CIESIN Reproduced, with permission, from: United Nations Environment Programme. 1990. The impacts of climate change on agriculture. United Nations Environment Programme Information Unit for Climate Change Fact Sheet 101. Nairobi, Kenya: United Nations Environment Programme (UNEP) Information Unit for Climate Change (IUCC).

Fact sheet 101

The impacts of climate change on agriculture

Climate change would strongly affect agriculture, but scientists still don't know exactly how. Most agricultural impacts studies are based on the results of general circulation models (GCMs -- see fact sheet 14).These climate models indicate that rising levels of greenhouse gases are likely to increase the global average surface temperature by 1.5-4.5 C over the next 100 years, raise sea-levels (thus inundating farmland and making coastal groundwater saltier), amplify extreme weather events s uch asstorms and hot spells, shift climate zones poleward, and reduce soil moisture. Impacts studies consider how these general trends would affect agricultural production in specific regions. To date, most studies have assumed that agricultural technology and management will not improve andadapt. New studies are becoming increasingly sophisticated, however, and adjustments experiments now incorporate assumptions about the human response to climate change.

Increased concentrations of CO2 may boost crop productivity. Inprinciple, higher levels of CO2 should stimulate photosynthesis in certain plants; a doubling of CO2 may increase photosynthesis rates by as much as30-100%. Laboratory experiments confirm that when plants absorb more carbon they grow bigger and more quickly. This is particularly true for C3 plants (so called because the product of their first biochemical reactions during photosynthesis has three carbon atoms). Increased carbon dioxide tends to suppress photo-respiration in these plants, making them more water-efficient. C3 plants include such major mid-latitude food staples as wheat, rice, and soybean. The response of C4 plants, on the other hand, would not be as dramatic (although at current CO2 levels these plants photosynthesize more efficiently than do C3 plants). C4 plants include such low-latitude crops as maize, sorghum, sugar-cane, and millet, plus many pasture and forage grasses.

Climate and agricultural zones would tend to shift towards the poles. Because average temperatures are expected to increase more near the poles than near the equator, the shift in climate zones will be more pronounced in the higher latitudes. In the mid-latitude regions (45 to 60 latitude), the shift is expected to be about 200-300 kilometres for every degree Celsius of warming. Since todays latitudinal climate belts are each optimal for particular crops, such shifts could have a powerful impact on agricultural and livestock production. Crops for which temperature is the limiting factor may experience longer growing seasons. For example, in the Canadian prairies the growing season might lengthen by 10 days for every 1C increase in average annual temperature.

While some species would benefit from higher temperatures, others might not. A warmer climate might, for example, interfere with germination or with other key stages in their life cycle. It might also reduce soil moisture; evaporation rates increase in mid-latitudes by about 5% for each 1C rise in average annual temperature. Another potentially limiting factor is that soil types in a new climate zone may be unable to support intensive agriculture as practiced today in the main producer countries. For example, even if sub-Arctic Canada experiences climatic conditions similar to those now existing in the country's southern grain-producing regions, its poor soil may be unable to sustain crop growth.

Mid-latitude yields may be reduced by 10-30% due to increased summer dryness. Climate models suggest that todays leading grain-producing areas -- in particular the Great Plains of the US -- may experience more frequent droughts and heat waves by the year 2030. Extended periods of extreme weather conditions would destroy certain crops, negating completely the potential for greater productivity through CO2 fertilization. During the extended drought of 1988 in the US corn belt region, for example, corn yields dropped by 40% and, for the first time since 1930, US grain consumption exceeded production. The poleward edges of the mid-latitude agricultural zones -- northern Canada, Scandinavia, Russia, and Japan in the northern hemisphere, and southern Chile and Argentina in the southern one -- may benefit from the combined effects of higher temperatures and CO2 fertilization. But the problems of rugged terrain and poor soil suggest that this would not be enough to compensate for reduced yields in the more productive areas.

The impact on yields of low-latitude crops is more difficult to predict. While scientists are relatively confident that climate change will lead to higher temperatures, they are less sure of how it will affect precipitation -- the key constraint on low-latitude and tropical agriculture. Climate models do suggest, however, that the inter tropical convergence zones may migrate poleward, bringing the monsoon rains with them. The greatest risks for low-latitude countries, then, are that reduced rain fall and soil moisture will damage crops in semi-arid regions, and that additional heat stress will damage crops and especially livestock in humid tropical regions.

The impact on net global agricultural productivity is also difficult to assess. Higher yields in some areas may compensate for decreases in others -- but again they may not, particularly if todays major food exporters suffer serious losses. In addition, it is difficult to forecast to what extent farmers and governments will be able to adopt new techniques and management approaches to compensate for the negative impacts of climate change. It is also hard to predict how relationships between crops and pests will evolve.

********************************************************************* Last revised August 1992 by the Information Unit on Climate Change, UNEP, Palais des Nations, CH-1211 Geneva 10, Switzerland.

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