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  3. Scientists studying the effect of large volcanic eruptions on global climate have long focused on the major quantities of

Scientists studying the effect of large volcanic eruptions on global climate have long focused on the major quantities of

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问题             Scientists studying the effect of large volcanic eruptions on global climate
       have long focused on the major quantities of carbon dioxide (C02), a gas known
       to contribute to the greenhouse effect, produced by these eruptions. It is well
Line    observed that such greenhouse gases trap heat radiated from the surface of the
(5)     earth, thereby forming a type of insulation around the planet. The greenhouse
       effect is essential for ecological equipoise because it maintains the temperature
       of the planet within habitable parameters, but there is growing concern that
       man-made production of gases such as CO~ from the burning of fossil fuels may
       be threatening the system’s tolerance, and have resulted in excessive warming
(10)    on a global scale.
           While volcanic eruptions indubitably metabolize and accumulate C02 in the
       atmosphere, it has been recently discovered that their impact is virtually trivial
       compared to the quantity produced by human activities, especially heavy
       industry. In reality, the more substantive climatic effect from volcanoes results
(15)    from the production of atmospheric haze, whereby large eruption columns inject
       ash particles and sulfur-rich gases into the troposphere and stratosphere, clouds
       that circumscribe the globe within weeks of the volcanic activity. Ash and
       aerosol clouds from large volcanic eruptions disseminate quickly through the
       atmosphere, and the small ash particles decrease the amount of sunlight
(20)    reaching the surface of the earth and lower average global temperatures, while
       the sulfurous gases combine with water in the atmosphere to form acidic
       aerosols that also absorb incoming solar radiation and scatter it back out into
       space.
           There is evidence that volcanoes’ stratospheric ash clouds has a lesser
(25)    effect on global temperatures than aerosol clouds, given that the major Mt. St.
       Helens eruption had lowered global temperatures by about 0.1 degree C, while
       two years later the much smaller eruption of El Chico had, by contrast, three to
       five times the global cooling effect worldwide. Despite its smaller ash cloud, El
       Chico emitted more than 40 times the volume of sulfur-rich gases produced by
(30)    Mt. St. Helens, revealing that the formation of atmospheric sulfur aerosols has
       a more substantial effect on global temperatures than simply the volume of ash
       produced during an eruption. Sulfate aerosols appear to necessitate several
       years to settle out of the atmosphere, one of the reasons their effects are so
       widespread and enduring. This corroborates the opinion of those scientists who
(35)    argue that without the cooling effect of major volcanic eruptions such as E1
       Chico, global warming effects caused by human activities would be far more
       substantial. It should be noted that major volcanic eruptions have additional
       climatic effects beyond global temperature decreases and acid rain, for ash and
       aerosol particles suspended in the atmosphere scatter light of red wavelengths,
(40)    often resulting in brilliantly colored sunsets and sunrises around the world.
The author mentions the eruption of Mount St. Helens (lines 25 - 26) primarily in order to emphasize which of the following points?
选项 A、The deep and enduring effect that volcanic eruptions have on the environment
B、The process by which classic volcanic eruptions tend to occur
C、The relative dearth of cases with scientists can judge the effects of volcanic eruptions
D、A sample case with which later volcanic explosions are compared
E、The global cooling effects of one of the best-studied volcanic eruptions
答案D
解析
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