Climate Change and Agriculture : Reinventing the crop cycle

Suman Sahai

Increasing temperatures, shifting rain patterns and frequency of extreme weather spells threaten global food systems. The new buzz phrase is `climate proofing of crops'.

Global climate change, if it occurs, will definitely affect agriculture. However, simulation of crop response models has been limited to a few major crops for a region, usually important grain crops, with yield effects extended to other crops. Also, model responses do not take into consideration carbon dioxide or CO, fertilisation and improved water-use efficiency, the effect of cloud cover (on both climate and photosynthesis), or the uncertain nature of climate change. Farmers will have to change crop management practices, grow tougher plant varieties and be prepared for constant change in the way they operate. Temperature and rain are the key factors that affect crops.

Higher Temperatures

Crop-producing areas may expand northwards to ice-bound regions in Greenland, Canada and Russia. At higher latitudes, global warming will extend the length of the potential growing season, because the snow bound period will be reduced. This could allow earlier planting of crops and the possibility of taking two crops instead of just one crop as now. On the other hand crops adapted to the growing-season temperature and day lengths of the plains and lower latitudes may not respond well to the changed conditions. Satellite data shows that the dry trop-ics, where rainfed agriculture provides 60 per cent of the world's food, will be the most vulnerable to climate change.

In warmer, plains regions, increased temperatures may speed up the rate at which plants release carbon dioxide in the process of respiration, resulting in less than optimal conditions for net growth. When temperatures exceed the optimal for biological processes, plants will respond with a drop in yield. If minimum night-time temperatures rise significantly--as is expected from green-house warming projections, higher night-time respiration -may also reduce potential yields. Another important effect could be accelerated physiological development, resulting in premature maturation and, therefore, reduced yield. Increased temperatures may also lead to more decomposition of soil organic matter.

Increased Co2

Photosynthesis is the foundation of plant growth. It is the process by which the energy of sunlight converts water from the soil and carbon dioxide from the air into sugar, starches, and cellulose, making the plant grow. Carbon dioxide enters a plant through its leaves. Greater concentration of carbon dioxide in the air will result in higher carbon dioxide uptake and greater conversion to carbohydrates.

Crop species vary in their response to carbon dioxide. Wheat, rice, and soybeans arc called C3 plants and respond readily to increased carbon dioxide levels. Corn, sorghum, sugarcane, and millet arc C4 plants that follow a different pathway and arc more efficient photosynthetically than C3 crops. So far these distinctions have been demonstrated only under experimental conditions such as growth chambers and greenhouses. Experimental studies of the long-term effects of carbon dioxide in realistic field settings have not yet been done on a comprehensive scale.

Already; rising carbon dioxide levels arc changing the metabolism of grasses and shrubs on rangeland, decreasing the protein levels in plants eaten by cattle. Higher temperatures are already, resulting in shorter picking sea-sons.

Higher levels of atmospheric carbon dioxide also induce plants to close the small leaf openings known as stomata through which carbon dioxide is absorbed and water vapour is released. Thus, under higher carbon dioxide conditions, crops may use less water (by closing their stomata) even while they, produce more carbohydrates. This dual effect is likely to improve water-use efficiency, which is the ratio between crop biomass and the amount of water consumed. At the same time, climatic effects, such as higher temperatures and changes in rainfall and soil moisture, could either enhance or negate potentially beneficial effects of enhanced atmospheric carbon dioxide on crop physiology.

Water Stress

Climate change and global warming will modify rainfall, evaporation, surface runoff, and soil moisture storage. If temperatures and rainfall patterns turn adverse, soil mois­ture stress will result from increased evaporation from the soil and accelerated transpiration in the plants. Moisture stress during flowering, pollination, and grain-filling is harmful to almost all crops, but the most susceptible crops are corn, soybeans, and wheat, which are likely to suffer the worst damage when there is shortage of soil moisture. The predictable response to this eventuality is to develop crop varieties with greater drought tolerance, those that can withstand moisture stress.

More crop pests

Insect pests proliferate more readily in warmer climates since conditions fbr growth and multiplication are more favourable compared to cooler conditions. The incidence of other crop diseases like fungal and bacterial infections is also likely to increase if the climate gets warmer.

In temperate areas, longer growing seasons will ena­ble insects to complete a greater number of reproductive cycles during the spring, summer, and autumn. Warmer winter temperatures may also allow larvae to survive the winter in areas where they are now limited by cold, thus causing greater infestation during the following crop sea­son.

Storms and hurricanes and changed wind directions are likely to change the spread of both wind-borne pests and of the bacteria and fungi that are the agents of crop disease. Crop-pest interactions may shift as the timing of development stages in both hosts and pests is altered. Livestock diseases may be similarly affected.

Climate proofing

Climate change is making crop scientists review their research agenda. Until now, their main focus was on improving yields. But with successive Intergovernmental Panel on Climate Change (IPCC) reports warning that increased droughts and floods will shift crop systems, 'climate-proofing' of crops has become crucial. The Consultative Group on International Agricultural Research (CGIAR) institutes are now investigating how to make crops' more resilient to environment stresses. But efforts are hampered because few climate models predict changes for individual regions, making it difficult to predict how climate change will affect growth and yields of specific crops in each region.

More importantly, under changing climate conditions, farmers' past experience will be a less reliable predictor of what is to come. In many areas of the world, the necessary adjustments (such as substituting crops, introducing or intensifying irrigation, and modifying field operations such as tillage or pest control) may be too costly for many farmers to implement. These and other uncertainties must be taken into account explicitly in climate change impact studies.