In 2010, global greenhouse gas emissions from the agricultural sector totalled 4.7 billion tons of carbon dioxide (CO₂) equivalent, up 13 per cent over 1990, according to a new report from the Worldwatch Institute based in Washington, DC. 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 totalled 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 energy efficiency of agriculture. From 1990 to 2010, the volume of agricultural production overall increased nearly 23 per cent.

According to the United Nations Food and Agriculture Organization, methane accounts for just under half of total agricultural emissions, nitrous oxide for 36 per cent, and carbon dioxide for some 14 per cent. 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 per cent 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 per cent of global emissions from this source. These emissions from the three regions increased 42 per cent on average between 1990 and 2010, reflecting an increase in range-based livestock populations; elsewhere, these emissions either decreased or stagnated.

Carbon dioxide emissions from cultivated organic soils account for some 14 per cent of total agricultural emissions, with Asia contributing 54 per cent of these emissions. Deforestation and clearing for agricultural land in many tropical South and Southeast Asian countries are a leading cause of these emissions. Asia is home to four out of the top five countries with the highest carbon dioxide emissions from cultivated organic soils, with Indonesia contributing 279 million tons, Papua New Guinea 41 million tons, Malaysia 35 million tons, and Bangladesh 31 million tons.

These data clearly indicate, says the report, that livestock production accounts for an enormous share of global greenhouse gas emissions. Together, emissions from enteric fermentation, manure left on pastures, manure applied to soils, cropland devoted to feed production, and manure treated in management systems contribute more than 80 per cent of total emissions. Meanwhile, emissions related to the direct human consumption of food crops represent less than 20 per cent of the total.

One obvious way to reduce agricultural emissions is for people to minimise their consumption of meat and dairy products. This would help stabilise or shrink livestock populations, lessen the pressure to clear additional land for livestock, and reduce the proportion of grain that is grown for livestock feed instead of for direct human consumption.

Farmers and landowners have numerous opportunities to mitigate these impacts as well, bringing environmental and even economic co-benefits. For example, applying fertiliser more efficiently, precisely, and at times when plants can absorb it can significantly reduce nitrous oxide emissions while lowering fertiliser costs. Planting fallow fields with nitrogen-fixing legume crops - such as soybeans, alfalfa, and clover - can also naturally rebuild nitrogen and other nutrients in soils.

Growing trees and woody perennials on land can sequester carbon while simultaneously helping to restore soils, reduce water contamination, and provide beneficial wildlife habitat. Reducing soil tillage can also rebuild soils while lowering greenhouse gas emissions. Some practices can even result in increased income for farmers: 'cap-and-trade' programmes allow farmers to monetize and sell certain sequestration practices, while government programs like the US Conservation Reserve Program pay farmers to set aside some of their land for long-term restoration.

As detailed in the 2012 Worldwatch report, 'Innovations in Sustainable Agriculture: Supporting Climate-Friendly Food Production', many mitigation practices use existing and accessible technologies and can be implemented immediately.

Read the latest full report at Vital Signs Online.