Climate Change and its Impact on Agriculture
Climate change is probably the most important cliché in the world. To understand what climate change is it is important to define climate as it is often an ambiguous term. Climate may be defined as a composite or general weather conditions that prevails over a long period of time in a particular geographical area. In simpler terms it is the average of everyday weather over a long period of time. Climate change refers to a lasting and significant change in the weather pattern and its distribution across the globe. Climate change is caused due to various factors such as accumulation of greenhouse gases like Carbon dioxide, water vapor, Methane, Nitrous oxide and Chlorofluorocarbons caused due to increase in the output of solar irradiance, volcanic eruptions and many human activities.
To substantially understand the effect climate change will have on agriculture we can begin with listing the possible changes in the environment that are likely to occur. Change in climate can be associated with change in temperature, precipitation, Carbon dioxide concentration, wind pattern and other climatic variables.
Impact of increase in temperature on crops:
Temperature is critical variant in any biochemical process as it determines the rate of the reaction. The physiology of plants is ultimately a biochemical reaction and hence it is bound to effect the growth of the plants. An increase in temperature will lead to increase in the rate of respiration of the plant. An increase in the temperature is seen to decrease the grain-filling period which can lead to decreased yield. The effects of high temperature can be more significant around anthesis as the various stages of reproduction that the leads to the formation of seeds such as pollen grain synthesis, transfer of pollen grains to the stigma, generations of pollen tube, fertilization and development of zygote are all temperature sensitive. This could explain the decrease in yield. Also a higher mean temperature is shown to affect the root biomass of the crops which in turn affects the quality of crop as it reduces the ability to absorb nutrients from the soil. Though these effects may not be universal and many crops may be able to adapt to the changes in temperature, it cannot be neglected that some crops might be adversely affected even by the slight increase in the mean temperature. Studies conducted by enlarge show that the effect of increase in mean temperature leads to lower yield of crops.
In experiments performed for Wheat crop under controlled conditions from 25 to 35°C, mean grain weight declined 16% for each 5°C increase in temperature (Asana and Williams, 1965). In pot experiments, grain yield decreased by 17% for each 5°C rise (Wattal, 1965). For every 1°C rise in temperature, there is a depression in grain yield by 8 to 10%, mediated through 5 to 6% fewer grains and 3 to 4% smaller grain weight.
Wardlaw in 1974 further broke down the study into the three main components of the plant system in order to explain the reasons for the reduced yield. The three components are: (a) source - flag leaf blade; (b) sink - ear; and (c) transport pathway - peduncle. He observed that photosynthesis had a broad temperature optimum from 20 to 30°C with photosynthesis declining rapidly at temperatures above 30°C. The rate of 14C assimilate movement out of the flag leaf, phloem loading, was optimum around 30°C; the rate of 14C assimilate movement through the stem was independent of temperature from 1 to 50°C. Thus, in wheat, temperature effects on translocation result indirectly from direct temperature effects on source and sink activities.
In a subsequent experiment with source-sink relationships altered through grain excision, defoliation and shading treatments, heat stress still reduced grain weight (Wardlaw et al., 1980). This result supports the earlier findings that temperature effects on grain weight are direct effects rather than assimilate availability (Bremner and Rawson, 1978; Ford et al., 1978; Spiertz, 1974). Furthermore, respiration effects do not appear to be the direct cause of decreased grain size in heat-stressed wheat (Wardlaw, 1974).
Reduction of grain weight by heat stress may be explained mostly by effects of temperature on rate and duration of grain growth. As temperature increased from 15/10°C to 21/16°C, duration of grain filling was reduced from 60 to 36 days and grain growth rate increased from 0.73 to 1.49 mg/grain/day with a result of minimal influence on grain weight at maturity. Further increase in temperature from 21/16°C to 30/25°C resulted in decline in grain filling during 36 to 22 days with a minimal increase in grain growth rate from 1.49 to 1.51 mg/grain/day. Thus, mature grain weight was significantly reduced at the highest temperature.
