One worldwide estimate, Livestock’s long shadow, places total GHG emissions attributable to livestock at 4.6 GTCO2e (Table 3.12) — perhaps 80% of total agricultural emissions worldwide, estimated at 5.1 to 6.1 GTCO2e by the IPCC’s AR4. (However, these two reports are not be directly comparable to each other in estimating method.)
Working from EPA’s inventory of greenhouse gas emissions, the following estimate is explained below: 55 – 65% of GHG’s from United States agriculture are attributable to domestic livestock (feed exports are not attributed to livestock in this computation). For 2005, this report (Tables 2-16 and 6-1) offered the following “Total” estimates expressed in gigatons of carbon dioxide equivalents (GTCO2e).
Total Livestock share
All U.S. Agriculture 0.659 0.372 to 0.437 (56% to 66%)
Methane 0.161 0.153
Enteric Fermentation 0.112 0.112
Manure Management 0.041 0.041
Rice Cultivation 0.007 0.000
Field Burning 0.001 0.000
Nitrous Oxide 0.375 0.178 to 0.243
Soil Management 0.365 0.168 to 0.233
Manure Management 0.010 0.010
Field Burning 0.001 0.000
CO2e from fuel and power 0.123 0.041
Allocation of Methane (definitive)
Almost all of the methane derives from livestock (enteric fermentation and manure management).
Allocation of Nitrous Oxide (speculative)
The EPA report provides explanataion and data which can be used to very roughly allocate the Nitrous Oxide component: Nitrous Oxide is produced by the action of soil microbes. The amount the microbes produce is a function of the amount of nitrogen in the soil. Nitrogen may be added to the soil directly, for example, by fertilization or by planting of nitrogen fixing crops. Or it may be added indirectly through atmospheric or ground water transfer of fertilizer applied elsewhere. Focusing on the Soil Management line of the Nitrous Oxide estimate, the EPA inventory breaks down the components as below. The shares allocable to livestock are roughed in based on the further discussion below:
Est. Livestock Share
N2O from Soil Management 0.365 0.168 – 0.233
Direct Emissions 0.311
Cropland 0.234 25-50%
Grassland 0.076 100%
Cropland 0.026 25-50%
Grassland 0.018 100%
Managed Manure 0.009 100% (loss of manure in transport, etc.)
Forestland 0.000 0%
Settlements 0.002 0%
Soil N20 attributable to Livestock: .168 to .233
Memo: CO2 from feed fertilizer manufacture: .007 (WB est. from sources below)
As to emissions from cropland, a recent study of the environmental impact of livestock cultivation, Livestock’s long shadow, estimates that 51 percent of nitrogen fertilizer used in the United States goes to feed crops (Table 3.3). This estimate appears to be high. USDA data appear to put the number closer to 25%, excluding exported products. Attribution of exported grain and soy to livestock could account for a portion of the difference.
The largest category of nitrogen source is not actually fertilizer, but “other” — soil processes that generate nitrogen. This is a quantity generated for the inventory through a biochemical model called DAYCENT and is not readily allocated. See page 6-21ff. of the EPA inventory. An acreage allocation may be the most appropriate guess for this category. USDA datasuggest that 45% of cropland is allocated to feed generation.
From all of this, the range 25-50% for crop land.
It appears to make sense to allocate 100% of grassland emissions to livestock as the definition of grasslands in the EPA inventory is “pastures and rangelands used for grass forage production where the primary use is livestock grazing” (page 6-24).
Allocation of Carbon Dioxide (highly speculative)
There is no easy route to allocating the contribution of livestock to farm carbon dioxide — heating, farm equipment, etc. 45% based on acreage of cropland devoted to feed crops is not too unreasonable, especially given that all of the energy directly expended in livestock operations is not accounted for. 1/3 of use seems conservative. Compare Livestock’s long shadow, circa Table 3.5 — impossible to do a world wide allocation; offers numbers from Minnesota: These numbers work out to 44% if one conservatively attributes only 25% of soy and corn production to livestock feed.
Carbon Dioxide from processing and transportation of live stock
Processing emissions are impossible to accurately estimate, but Livestock’s long shadow, places it “on the order of a few million tonnes CO2,” in other words, apparently less than 0.010 GTCO2 and not essential to estimate. Similarly transporation also appears to well under this level. See pages 99 to 101.