Global greenhouse gas emissions from the agricultural sector totalled 4.25 billion tonnes of carbon dioxide (CO2) equivalent in 2010 (the last year for which data are available), an increase of 13 percent over 1990 emissions. Put into perspective, agricultural emissions compare to 6.13 billion tonnes CO2e from transport and 11.32 billion tons from heat and energy production.

Washington DC-based policy organisation Worldwatch Institute, which produced a report on the subject this week said, though there are suggestions that the rate of emission fell – as growth in agricultural production between 1990 and 2010 outpaced growth in emissions by a factor of 1.6, suggesting improvements in energy efficiency – the high emissions levels are still concerning, given the findings of studies related to the potential effects of anthropogenic climate change released this week.

image expired

Although Worldwatch said that emissions from animal agriculture are responsible for the lion's share of farming sector emissions globally, figures from the UK and Europe seem to show that other areas of agriculture are releasing more damaging greenhouse gasses, due to changes in Europe's livestock sector.

Breakdown of GHG emissions

The three most common gases emitted in agriculture are nitrous oxide, CO2, and methane.

Although in the UK, nitrous oxide emissions account for the largest proportion of emissions from agriculture (at 55 percent of emissions from the sector), worldwide Methane is the most commonly emitted greenhouse gas, accounting for around 50 percent of total agricultural emissions. In the UK, Defra figures show methane emissions account for around 36 percent of emissions from agriculture. According to worldwatch, enteric fermentation (the digestion of organic materials by livestock) is the largest source of methane emissions and of agricultural emissions overall.

Nitrous oxide is most commonly released by microbes breaking down nitrogen in soils and manures; although this process happens naturally at a background level, emissions increase when nitrogen-rich synthetic fertilisers are used; Worldwatch said "Nitrous oxide production is particularly high in cases where the nitrogen available in soils exceeds that required by plants to grow". Globally, nitrous oxide is responsible for around 36 percent of agricultural greenhouse gas emissions.

Patterns observed in Worldwatch's research, including elevated nitrous oxide emissions from cultivated organic soils, were indicative of high levels of deforestation and clearing for agricultural land. Four out of the top five countries with this kind of emissions were in Asia.

Finally, carbon dioxide is released from soils when organic matter decomposes aerobically (with oxygen). The largest source of CO2 emissions within agriculture is the drainage and cultivation of "organic soils"----soils in wetlands, peatlands, bogs, or fens with high organic material. When these areas are drained for cultivation, organic matter within the soil decomposes at a rapid rate, releasing CO2. This process accounts for around 14 percent of total agricultural greenhouse gas emissions.

The proportion in which gasses are released from agricultural activity are particularly concerning, as methane is 12 times more potent than CO2 over its atmospheric 'lifetime', while nitrous oxide has 112 times the global warming potential. The Global Warming Potential of emissions is measured using carbon dioxide as a baseline; the gasses' potential to trap heat is measured relative to the effects caused by the same mass of CO2.

Overall emissions on the rise

Worldwatch revealed this week that emissions from enteric fermentation rose by 7.6 percent worldwide between 1990 and 2010, and pointed out that regional variation was high. Over the period, such emissions from Africa and Asia rose by 51.4 percent and 28.1 percent respectively. However, emissions in Europe and Oceania fell by 48.1 percent and 16.1 percent.

The American policy organisation suggested "Europe's significant reduction in emissions parallels the decline in its beef production between 1990 and 2010, but it may also reflect increased use of grains and oils in cattle feed instead of grasses."

Worldwatch author Laura Reynolds elaborated that this may result in reduced emissions but could potentially engender other problems; "Adding oils or oilseeds to feed can help with digestion and reduce methane emissions. 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," she said. "Aside from reducing livestock populations, there is no other clear pathway to climate-friendly meat production from livestock."

Reynolds added that, although she advocates reducing livestock populations to limit global emissions from agriculture, there are numerous other ways to mitigate emissions, many of carry environmental and even economic co-benefits.

The Worldwatch author advocated moves into agro-forestry – growing trees and woody perennials on land to sequester carbon while simultaneously helping to restore soils, reduce water contamination, and provide beneficial wildlife habitat. This can also be achieved alongside other farming activities; the approach is receiving growing attention and public support, particularly in France and the United States.