C.
Conservation reduces petroleum imports, saves money, and reduces air pollution. In short, conservation is responsible
stewardship of the environment and it makes practical sense. But its effect on global warming is less certain. Although a
growing number of scientists believe the increasing amount of CO2 in the atmosphere is the major impetus behind global
warming, the issue is not yet settled. For example, recent findings published in Nature conclude that the sun is more active now
than at any time in the past 8,000 years. Other natural cycles may also be playing a role. Deforestation and other land use
changes contribute to warming. Towns and cities and the roads that connect them form heat islands that cause significant local
and even regional warming.
Scientists are especially uncertain about the global warming roles of water vapor, clouds and particles in the atmosphere.
Water vapor is a much more potent greenhouse gas than carbon dioxide. If the atmosphere were devoid of the warm air trapped
by water vapor, the Earth would be so cold that the oceans would be frozen. Warm air can hold more water vapor than cool air.
Increasing the water vapor in the atmosphere can trap more heat, thus increasing temperature. On the other hand, increasing the
water vapor can also increase cloud cover, which reduces the solar irradiance that increases the temperature. The interactions of
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these poorly understood mechanisms contribute to the uncertainty of global warming models. So does the presence of particles
in the atmosphere, which can either reflect sunlight (a cooling effect) or absorb it (a warming effect).
The melting of glaciers is powerful evidence that the Earth is indeed warming. Or is it? Consider Alaska’s Bering Glacier,
the largest in North America. According to NASA’s Earth Observatory website, warmer temperatures and changes in
precipitation over the past century have thinned the Bering Glacier by several hundred meters. Since 1900 the glacier has
retreated some 7.5 miles. It seems only logical to blame the retreat of a massive glacier on global warming, but checking the
temperature record suggests something else must be at work. The temperature has been measured on either side of the Bering
Glacier since 1909, and the temperature record at both sites does not show obvious warming or cooling trends that are found
elsewhere around the Earth. So why is the giant glacier receding?
James Hansen, NASA’s primary global warming advocate, has shown that soot may be causing significant melting of
global ice and snow. Major sources of soot include agricultural fires around the world, coal-burning power plants in China,
cooking fires in India, and massive forest fires in Alaska, Canada, and Russia. The smoke from these fires is clearly visible in
NASA satellite images. The possible role of soot in melting glaciers might someday be viewed as a major discovery,
particularly if it explains the melting of glaciers, such as Bering Glacier, in the absence of any warming trends. Meanwhile,
global warming advocates continue to claim that warming alone is melting glaciers.
This brings us to the issue of the magnitude of global warming, for there is great controversy over the accuracy of
measurements used to determine temperature trends. The central issue is that many stations that measure temperature around
the world have become surrounded by development that causes an artificial warming effect. Climate modelers have attempted
to remove this heat island effect, but there is controversy over their success.
【題組】47. Why does water vapor contribute to temperature cooling?
(A) Water vapor can reduce heat in the atmosphere by absorbing sunlight.
(B) Water vapor can lower the temperature by integrating with cold air.
(C) Water vapor can decrease sunlight exposure by enlarging cloud coverage.
(D) Water vapor can cool down the air by trapping warm air and sparing cold air.
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