Thursday, June 17, 2010

Human Impact on Earth’s Atmosphere Part.IV

Humans have farmed for thousands of years but in just the last few decades burgeoning populations have called for more agricultural ingenuity than ever before. The breeding of new crop varieties, the use of fertilizers and pesticides and the bringing of more and more land under cultivation have kept production ahead of population growth throughout most of history. Since 1985, however, the limit seems to have been reached and per capita grain production has started to fall. In 1987 world grain reserves were sufficient for 100 days; by1989 there were only enough for 54 days, yet there are 93 million more people to feed every year. There have been hidden costs to the increases in productivity. It is though that about 40,000 people in the developing world die of pesticide poisoning every year. Twenty four billion tones of top soil are lost from crop lands each year, eroded by wind and water. Irrigation is lowering the water table beneath eight states in the Great Plains of America by a meter a year, and the diversion of rivers in the former Russia has reduced the Aral Sea to about a third of the sixe it was 25 years ago. One tenth of the earth’s land surface is currently given over to agriculture but there will have to be yet further changes before food production is sustainable.
 The earth’s atmosphere is now changing more quickly than at any time in the past. In the last 150 years there has been a 25 percent increase in the carbon dioxide and a 100 percent increase in methane in the atmosphere, largely as a result of the burning of fossil fuels’ the expansion of agriculture and rapid deforestation. Over the same period, the world has warmed by an average of 0.5. C, as green house gases trap the Suns heat within the atmosphere. Computer models predict a continued warming of     0.2. C a decade unless steps are taken to limit emissions. The warming would be greater were it not for pollutants such as sulphur dioxide which scatter sunlight back onto space. Other gases- notably CFCs are damaging the tenuous layer of ozone in the stratosphere which screens out ultraviolet radiation from the sun. Steps have been taken to phase out CFCs. Legislation to reduce other green house gases will be harder to implement since the practices that produce them are central to modern life. Many nations have pledged to reduce emissions to their 1990 levels, but stronger measures will be needed if global warming is to be averted.

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