Apart from physiological effect on plant growth the increase in temperature also influences the nature of soil. An increase in temperature would result in decrease moisture content in the soil which in turn would decrease the mobility of nutrients and hence the available nutrient for the crop.
Impact of increase in Carbon Dioxide concentration:
The increase in CO2 concentration in the atmosphere will favor the increase in the rate of photosynthesis and result in reducing water utilization by plants. Since the CO2 assimilated by plants is used to build carbohydrates, elevated levels will lead to production of more grains. This effect is often termed as ‘CO2 fertilization effect’. Bunce (1995) observed that the biomass and leaf area ratio of Glycine maxincreased and that photosynthetic rates decreased in plants exposed to high night-time [CO2] relative to plants grown at normal ambient [CO2]. Similarly, Griffin et al. (1999) found that Glycine max exposed to high night-time [CO2] had lower leaf respiration rates and greater biomass than plants grown at ambient or elevated [CO2]. Reuveni et al. (1997) speculated that increases in the biomass of Lemna gibba grown at high night-time [CO2] relative to control plants, was due to a reduction in alternative pathway respiration (although the alternative pathway activity was not measured). Reduction in alternative pathway activity might more fully couple respiration rates with growth and maintenance, enhancing growth.
On the other hand scientists at the Ohio University have found that the beneficial effects of increased levels of [CO2] will be negated by the crops that are traditionally not bred for vertical growth. This leads to an increase in grain mass to plant mass ratio which causes the crops to collapse under its own weight. Also reduced utilization of water could result in lesser transpiration rate due to reduced stomatal opening which could further result in lesser water movement within plant leading to an increase in temperature within the plant. This increase in temperature will affect the rate of biochemical reactions within the plant. While some process might benefit from the increase in temperature there could be other reactions that have negative effects.
Transpiration plays a role in precipitation; hence reduced transpiration can lead to change in precipitation levels to some degree. This reduced movement of water could also affect the mobilization of nutrients from soil to plant and with the tissues of plants itself. Though the effects of elevated levels of carbon dioxide level are largely found to affect individual biochemical reactions in plants, their overall effect on plant growth has found to be inconclusively in many works. Largely it seems that the overall effect of increase in the concentration of CO2 may be beneficial to the plants, but its other side effects, mainly increase in temperature would nullify the positive effects if any.
Changes in precipitation pattern:
Precipitation patterns are directly affected by global warming. A small increase in temperature can bring about a big change in the water vapor content, cloud formation and wind movement which together can alter the weather pattern as we know it today. With increase in temperature the moisture from the Earth’s surface will evaporate faster leaving the land drier and exposed to long periods of drought. On the other hand the moisture content in the atmosphere would increase. This in turn will result in thunderstorms, tropical cyclones, extratropical rain or snow storms which can cause massive floods. Hence the already dry lands become drier and the wet areas become wetter. With increase in temperature the percentage of precipitation as snow will decrease and more rainfall would mean massive flooding that could cause soil erosion. On the whole climate change is going to bring in much uncertainty in predicting weather patterns which would make it difficult to predict the right timing for sowing seasonal crops.
Climate variable would include heat waves, storms, changing wind pattern, flashfloods etc. These variables as mentioned previously lead to uncertainty in predicting the right sowing seasons. The faltering of monsoon due to changes in wind pattern and short but high levels of precipitation can also cause massive crop failure; especially fruits and vegetables.
Climate change has always been a part of earth’s history but its significance with respect to human life is certainly in the near future if not now. We have thrived as a species by mastering the art of agriculture but now changing patterns in the weather has challenged our ability feed ourselves. The challenges posed by climate change are plenty in terms of the type of crop grown, its region, variety etc. Human beings will have to adapt to new system of agriculture assisted sophisticated technology and superior varieties of crops. This will also impact the economics of agriculture as food produce are likely to be dearer. Agriculture may change the way we know it in the coming future.
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