Food & Climate 

Basics

Contribution of agriculture to greenhouse gases.

There has been great intensification in agriculture in the last century, nowadays fields are so vast and production is so high, that it is not possible to think about agriculture without the help from machinery and pesticides in most parts of the world.

Although carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) occur naturally in the atmosphere their recent increase is largely a result of human activities. The contribution of CO₂ released to the atmosphere by agriculture represents 15-20% of the total amount released by human activities.

All these are the so called Greenhouse gases (GHG), as they are major contributors to increase the Greenhouse effect on Earth.

Use of fossil fuels

Modern intensive agriculture needs much more energy input than did traditional farming methods, as it relies in the use of machinery for most operations (tillage, harvesting, transport of grains or use of pesticides). 17% of the total energy used in the USA economy is consumed in food systems (6% agricultural production, 6% processing and packaging and 5% for distribution and preparation) this means an annual use of around 1500 litres/person of fuel only for food.

CO₂ in soils and forests

But not only the use of fossil fuels emits greenhouse gases to the atmosphere. The intensive tillage of agricultural land, and deforestation are very effective ways to increase those emissions.

Forests are great sinks for CO₂. As you know, vegetation needs to absorb CO₂ in order to grow (for the photosynthesis process) and trees need a much bigger amount than simple crops, as they have to develop much bigger structures, also forests keep moisture quite high and this helps in the storage function. Then, the cut down of forest areas decreases the possibility of CO₂ storaging, not only in the trees but also in all the associated vegetation and the decrease in humidity, also the decomposition of all that organic matter liberates great amounts of CO₂. It is said that almost 15% of all the greenhouse gases emissions are produced by forest destruction.

 The soil is also an important sink, as it keeps a great amount of organic matter that contains Carbon, intensive and aggressive tillage of land under cultivation releases CO₂ very fast. When the soil is twisted upside down more Oxygen can get into it and more CO₂ can get out f it, as organic matter decomposes more easily and liberates that CO₂ (plant litter and living animals)

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

World reduction of forests (Font:FAO 1997) Changes in the size of Tropical Forests between 1990 and 1995. Annual change (millions of ha and %)

You might get a better idea of the size changes knowing that 30 millions of ha is aproximately the size of countries like Italy, Poland or Norway.

N₂O and CH₄ from farming wastes

Nitrous Oxide is produced from a wide variety of biological sources in soil, water and animal wastes. During the last two centuries, human activities have increased N₂O concentration by 13%. The main activities producing N₂O are fossil fuel combustion, agricultural soil management, industrial sources and the use of N fertilizer, which produces also secondary effects.
The main sources of CH₄ are ruminant livestock and rice cultivation. But fluxes of GHG from animal storage and handling are very different from place to place depending on the species used, diet, type of storage and other environmental conditions. The emissions of all these gases have not been measured in a sufficiently long period of time and scientists cannot be sure then of their behaviour in different climatic conditions, so there is still a lot of work to do in this field.

Rice and CH₄ production

Why is rice production such a big source of Methane? This molecule (CH₄) is produced by microscopical organisms that live in water (in what is usually called “anaerobic conditions”, as there is a very low concentration of oxygen) As you probably know, rice is one of the major crop productions in the world and it is cultivated in flooded fields, this is the reason why rice fields are large producers of methane Agricultural sources of CH₄ account for as much as one third of the total the atmosphere is able to absorb.

What agriculture can do about GHG emissions

As we have just seen, agriculture is a large producer of GHG, but this can be changed with a bit of effort. Agriculture can contribute to mitigate GHG emissions by adopting practices that promote capturing of CO₂ in soils, crops biomass or trees, which could be the use of less aggressive tillage and the stop of deforestation for more arable land.
Further reductions can be achieved in the use of fossil fuels for farming although this is not a very signifiactive field. The use of less aggressive techniques (lessen the use of chemicals to an adequate amount or improve their effectiveness) in general would allow a more “clean” agriculture

Dry cultivation of rice would be a solution for methane emission, but yield would decrease dramatically, so this does not seem to be a vialble solution. On the other hand there are other posibltilies like the change in rice variety, water management or the use of soil aditives, which don't affect or even increase yield. Overall, agricultural-related emissions could be reduced by up to 35% by the year 2020.

Related pages:

More explanations about greenhouse effect:
Lower atmosphere - Basics - Greenhouse, light and biosphere - Greenhouse effect and light
Lower atmosphere - Basics - Greenhouse, light and biosphere - Greenhouse gases

Author:  Marta Moneo and Ana Iglesias- Universidad Politécnica de Madrid - España
1. Scientific reviewer: Alex de Sherbinin - CIESIN, Columbia University - USA
2. Scientific reviewer: Lily Parshall - Goddard Institute for space studies, Columbia University - USA
Educational reviewer: Emilio Sternfeld - Colegio Virgen de Mirasierra - España
Last update: 12/05/2004