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Food & Climate
Basics |
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Contribution of agriculture to greenhouse gas levels
There has been great intensification in agriculture in the last century. In most parts of the world, fields are huge and crop yields are high and agriculture relies on the use of machinery and pesticides.
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1. Contributions of different human activities to atmospheric greenhouse gas levels. By Marta Moneo. Data obtained from FAO. Please click to enlarge (26 K).
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Although carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) occur naturally in the atmosphere, their recent increases in concentration are largely a result of human activities. The contribution of CO2 released to the atmosphere by agriculture is between 15 and 20% of the total amount released by humans.
Carbon dioxide, nitrous oxide and methane are all greenhouse gases and all contribute to the enhanced greenhouse effect.
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Use of fossil fuels
Since modern intensive agriculture relies on machinery for most operations (ploughing, irrigation, harvesting, crop transport and pesticide spraying), it uses much more energy than traditional farming methods. 17% of the total energy consumed in the USA is used to produce food (6% agricultural production, 6% processing and packaging and 5% for distribution and preparation). This means that around 1500 litres of fuel are used per person each year just to supply their food.
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2. Use of fossil fuels for agricultural machinery. Picture from USDA. NRCS.
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Carbon dioxide in soils and forests
It's not only fossil fuel use that emits greenhouse gases into the atmosphere. Intensive ploughing of agricultural land and deforestation are also very effective ways to increase CO2 emissions.
Forests take up huge amounts of CO2. Vegetation needs to absorb CO2 in order to grow (through photosynthesis) and trees need much larger amounts than simple crops as they have to develop much bigger structures. Forests also keep moisture levels quite high and his helps water storage. So loss of forested areas means that less CO2 is taken out of the air and stored in plant tissues and it also means less water is stored. In addition, decomposition of the trees and other plants once they have been cut down releases large amounts of CO2 back into the air. Almost 15% of all greenhouse gases emissions are thought to come from the destruction of forests.
Soil contains a large amount of organic matter and is, therefore, also an important carbon store. When the soil is intensively ploughed, more oxygen can get into it. This extra oxygen increases the rate at which the organic matter is broken down into CO2.
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Changes in the size of tropical forests between 1990 and 1995
Annual change (millions of ha) [%]
Tropical Africa -18.5 -0.7
Tropical Asia -15.3 -1.1
Tropical America -28.5 -1.3
Reduction in forest area over the world between 1990 and 1995 (Font: FAO 1997). To get a better idea quite how large these annual losses of forest area are; 30 million hectares is approximately the size of Italy, Poland or Norway!
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Nitrous oxide and methane from farming wastes
Nitrous oxide (N2O) is produced biologically in soils, water and animal wastes. Over the last two centuries, human activities have increased N2O concentrations by 13%. The main sources of N2O are fossil fuel combustion, agricultural soil management, industry and the use of nitrogen based fertilizers.
The main sources of methane (CH4) are ruminant livestock (cows and sheep) and rice cultivation. Fluxes of greenhouse gases from animals are very different from place to place depending on the species, their diet, whether they are kept outside or in barns and on other environmental conditions. We are not really sure at the moment which factors are the most important in governing methane emissions from livestock.
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3. Rice cultivation in Philippines
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Rice and methane production
Why is rice production such a big source of methane? Methane (CH4) is produced by microscopical organisms which grow in anaerobic conditions. Anaerobic means that there is no oxygen present. Anaerobic conditions occur in waterlogged soils. Rice is grown in flooded fields so rice paddies are an ideal environment for these methane producing organisms to grow. About a third of the total amount of methane in the atmosphere comes from agricultural sources.
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What can agriculture can do about greenhouse gas emissions?
As we have just seen, agriculture is a large producer of greenhouse gases. With a little effort this could be changed. Agriculture can help to reduced greenhouse gas emissions by adopting practices that allow more CO2 to be stored in soils, crops and trees by ploughing less and slowing the rate of deforestation.
Further reductions in greenhouse gas emissions could be achieved by using less fossil fuels. More effective use of chemicals would lead to "cleaner" agriculture.
Dry cultivation of rice would reduce methane emissions but rice yield would decrease dramatically so this isn't a very viable solution given our growing world population. However, development of rice varieties which need less water and better water management may reduce methane emissions in the future.
Changing agricultural practices could reduce overall agricultural related emissions of greenhouse gases by up to 35% by the year 2020.
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