Agriculture sector alone represents 23 per cent of India's Gross National Product (GNP), plays a crucial role in the country's development and shall continue to occupy an important place in the national economy. It sustains the livelihood of nearly 70% of the population. It seems obvious that any significant change in climate on a global scale will impact local agriculture, and therefore affect the world's food supply. Considerable studies have been carried out to investigate how farming might be affected in the different regions. Several uncertainties limit the accuracy of current projections. One relates to the degree of temperature increase and its geographic distribution. Another pertains to the concomitant changes likely to occur in the precipitation patterns that determine the water supply to the crops, and the evaporative demand imposed on the crops in carbon dioxide enriched atmosphere. The problems of predicting the future course of agriculture in the changing world are compounded by the fundamental complexity of natural agricultural systems, and socio-economic systems governing the world food supply and demand. Many cli-matologists predict a significant global warning in the coming decades due to rising atmospheric carbon dioxide and other green house gases. As a consequence, major changes in the hydrological regimes have been also forecast to occur. Changes in the temperature, solar radiation, and precipitation will have an effect on crop productivity and livestock agriculture. Climate change will also have an economic impact on agriculture, including changes in farm profitability, prices, supply, demand, trade and regional comparative advantages. The magnitude and geographical distribution of such climate induced changes may affect our ability to expand the food production area as required to feed the burgeoning population of more than 10,000 million people projected for the middle of the next century.
Agriculture is sensitive to short-term changes in weather and to seasonal, annual and longer-term variations in climate. For the long-term changes, agriculture is able to tolerate moderate variations in the climatic mean. Changes beyond these bands of tolerance may require shifts in cultivars and crops, new technologies and infrastructure or ultimately conversion to different land uses. Crop yield is the culmination of a diversified range of factors. The variations in the meteorological parameters are more of transitory in nature and have paramount influence on the agricultural systems, although other parameters, like soil characteristic, seed genetics, pest and disease and agronomic practices also do impact crop yields. Among these factors, pest and diseases cause a significant loss to world food production under different climatic conditions. Development and distribution of pest and diseases are governed by temperature patterns, rainfall or humidity and seasonal length to a great extent. Especially, winter temperatures are important for the survival of pest and studies have shown that increase in temperature accelerates the development of pests in general. Pest-crop interaction will be also directly affected by the rising CO2 levels through the alteration of host plant attributes, such as C/N ratios and secondary plant nutrient chemistry. In terms of crop production, these fluctuations must be taken into the account while planning agricultural operations. The climate elements which affect the plant growth and development, hence the agriculture as a whole, are carbon dioxide concentration, temperature, radiation, precipitation and humidity.
Analysis of the food grains production/productivity data for the last few decades reveals a tremendous increase in yield, but it appears that negative impact of vagaries of monsoon has been large throughout the period. In this context, a number of questions need to be addressed as to determine the nature of variability of important weather events, particularly the rainfall received in a season/year as well its distribution within the season. These observations need to be coupled to management practices, which are tailored to the climate variability of the region, such as optimal time of sowing, level of pesticides and fertilizer application.
The mean temperature in India is projected to increase by 0.1-0.3°C in kharif and 0.3-0.7°C during rabi by 2010 and by 0.4-2.0°C during kharif and to 1.1-4.5°C in rabi by 2070. Similarly, mean rainfall is projected not to change by 2010, but to increase by up to 10% during kharif and rabi by 2070. At the same time, there is an increased possibility of climate extremes, such as the timing of onset of monsoon, intensities and frequencies of drought and floods.
The rise in the concentration of green houses gases was caused primarily by human and industrial activities. The increased agricultural activities and organic waste management are presumed to be contributing to the building up of both methane and nitrous oxide in the atmosphere. However, agriculture in general and Indian agriculture in particular is not contributing significantly to global climatic change, as GHG emissions from agriculture indicate. India's total contribution to global methane emission from all sources is only 18.5 Tg per year. Agriculture (largely rice paddies and ruminant animal production) is a major source of CH4 emission and contributes 68% to it. The continuously flooded rice fields emit methane, because anoxic conditions favor methanogenesis. Since India and China are the major rice producing countries, US-EPA attributed 37.8 Tg Methane/year to the Indian rice paddies. Based on this estimate, an international opinion was made that Asia and in particular, India and China are contributing significantly to global warming and they should do something to prevent this phenomenon. Sinha et al. (1998) estimated that global annual methane emission from rice cultivation is less than 13 Tg. IPCC (1996) has now revised the estimates of global methane emission from rice to 60 Tg/year. These estimates are still very high and can be further brought down. The contribution of Indian paddies to global CH4 budget was estimated to be only 4.2 Tg/year (Bhattacharya and Mitra 1998). The main reasons of low methane emissions from rice fields in India are that the soils of major rice growing areas have very low organic carbon are also and not continuously flooded.
Atmospheric concentration of N2O is increasing at a rate of 0.22±0.02% per year (Machida et al. 1995; Battle et al. 1996; Mosier et al. 1998). The emission of N2O is of serious concern, because of its long atmospheric lifetime of 166±16 years (Prinn et al. 1990). But despite its lower concentration and less rapid rise, N2O is becoming an important GHG, because of its longer lifetime and greater global warming potential than CO2 (300 times more than that of CO2 molecule). About 5% of total greenhouse effect can be ascribed to N2O and it is also responsible for the destruction of stratospheric ozone (Rodhe 1990). Estimates of total nitrous oxides from Indian agriculture are very low due to low soil fertility and lower amounts of fertilizers used in agriculture as compared to the western countries. In India, CO2 fixation becomes more important, because we use almost 190 million hectare of land for farming. The estimated dry biomass production from agriculture in India is almost 800 million tons every year. This is equivalent to the fixation of 320 Tg of C or 1000 Tg of CO2 per annum. Only a part is retained over time due to low body weight of human beings and other consumers and the rest is released to the atmosphere.
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