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  3. We are all seeing rather less of the sun. Scientists looking at five decades of sunlight measurements have reached the disturbin

We are all seeing rather less of the sun. Scientists looking at five decades of sunlight measurements have reached the disturbin

admin2015-04-10  64
问题     We are all seeing rather less of the sun. Scientists looking at five decades of sunlight measurements have reached the disturbing conclusion that the amount of solar energy reaching the earth’s surface has been gradually falling. Paradoxically, the decline in sunlight may mean that global warming is a far greater threat to society than previously thought.
    The effect was first spotted by Gerry Stanhill, an English scientist working in Israel. Comparing Israeli sunlight records from the 1950s with current ones, Stanhill was astonished to find a large fall in solar radiation. "There was a staggering 22% drop in the sunlight, and that really amazed me, " he says.
【C1】______
    Gerry called the phenomenon global dimming, but his research, published in 2001, met with a skeptical response from other scientists.
【C2】______
    Dimming appears to be caused by air pollution. Burning coal, oil and wood, whether in cars, power stations or cooking fires, produces not only invisible carbon dioxide but also tiny airborne particles of soot(浮在空中的烟尘), ash, sulphur compounds and other pollutants.
【C3】______
    Scientists are now worried that dimming, by shielding the oceans from the full power of the sun, may be disrupting the pattern of the world’s rainfall. There are suggestions that dimming was behind the droughts in sub-Saharan Africa. There are disturbing hints the same thing may be happening today in Asia.
【C4】______
    They know how much extra energy is being trapped in the earth’s atmosphere by the extra carbon dioxide(CO2)we have placed there. What has been surprising is that this extra energy has so far resulted in a temperature rise of just 0. 6°C.
【C5】______
    If so, then this is bad news. Even the most pessimistic forecasts of global warming may now have to be drastically revised upwards. That means a temperature rise of 10°C by 2100 in France, could be on the cards, and rendering many parts of the world uninhabitable. That is bound to happen unless we act urgently to curb our emissions of greenhouse gases.
[A]It was only recently, when his conclusions were confirmed by Australian scientists using a completely different method to estimate solar radiation, which climate scientists at last woke up to the reality of global dimming.
[B]But perhaps the most alarming aspect of global dimming is that it may have led scientists to underestimate the true power of the greenhouse effect.
[C]Intrigued,he searched out records from all around the world, and found the same story almost everywhere he looked. The sunlight fell by 10% over the USA, nearly 30% in parts of the former Soviet Union, and even by 16% in parts of the British Isles.
[D]As it turns out, people’s activity causes not only global warming but also "global dimming". It is connected with air pollution as a result of which the sunlight is reflected back to the outer space and massive settled clouds get formed.
[E]Global dimming is a daytime effect. At night the sun’s radiation is completely blocked by the earth.
[F]This visible air pollution reflects sunlight back into space, preventing it reaching the surface. But the pollution also changes the optical properties(光学性质)of clouds. Because the particles seed the formation of water droplets(生成点滴), polluted clouds contain a larger number of droplets than unpolluted clouds. Recent research shows that this makes them more reflective than they would otherwise be, again reflecting the sun’s rays back into space.
[G]This has led many scientists to conclude that the present-day climate is less sensitive to the effects of carbon dioxide than it was, say, during the ice age, when a similar rise in CO2 led to a temperature rise of 6°C. But it now appears the warming from greenhouse gases has been offset by a strong cooling effect from dimming.
【C5】
选项
答案G
解析 紧接上文，分析全球变暗缓解了温室效应的进一步加剧。科学家断言，目前的天气对二氧化碳的反应不如从前那么敏感。例如在冰河季(Ice Age)，大气中要是增加了同等数量的二氧化碳，地球温度会升高6摄氏度，而不是0．6摄氏度。很明显全球变暗部分抵消了温室气体所产生的效应。
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