Naturally occurring greenhouse gases keep the earth warm enough to be habitable. Increasing their concentrations and adding new ones will gradually make the earth quite a different place.	According to the best estimate by the Intergovernmental Panel on Climate Change, if present trends continue, sea levels may rise by about 40 centimetres by the end of the next century. Small island states, such as the Bahamas, Maldives and Tonga, will be most affected.	Deforestation reduces a vital store of carbon dioxide.

Agriculture depends on the climate more than any other human activity, and so is particularly vulnerable to climatic change. The Intergovernmental Panel on Climate Change (IPCC) estimates that as a result of increasing human-induced emissions of carbon dioxide, methane, nitrous oxide and other "greenhouse gases", average temperatures may climb by about 0.3 degrees centigrade per decade over the next century, while sea levels could rise by at least 2-4 centimetres per decade. This will have an impact, still to be quantified, on agriculture, forestry, fisheries, food security, biodiversity and rural environmental conditions.

Not all of the effects of global warming would be harmful to agriculture. Higher concentrations of carbon dioxide can have a fertilizing effect under optimal growing conditions: 10-20 percent of improved crop productivity over the past century could be the result of the gradual increase in the level of the gas; and crop productivity could increase further, by up to 30 percent, if the concentration of carbon dioxide doubles as foreseen over the next 50 years. It could also offset the damage done to plant growth by other pollutants, and increase the efficiency with which crops use water. Rising temperatures could increase the yield of some plants, while diminishing others. Rainfall could also increase, by about 10 percent, but its distribution and intensity would change; some areas would benefit, others would be harmed, but it is not yet certain which ones.

Overall, global warming is expected to add to the difficulties of increasing food production. The weather and climate would become more unpredictable, making farming and planning more difficult. Present agricultural zones would shift, sometimes by hundreds of kilometres in latitude and by hundreds of metres in altitude on hills and mountains. Some plant and animal species, particularly those such as trees with long life cycles, might not be able to adjust to this and poorer farmers, in particular, would find it hard to adapt. Fishing areas may also shift, leading to disruption, although the overall productivity of the oceans might stay about the same. Diseases and pests would possibly increase. Biological diversity could be at risk in natural environments such as tropical forests and mangroves. And the rise in sea levels would increase flooding, submerging or waterlogging coastal plains which are among the most productive, and highly populated, lands.

Global warming is likely to accentuate the existing imbalance in world food production between the developed and developing countries. Cooler, temperate regions - home to the industrialized countries - are expected to receive most of the benefits from global warming, while tropical and subtropical ones are likely to suffer most. Farmers in wealthier countries are also most likely to be able to adapt to climate change. Sub-Saharan Africa, where food production already lags behind the rest of the world, is expected to be hardest hit.

Carbon dioxide (CO₂) is the most important greenhouse gas after water vapour. Much atmospheric CO₂ originates from use of fossil fuels for the production of energy in industrialized countries but about 30 percent has been estimated to result from deforestation and other land use practices such as rangeland burning. Some 35 percent of worldwide methane emissions are now estimated to arise from fermentation in rice paddies and in the digestive systems of cattle and other ruminants. And agriculture, including the application of nitrogenous fertilizers, may account for as much as 90 percent of nitrous oxide emissions.

The growth of carbon dioxide in the atmosphere can be slowed by reducing the rate of deforestation. Using biofuels derived from plants, instead of fossil fuels, will also reduce these emissions. Changes in land management techniques such as reforestation would stimulate the annual terrestrial uptake of atmospheric CO₂ and its storage in the organic matter of arable or grassland soils. FAO is helping governments and people to reduce emissions of methane and nitrous oxide, which have no positive effects on plant growth, by improving the use of nitrogenous fertilizer, modifying irrigated rice cultivation and feeding cattle a well-balanced diet-including, for example, straw treated with urea - that produces less methane than diets of untreated roughage.

The Organization also monitors the condition of tropical forests, helps to combat deforestation and promotes the planting of trees. It is developing plans for preparing for disasters and early warning systems for droughts, outbreaks of pests and diseases, and other "extreme events" affecting food and agriculture. It is promoting the development of more resilience in agriculture: for example, by encouraging diversification and developing improved crop varieties and animal breeds. And it is stimulating further research to assess the impact of global warming on food production.

How the greenhouse effect works

Worldwide forecasts

Potential changes in surface temperature according to the Intergovernmental Panel on Climate Change (IPCC)

Source: IPCC

Concentration of greenhouse gases in the atmosphere and their contribution to global warming

Concentration of greenhouse gases in the atmosphere

Contribution of greenhouse gases to global warming, 1980-90

Projected emissions of greenhouse gases