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  3. [A]Global climate change—average temperatures have risen by roughly 0. 6 degree Celsius since the beginning of the 20th century—

[A]Global climate change—average temperatures have risen by roughly 0. 6 degree Celsius since the beginning of the 20th century—

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问题      [A]Global climate change—average temperatures have risen by roughly 0. 6 degree Celsius since the beginning of the 20th century—is caused by greenhouse gases in the atmosphere. These gases, chief among them carbon dioxide(CO2), act as a blanket, trapping the sun’s heat that would otherwise be radiated back into space. Rising greenhouse gas levels in the atmosphere means rising average temperatures for the planet, causing climate change.
     [B]Regardless, the solar change is dwarfed by the impact from the extra heat trapped by CO2 alone since 1750: an additional 1. 66 watts per square meter, an effect that other greenhouse gases, such as methane, strengthen further. In other words, whereas the new satellite measurements call into question computer models of solar output, it does not change the fundamental physics of human-induced global warming.
     [C]The sun controls Earth’s climate, bathing us in light ranging from ultraviolet to visible that warms the planet and drives the heat engines we know as weather systems and ocean currents. The sun is changeable, cycling from maximum to minimum outputs over a roughly 11-year cycle, increasing or decreasing the amount of light that reaches Earth as a result of the poorly understood aspects of the sun’s seething nuclear fusion. Now new satellite measurements reveal that from 2004 to 2007—the declining phase of an unusually low and prolonged solar minimum—the sun put out even less ultraviolet light than expected but compensated by putting out more visible light.
     [D]Still, the finding suggests that scientists’ understanding of solar cycles and their impact on climate needs more work. " The result reverses understanding of solar cycle climate effects," which had been that the sun generally warms the climate on the way up from minimum to maximum and generally cools the climate on the way down from maximum to minimum, explains atmospheric scientist Piers Forster of the University of Leeds in England. "But the opposite seems to have been true of the last solar cycle. "
     [E]" The amount of visible radiation entering the lower atmosphere was increasing, which implies warming at the surface," says atmospheric physicist Joanna Haigh of Imperial College London, who led the research, published in Nature on October 7. "The solar radiative forcing of climate increased by 0.1 watt per square meter. " That means the sun, at least for those three years, played a larger role in ongoing climate change than previously thought.
     [F]In addition, the larger than expected loss of UV light meant less stratospheric ozone up to 45 kilometers above the surface, but more above that line. That distinguishes this solar cycle from the preceding two and " suggests that the declining phase of solar cycle 23 is behaving differently to previous solar cycles," the team wrote.
     [G]But the change from 2004 to 2007 in the sun’s output of visible light, and the attendant warming at Earth’s surface of 0.1 watt per square meter, is roughly equivalent to the overall forcing of the sun on the climate over the past 25 years—estimated by the U. N. Intergovernmental Panel on Climate Change to be an additional 0. 12 watt per square meter. That suggests scientists may have overestimated the sun’s role in climate change.

选项
答案G
解析 作者在讲述了一项太阳可见光增加，提升大气温度的新研究后，插入了全球变暖的背景介绍，其中重点解释了温室气体的影响。由上一空的分析可知，在剩下的三个选项中，[B]是数据分析的总结段，而[D]则开始讲研究的启示，都应该在[G]之后出现。而再由[G]段首的转折词But，以及本段内容，可以看出它是与[E]段研究分析相对立的观点，这也符合英文议论文体的基本特点，即提出正反不同的观点。故选项[G]为答案。
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