Agriculture and climate change
Agriculture as source and sinkAgriculture is a major source of greenhouse gas emissions. It releases large quantities of carbon dioxide through the burning of biomass, mainly in areas of deforestation and grassland.
Agriculture is also responsible for up to half of all methane emissions. Though it persists for a shorter time in the atmosphere, methane is about 20 times more powerful than carbon dioxide in its warming action and is therefore a major short-term contributor to global warming. Current annual anthropogenic emissions are around 540 million tonnes and are growing at around 5 percent per year.
Livestock alone account for about a quarter of methane emissions, by way of gut fermentation and the decay of excreta. As livestock numbers grow, and as livestock rearing becomes increasingly industrial, the production of manure is projected to rise by about 60 percent by 2030. Methane emissions from livestock are likely to increase by the same proportion.
Irrigated rice farming is the other main agricultural source of methane, accounting for about a fifth of total anthropogenic emissions. The area used for irrigated rice is projected to increase by about 10 percent by 2030. However, emissions may grow more slowly, because an increasing share of rice will be grown with better controlled irrigation and nutrient management, and rice varieties may be used which emit less methane.
Agriculture is a key source of another important greenhouse gas: nitrous oxide. This is generated by natural processes, but is boosted by leaching, volatilization and runoff of nitrogen fertilizers, and by the breakdown of crop residues and animal wastes. Livestock account for about half of anthropogenic emissions. Annual nitrous oxide emissions from agriculture are projected to grow by 50 percent by 2030.
Agriculture can help mitigate climate changeFarming can also be a sink for carbon. However, it is generally believed that soils, like other biological sinks (e.g. vegetation), have an inherent upper limit for storage. The total amount that can be stored is crop and location-specific and the rate of sequestration declines after a few years of growth before eventually reaching this limit. In 1997-99 an estimated 590 to 1 180 million tonnes of carbon were locked up in cropland soils alone, in the form of soil organic matter from crop residues and manure. Projections of increased crop production imply that by 2030 this total could rise by 50 percent.
Other changes could boost the total even further. If only 2 million of the current 126 million ha of saline soils were restored each year, they could account for an extra 13 million tonnes of carbon annually. In developed countries, permanent set-aside land can sequester large amounts of carbon if it is left unmanaged, or reforested.
Depending on agroclimatic conditions, NT/CA can lock up 0.1 to 1 tonne of carbon per ha per year, in addition to cutting carbon dioxide emissions by over 50 percent through the reduced use of fossil fuel in ploughing. The growth potential for NT/CA is considerable. If another 150 million ha of rainfed cropland is converted to NT/CA by 2030 and the average sequestration rate on land managed in this way is 0.2 to 0.4 tonne per ha per year, a further 30 to 60 million tonnes of carbon could be soaked up annually during the first few years after conversion.
Should any of these practices be discontinued, the sequestered carbon would be released over a period of a few years. Agricultural carbon sinks of this kind are needed to "buy time" in which to cope with carbon dioxide emissions at source.
Climate change will have very diverse impacts on agricultureClimate change will affect agriculture, forestry and fisheries in complex ways, positive as well as negative. Global carbon dioxide concentrations in the atmosphere are expected to rise from 350 ppm to over 400 ppm by 2030. Carbon dioxide causes plant stomata to narrow, so water losses are reduced and the efficiency of water use improves. Increasing atmospheric concentrations of carbon dioxide will also stimulate photo-synthesis and have a fertilizing effect on many crops.
In the next three decades, climate change is not expected to depress global food availability, but it may increase the dependence of developing countries on food imports and accentuate food insecurity for vulnerable groups and countries.