By Laura Reynolds
In 2010, global greenhouse gas emissions from the agricultural sector totaled 4.7 billion tons of carbon dioxide (CO₂) equivalent, up 13 percent over 1990. Agriculture is the third largest contributor to global emissions by sector, following the burning of fossil fuels for power and heat, and transportation. In 2010, emissions from electricity and heat production reached 12.5 billion tons, and emissions from transport totaled 6.7 billion tons.
Despite their continuing rise, emissions from agriculture are growing at a much slower rate than the sector as a whole, demonstrating the increasing carbon efficiency of agriculture. From 1990 to 2010, the volume of agricultural production overall increased nearly 23 percent, according to data compiled by the United Nations Food and Agriculture Organization (FAO) for its program, FAOSTAT. FAO released a new GHG Emissions database for agriculture, forestry and other land use changes in Dec 2012, which can be found here.
According to FAO, methane accounts for just under half of total agricultural emissions, nitrous oxide for 36 percent, and carbon dioxide for some 14 percent. The largest source of methane emissions is enteric fermentation, or the digestion of organic materials by livestock, predominantly beef cattle. This is also the largest source of agricultural emissions overall, contributing 37 percent of the total.
Livestock contribute to global emissions in other ways as well. Manure deposited and left on pastures is a major source of nitrous oxide emissions because of its high nitrogen content. When more nitrogen is added to soil than is needed, bacteria convert the extra nitrogen into nitrous oxide and release it into the atmosphere. Emissions from manure on pasture in Asia, Africa, and South America together account for as much as 81 percent of global emissions from this source. These emissions from the three regions increased 42 percent on average between 1990 and 2010, reflecting an increase in range-based livestock populations; elsewhere, these emissions either decreased or stagnated.