of emissions from soils under different conditions and management practices. There are large uncertainties in GHG emissions from agricultural cropping systems due to high spatial and temporal variability, measurement methods, cropping systems, management practices, and variations in soil and climatic conditions among regions.

Assessment of confidence based on evidence: There is very high certainty that N2O and CH4 are the major agricultural non-CO2 emission sources. There is high confidence in the numerical estimates.

This finding was derived from figure P.2: P.2. Likelihood and Confidence Evaluation

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