填空题 When we burn fossil fuels (principally coal and oil) we send extra quantities of carbon dioxide into our atmosphere. Since 1958, the proportion of CO ₂ in our air has risen 25 percent. Many scientists think that within a century this simple gas could devastate our world.
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Consider CO ₂ , for example. Today, it causes about half the total greenhouse effect. Each year our skies receive five billion tons of CO ₂ , from the burning of fossil fuels, and up to half again as much from the clearing and burning of almost 33 million acres of tropical forest. At the present rate of increase, the amount of this gas alone could double during the next century.
The University of Chicago atmospheric scientist V. Ramanathan calculates that the earth"s average temperature already has risen during this century by one degree F., almost certainly because of the increase in greenhouse gases. Even without further atmospheric pollution, he estimates that trapped heat from gases we"ve already put in our skies will boost global temperatures another one to five degrees over 1980 levels in the next century. If our emissions of these gases continue to increase as they have, he and others predict that by the year 2030 the earth"s average temperature could climb by nine degrees F. over 1900 levels.
A two-or-three-degree warming seems small until we realize that it approximates the rise that 100,000 years ago ended the last major ice age. If we don"t slow the rate of warming, here"s what a number of researchers fear you and your children will face: the droughts and heat waves that blistered much of the United States in 1988 will become routine summer weather. As rainfall across the country"s wheat and corn belts diminishes by 40 percent, droughts will become common and the Dust Bowl era will return. Summer will mean plagues of insects devouring every moist crop leaf they can find. Prairie and forest fires will become far more frequent and harder to control. Giant hurricanes, with 50 percent more destructive potential than those today, will hit farther north and during more months of the year.
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How much of this terrifying scenario is science, and how much is science fiction? Many scientists believe that a number of these greenhouse problems will come to pass unless actions are taken to slow some of today"s trends. Others aren"t so sure.
"We scientists can"t even be 100-percent certain that the world has gotten warmer during this century," says Tim Barnett, a marine scientist at the Climate Research Group at Scripps Institution of Oceanography in La Jolla, California. Early in this century inferior techniques and instruments were used for measuring temperatures, and today many of our weather stations are near urban "hot" areas that may be under-reporting the effects of trees and other natural modifiers on temperatures.
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Help From Sea Plankton. The earth, scientists know from fossil records, has over millions of years faced times when large-scale volcanic activity or sea-floor hot springs have loaded the atmosphere with CO ₂ and heated the climate. Yet somehow a runaway greenhouse effect was prevented. The oceans themselves appear to have an enormous capacity for absorbing CO ₂ , but even more amazing is the life that came from the oceans and its role in regulating CO ₂ levels in the atmosphere.
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As a warming climate increases plankton breeding, growing communities of the small organisms emit more and more dimethyl-sulphide (DMS) gas into the air. DMS triggers the formation of unusually small water droplets, which can reflect more sunlight than ordinary clouds do, thus helping to cool the earth"s climate.
A. The earth"s climate, adds Michael MacCracken, head of atmospheric sciences at Lawrence Livermore National Laboratory in California, "is like a Rube Goldberg machine." The trouble with making any predictions about climatic change is that thousands of factors influence climate—including many that trigger a cascade of other unexpected changes.
B. How? Carbon dioxide in our atmosphere, like the glass in a greenhouse, lets sunlight pass through, then catches and retains some of the sunlight"s energy as heat. This greenhouse effect helps warm the earth"s climate. If CO ₂ and other greenhouse gases such as methane, nitrous oxide and chlorofluorocarbons vanished tomorrow, the earth would become overnight a frozen, lifeless world like Mars. In fact, all these gases have been increasing since the start of the Industrial Revolution.
C. As the world"s climate warms, according to this theory, polar icecaps will melt and ocean levels will rise by up to four feet during the next century, threatening such cities as New York, London, Beijing and Seoul. Farmland will be devastated, water supplies contaminated, and wildlife habitats decimated.
D. If you have held a piece of blackboard chalk, for example, the fevers of past greenhouse warming. About 160 million years ago, ocean plankton took carbon dioxide from the atmosphere and used the carbon to make their protective shells of limestone. When the plankton died, their shells sank to the ocean bottom, locking the carbon away in mineral deposits that one day would rise from the sea as white cliffs of chalk.
E. Handler speculates that tropical volcanoes trigger other kinds of weather as well. Twelve of the biggest such eruptions in this century, he notes, were followed immediately by weather disturbances called El Nino-Southern Oscillations. Occurring three or four years, El Ninos are marked by a shift in Pacific equatorial winds associated with an unusual warming of the eastern tropical Pacific Ocean. Most climatologists believe that the higher ocean-surface temperature in turn alters atmospheric pressures, temperatures and wind currents, with the most massive E1 Ninos causing wild distortions in weather throughout the world.