Climate Change & Agriculture
Climate is the most significant factor in determining plant growth and productivity. Depending upon the response of the climate system and the way the global economy evolves over the next 100 years global mean temperatures are projected to increase by between 1.4 and 5.8°C by 2001. This will have some effect on the climate throughout the world, altering factors limiting to plant growth, such as temperature, precipitation and nutrient supply. Changes in climate could also bring an increase in climatic extremes, warming in the high latitudes, poleward advance of the monsoon rainfall, and reduced soil water availability.
Climatic Impacts on Agriculture
Crop growth is often limited by temperature. The current scientific consensus is that temperatures are expected to increase more in the higher latitudes where vegetational shifts will be greater. In Britain an increase in temperature of between 0.9 and 2.4°C by 2050 is expected. A rise of 1.5°C would be equivalent to a decrease in altitude of approximately 200m or a shift southwards in latitude of 200-300km. Although the current geographical boundary (with regards to temperature) for ripening maize excludes most of the UK, a temperature increase of only 0.5°C would allow maize cultivation across southern England. In the regions where ambient temperature does not limit growth, such as the corn belt of North America, the European lowlands and the Ukraine, an increase in temperature would potentially decrease crop yields because of the time reduction for crop development. Small increases in temperature would extend the range of temperature limited pests. For example, the European Corn Borer (a major pest of grain maize) may shift between 165 and 500km northwards with a rise of 1oC. With a warmer climate, favourable conditions for migrant pests would be extended over a greater geographical range
Moisture and water availability will be affected by a temperature increase, regardless of any change in precipitation. Higher temperatures increase the evaporation rate, thus reducing the level of moisture available for plant growth, although other climatic variables are involved. A warming of 1°C, with no change in precipitation, may decrease yields of wheat and maize in the core cropping regions 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.
A Change in Extreme Weather Events
A shift in global climate could bring about a change in the frequency and intensity of drought in already drought-prone regions. Years of successive extremes quickly lead to hunger and eventually widespread famine. An example of this is the devastating drought that affected the Sahel in northern Africa during the 1970s and 1980s.
Effects of Sea Level Rise
Global sea levels could rise by between 9-88cm by 2100 (above 1990 levels). The greatest threat to low lying agricultural regions from sea level rise is that of inundation. Southeast Asia would have the greatest threat of inundation because of the deltaic nature of the land. The indirect effect of sea level rise, salination of surface and groundwater 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.
Effect of Increased Carbon Dioxide Concentration on Plant Growth
Plants grow as a result of photosynthesis - the mechanism whereby the plant converts carbon dioxide (CO2) from the atmosphere into organic compounds. Climate change is associated with increasing atmospheric concentrations of carbon dioxide; with higher levels stimulating the rate of photosynthesis, the growth rate and productivity of a plant 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 e.g. wheat, rice and soybean. Under a doubling of carbon dioxide (with no other limiting factors) some studies have shown that growth rate in these crops increases 10 to 50%. Crops of more importance as a source of food in the tropical regions are more efficient at photosynthesis under present day carbon dioxide concentrations but show little positive response to an increased concentration of the gas. Examples of these food staples include sorghum, maize, sugar cane and millet, and together account for about one fifth of the world's food production.
Effects on Production
In the major grain-producing regions production could decrease as a result of warming and drying. U.S. grain production could be reduced by 10 to 20%. This would leave enough for national consumption but there would be no surplus for export. Production in the Canadian prairies and southern Russia would also decrease. In Europe, if rainfall significantly increases, there is the possibility of an increase in production in the UK and the Benelux Countries. However, production would be reduced in southern Europe if rainfall decreases as the climate models suggest. In Australia production could increase, but only if there is increased rainfall to counteract the increase in temperature. Production could increase in some regions where temperature is currently limiting to grain growing. Such areas include northern Scandinavia, northern Europe and Russia. In the southern hemisphere, southern New Zealand and southern parts of Chile and Argentina may also experience increases in crop production. The potential benefits of increased productivity in these regions would however be significantly limited by the poor soils and unsuitable terrain.
Adaptations in Agriculture
In order to maintain agricultural output to meet the demand for food, farmers will have to adjust as and when necessary to the possible changes imposed by changing climate. The most apparent future changes would be those in land use and management of agricultural systems. Changes in land use could include the area farmed, the type of crop cultivated and planting locations. The extent of such changes, if any, will depend on the region and type of farming. The crops grown must suit the climatic conditions. Any future change in the location of planting will depend on a complex variety of factors, including temperature and precipitation changes. However, on a broad-scale cultivation is likely to shift poleward with a more noticeable shift in mid- and high latitudes.
Higher temperatures would increase the demand for irrigation of agricultural land. In many regions of the world the demand for water currently exceeds supply, thus demand for irrigation purposes will only increase the current pressure on water resources. The arid and semi-arid regions are likely to see an increased demand for water and would require improved water management. The future use of fertilisers could increase where increased precipitation leads to leaching of nutrients from the soil (this would increase the potential for groundwater pollution). Where warming increases the productive potential without the need of additional fertilisers, usage could fall. However, the use of fertilisers would depend on the cost and availability. The cost of controlling pests and diseases would also be altered but it is very difficult to specify such changes. All of these practices would lead to changes in farm husbandry, such as the time of sowing and harvesting, the application of fertilisers, pesticides and herbicides. This would then help to reduce the negative effects and exploit the beneficial effects of climate change in agricultural systems.
The ability to adjust to the effects of climate change will vary greatly between countries and regions. Economic and technological constraints would limit the rate of adaptability; thus farmers on marginal lands and those in developing countries (with poorer economies) might not be capable of implementing required changes. The effects of climate change look set to further widen the gulf between developed and developing nations.
The effects of climate change on agriculture are still uncertain. This fact sheet provides only a broad indication of the potential effects and the changes which may arise from them. The continuation of current, and new, initiatives to research the potential effects of climate change at farm, regional, national and international level will help provide a more detailed picture of how world agriculture could change. Only then may we see the implementation of policies and other adaptations in agricultural systems that would minimise the negative effects of climate change and exploit the beneficial effects.