'No clear pathway' to climate-friendly livestock production: Worldwatch
However, food production has become more energy efficient, as the volume of agricultural production increased 23% from 1990 to 2010. The sector is the third biggest contributor to overall GHG emissions - at 4.69bn tons of carbon dioxide equivalent in 2010 - after transport, and electricity and heat production.
Methane accounts for about half of all agricultural emissions, and animals' digestion - enteric fermentation - is the biggest source of methane, making animals' digestive process the biggest single contributor to the sector's GHG emissions.
"Adding oils or oilseeds to feed can help with digestion and reduce methane emissions," said Worldwatch Food and Agriculture researcher and study author Laura Reynolds. "But a shift from a grass-based to a grain- and oilseeds-based diet often accompanies a shift from pastures to concentrated feedlots, which has a range of negative consequences such as water pollution and high fossil fuel consumption.
"Aside from reducing livestock populations, there is no other clear pathway to climate-friendly meat production from livestock."
Emissions from enteric fermentation rose 7.6% from 1990 to 2010, but there were big regional variations. Emissions from this source in Europe actually fell 48.1% in tandem with declining beef production, but the report said increased use of grains and oils in feed, instead of grasses, could also have played a part.
Emissions from enteric fermentation increased by 51.4% in Africa during the period, and by 28.1% in Asia.
After methane, nitrous oxide and carbon dioxide are the other most common gases produced through agriculture.Nitrous oxide is responsible for 36% of agricultural GHG emissions, the report found, and is particularly high when synthetic fertilizers are applied to soil.
Meanwhile, carbon dioxide accounts for about 14% of agricultural emissions, and the largest source is drainage and cultivation of organic soils - those in wetlands, peatlands, bogs or fens, which are rich in organic material.
Worldwatch said that apart from reducing livestock populations, other ways to reduce global emissions from agriculture include planting trees to restore soils and reduce water contamination, and reducing soil tillage.
Global GHG emissions from transport reached 6.76bn tons in 2010, while emissions from electricity and heat production totalled 12.48bn tons, according to the report.
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