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For more than a decade, scientists have observed a gradual rise in Earth's average surface temperature. There is wide — though not unanimous — agreement that the main source of global warming is the burning of fossil fuels — oil, coal and natural gas — to run vehicles, power industry, generate electricity and heat buildings. Burning fossil fuels releases carbon dioxide and several other so-called greenhouse gases, which trap the sun's heat inside Earth's atmosphere, much as glass traps heat inside a greenhouse.
While the rise in temperature — estimated at about 3 to 6 degrees Fahrenheit by the end of the century — may seem insignificant, scientists warn it could have a dramatic and possibly catastrophic effect on the global environment. One of the biggest impacts could be on the world's supply of fresh water.
Rising surface temperatures melt glaciers and permanent snow cover in the polar regions and on high mountaintops. As polar ice melts into the oceans, sea levels rise, threatening to flood coastal areas and islands. Seawater already is beginning to encroach on fresh water aquifers on some Pacific islands, such as Palau and Samoa, while the Netherlands, much of it below sea level, is bracing for new assaults on its legendary system of dikes.
Melting of glaciers also is beginning to threaten water supplies. As Himalayan snow and ice disappears, so does the high-altitude vegetation needed to slow runoff to the Ganges and other rivers of the Indian subcontinent. That, in turn, leads to recurrent cycles of flooding and low water levels in downstream areas of India and Bangladesh. Similarly erratic water-supply patterns are being observed in regions along South America's Andes range and the Rocky Mountains in the United States and Canada.
Global warming also is altering the world's precipitation patterns, scientists say. The tropics and subtropics are expected to receive lower and more erratic rainfall in coming decades, while the United States and other temperate zones are expected to experience more rain and snow. Warming is also expected to increase the frequency of extreme weather events, such as floods, droughts, typhoons and hurricanes. At times of drought, stream water may diminish and carry higher concentrations of pollutants, reducing still further the amount of usable fresh water.
On the positive side, rising temperatures may increase the amount of land that can be used for food production in the future by warming well-watered regions in northern Canada and Siberia that currently are too cold for agriculture. But at the same time there already are signs that warmer temperatures are accelerating drought and desertification in food-producing regions of Africa. Indeed, the overall impact of global warming on water supplies is expected to be negative. The United Nations predicts that climate change alone will account for about 20 percent of the expected increase in global water scarcity.
Climate experts say the combination of climate change and increased global demand for water makes it all the more urgent to find ways to improve access to clean water.
“There's more and more concern that future climate patterns may not reflect what we've seen in the past,” says Don Wilhite, director of the National Drought Mitigation Center in Lincoln, Neb. “We really need to be looking at some other scenarios of future climate, which may include more extreme weather events and more variability. With more and more of us trying to live on finite water resources, climate change amounts to a double whammy for future water resources.”
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