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GREEN HOUSE EFFECT A glass house used for raising delicate plants is called green house. A green house has higher temperature inside than outside though the interior receives less radiations, it is called green house effect. The temperature of the earth’s surface is determined by the energy balance between the heat energy reaching the earth’s surface and the heat energy that is radiated back into space. Much of the short wavelength visible light from the sun that reaches the earth is reradiated as a long wavelength infra red radiation. Some atmospheric gases, like carbon dioxide, chlorofluoro carbons, methane, nitrous oxide etc., which are IR active selectively absorb longer wavelengths (infra red) and reflect heat back to earth. Since this effect is similar to the one under the glass house is known as green house effect. The green house gases act as a thermal blanket around the globe, raising the earth’s surface temperature beyond the equivalent temperature calculated from the models. If the earth did not already have a green house effect its temperature would have been -18o C. Since actual average temperature of the earth is15o C, it seems that the green house effect adds 33o C of warming to the surface of the earth. Gases Atm. Conc (ppm) Annual conc. Increase (%) Relative green house efficiency Current green house contribution Table 4.1 The following table summarises some of the characteristics of these green house gases. CO2 CFCs CH4 N2O 351 0.00225 1.675 0.31 0.4 5 1 0.2 1 15000 25 230 57 % 25% 12% 6% Sources CO2 : Burning of fossil fuels, automobile exhaust, CFCs: Air conditioners, fire extinguishers refrigerators, propellants in aerosol spray cans. CH4: Garbage dumps, paddy fields, natural gas, petroleum refineries. N2O: Lightening, nylon production, break down of nitrogen fertilizers, respiration of anaerobic fungi . As is now well known, anthropogenic sources of a number of gases are enhancing the green house effect, leading us into a further uncertain global climate. By1990 the concentration of CO2 had reached 355ppm and was climbing at about 1.5ppm per year 4.7.1 Effects of Global Warming: (i) Effects on weather and Climate: In 20th century, the global mean temperature has increased by about 0.6C. The average temperature of the earth may increase by 1.4 to 5.8C by the year 2100 from the year 1990. It is expected that the rise in temperature will be more marked in the regions of middle and higher altitudes. The moisture carrying capacity of the atmosphere is also expected to increase due to warming of the atmosphere. The troposphere will warm up and the stratosphere will cool down. This would cause wide-spread change in the precipitation pattern due to changed pattern of air mass movement. The precipitation will increase at higher altitudes and but will decrease at lower altitudes. The frequency of extreme drought and floods will increase. (ii) Rise in Sea Level: The global warming also contributes to rise in sea level due to thermal expansion of ocean and melting of glaciers and Green land ice sheets. The level of sea has been rising by 1 to 2 mm per year during the 20th century. If the rise in sea level goes on increasing with the present rate, the global mean sea level will increase upto 0.88 mm from the 1990 level. A rise of even half a meter in the sea level would affect human population, one-third of which lives within 60 km of a coast line. Many of the world’s important cities and coastal areas are likely to be hit by storms and floods. Several low lying islands may be submerged. Many important birds and fishes inhabiting in coastal salt marshes and estuaries, will become extinct due to destruction of their breeding grounds. Thus, rise in sea level will have a negative impact on human settlements, tourism, fisheries, agriculture, water supply and coastal ecosystems. (iii) Effect on Range of Species Distribution: Each plant and animal species occurs within a specific range of temperature. The global warming will shift the temperature ranges, which would affect altitudinal and latitudinal distribution pattern of organisms. Many species may shift slowly pole ward or towards high elevation of mountain areas. Rapid rise in temperature may cause large scale death of many trees, as they are sensitive to temperature stress, and the area may be occupied by scrub vegetation. Many species may disappear, as they are unable to migrate fast enough to track temperature change. (iv) Food Production: Global warming will reduce crop yield due to increase in incidence of plant diseases and pests, explosive growth of weeds and enhanced basal rate of respiration of plants. In tropical and subtropical regions even a small rise in temperature will have detrimental effect on crop productivity. It is estimated that the yield of rice along in South East Asia will decrease by 5% for each 1C rise in temperature. Control Measures: 1. The dependence on fossil fuel should be reduced and use of non-conventional energy should be encouraged. 2. Afforestation and reforestation should be encouraged. 3. Reduction of N2O emission by minimizing the use of nitrogen fertilizers. 4. Development of substitutes for chlorofluorocarbons.