Climate is the most significant factor in determining plant growth and productivity. Without intervention to reduce emissions of greenhouse gases, global average surface temperature is projected to increase by about 0.2C per decade during the 21st century. This rapid change in climate will have major implications for agriculture around the world.
Crop growth is often limited by temperature. Temperatures during the 21st century are expected to increase more in the higher latitudes where shifts in vegetation will be greater. In Britain an increase in temperature of 1.5C by 2050 is the equivalent of a decrease in altitude of approximately 200m. This is the same as a shift southward in latitude of 200-300 km. Such an increase in temperature would allow widespread maize cultivation across southern England to take place. In other regions however, a rise in temperature may not be so beneficial. Small increases in temperature would extend the range of temperature-limited pests. The European Corn Borer for example, a major pest of grain maize, may shift between 165 and 500 km northwards with a rise of 1C.
Moisture and water availability will be affected by a temperature increase, regardless of any change in rainfall. Higher temperatures increase the evaporation rate, thus reducing the level of moisture available for plant growth, although other climatic elements are involved. A warming of 1C, with no change in precipitation, may decrease yields of wheat and maize in the core cropping regions such as the US by about 5%. A very large decrease in moisture availability in the dryer regions of the world would be of great concern to the subsistence farmers that farm these lands. Reduced moisture availability would only exacerbate the existing problems of infertile soils, soil erosion and poor crop yields. In the extreme case, a reduction in moisture could lead to desertification.
Sea levels have been projected to rise by anywhere up to a metre by 2100, although considerable uncertainty is attached to this. The greatest threat to low-lying agricultural regions from sea level rise is that of inundation and flooding. Southeast Asia would have the greatest threat of inundation because of the deltaic nature of the land. Furthermore, the pollution of surface and groundwater with salty seawater is another potential problem facing farmers situated in low-lying regions. The costs of agricultural production would increase, resulting in higher food prices for the consumer.
Although climate changes may have some detrimental impacts on agricultural production around the world, the increase in atmospheric carbon dioxide concentrations could be beneficial. Plants grow as a result of photosynthesis - the mechanism whereby the plant converts carbon dioxide from the atmosphere into food. With higher levels of carbon dioxide stimulating the rate of photosynthesis, the growth rate and productivity of plants could be expected to increase. This would be beneficial for global food stocks. Most crops grown in cool, temperate regions respond positively to an increased concentration of carbon dioxide, including some of the current major food staples such as wheat, rice and soybean. Some studies have shown that growth rate in these crops may increase up to 50% if carbon dioxide in the atmosphere is doubled. Crops grown in the tropical regions of the world, including sorghum, maize, sugar cane and millet, which together account for about one fifth of the world's food production, do not respond as well to increases in carbon dioxide.
In order to maintain agricultural output to meet the demand for a growing world population, farmers will have to adjust and adapt as and when necessary to the possible changes imposed by changing climate. Higher temperatures would increase the demand for irrigation of agricultural land. Unfortunately, in many arid and semi-arid regions of the world the demand for water already exceeds supply. Increased spread of pests and disease may also place additional demands on the need for fertilisers, pesticides and herbicides which are costly. The ability to adapt to the effects of climate change will vary greatly between countries and regions. Economic and technological constraints will limit the rate of adaptability, with poorer economies lagging behind. Consequently, without intervention the effects of climate change in the 21st century look set to further widen the gulf between developed and developing nations.