Estimators fairly consistently place agriculture (actual production of crops and animal products) at about 9-10% of the U.S. total of greenhouse gas emissions. See further below. Per the EPA inventory below (Table 2-16), emissions from agriculture have been relatively stable (up only 2%), while total emissions from all sources have been growing over the past 15 years (up 16%).
The processing, packaging, transportation, retail and household sectors are harder to estimate. However, a study from theCenter for Sustainable Systems at Michigan appears to use available information well to make estimates of energy use at these stages of the process. See page 41, Figure 5 and Appendix B.
Combining these estimates with the EPA data on greenhouse gas emissions, we can derive the following estimate of the GHG impacts of the food sector (including home preparation) — all together, it appears to account for approximately 18% of all greenhouse gases.
Food related greenhouse gas generation in the United States (mixed estimates around 2000)
Energy Use GTCO2 Other GHG’s Total % of
(QuadBTU) + (GTCO2e) GHG’s U.S.
Agriculture Production 2.2 0.161 0.536 0.697 9.6%
Transportation 1.4 0.103 * 0.103 1.4%
Processing 1.7 0.125 * 0.125 1.7%
Packaging 0.7 0.051 * 0.051 0.7%
Food retail 0.4 0.029 * 0.029 0.4%
Commercial food service 0.7 0.051 * 0.051 0.7%
In home processing 3.3 0.242 * 0.242 3.3%
Waste disposal * * * * *
TOTAL 10.4 0.763 0.536 1.299 17.9%
* Not estimated, small for all items except waste disposal
+ Converting quadBTUs to gigatons of CO2 at 20×3.67/1000 (see DOE conversion tables).
U.S. Department of Energy on GHG’s from Agriculture:
Emissions of Greenhouse Gases in the United States 2005 (November 2006). This annual report offers the following relevant estimates for 2005 at page xiv (expressed in Gigatons of Carbon Dioxide Equivalents — GTCO2e):
All U.S. Emissions 7.147
Carbon Dioxide 6.009 (does not break out agriculture sector)
From agriculture 0.183 (mostly cattle gut gas and animal waste)
Nitrous Oxide 0.367
From agriculture 0.280 (from fertilized soil and animal waste)
HFCs, PFCs and SF6 0.160
See also breakouts at Table 22 and 23 on page 44 to 45 of methane sources. The report acknowledges that the nitrous oxide estimates are particularly soft. See Table 33 and pages 71-2 for the point that in the United States, conversion of forest to crop land is not a significant source of net emissions. In fact, growing forests make for a net carbon flux out of the atmosphere and offset roughly 11% of GHG emissions in the United States.
Emission and Reduction of Greenhouse Gases from Agriculture and Food Manufacturing (1999). This study seeks to measure both the agriculture and the food processing sectors. It omits transportation. It concludes that agriculture accounts for 10% of GHG’s in the United States, and that the processing sector may account for another 1-2%. Most of the GHG impact from agriculture is from Methane (CH4) and Nitrous Oxide (N2O), potent greenhouse gases. The impact of these gases is usually referred to in carbon dioxide equivalents. Table 2 and Table 11 and Figure 9 of the report offer the following estimates (expressed in Gigatons of Carbon Dioxide Equivalents — GTCO2e) for 1997:
All U.S. emissions: 6.56
All U.S. agriculture: 0.644
Carbon Dioxide 0.163 (fertilizer production, farm equipment)
Methane 0.198 (almost all from animal gut gas and waste)
Nitrous Oxide 0.283 (from fertilized soil, including spread manure)
All U.S. food processing 0.108 (drying and other heat processes dominate)
This report also acknowledges that the nitrous oxide estimates are particularly soft, but comes to a number consistent with the more recent Energy report.
U.S. Environmental Protection Agency on GHG’s from Agriculture:
Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2005 (April 2007): This report is an authoritative overview for 2005 and includes the following estimates in (expressed in Gigatons of Carbon Dioxide Equivalents — GTCO2e):
All U.S. emissions: 7.260
All U.S. Agriculture 0.536 (excluding fuel combustion and electricity)
Methane 0.161 (almost all from animal gut gas and waste)
Nitrous Oxide 0.375 (almost all from soil management — fertilizer)
All U.S. Agriculture 0.659 (with allocated fuel and electricity)
CO2e added in this est. 0.123 (electricity and some fuel, but not fertilizer)
Note, table 6-3 allocates 81.0 of 115.7 TGCO2e of enteric methane to beef cattle (most or rest to dairy). The report offers a much more detailed analysis of the uncertainties associated with N2O emissions.
International Panel on Climate Change on GHG’s from Agriculture (Working Group III of Fourth Assessment Report (May 2007).
Estimates worldwide that agriculture accounts for 10 to 12% (5.1 to 6.1 GTCO2e) of world wide GHG emissions in 2005. This estimate excludes fuel and electricity use.