2018年03月03日最新
托福阅读考试日期：
2018年03月03日
新托福阅读题目回忆版本一：
前两段说介绍地球上的冰川，为什么会有冰川还是有争论的，到目前为止还没明确的答案。然后提出了M的三个解释，一是说到地球绕太阳转的倾斜角度会导致温度的变化;二是旋转时的不稳定性也会导致温度的区别;第三个是;第三段说的是根据M的假设，冰川会一直存在，而不知识在特定时期出现。说明他的假设是有漏洞的不能够证明冰川为什么只会在特定的时期存在。第四段讲到别的假设，有人说火山灰能够导致这个现象，但是事实证明那时候没有火山活动的出现。被推翻后另一个假说是因为大气中co2的减少导致温度的下降。事实证明在有冰川期的时候的确大气里的co2含量会别的时候低。第五段说其实蚊虫比较不容易落在水平黑白间隔的地方，但斑马是垂直黑白间隔的，所以这个理论也是存有疑惑的。
【难度解析】
较难
新托福阅读题目回忆版本二：
Although the history of glaciation during the Pleistocene epoch (2 million to 10,000 years ago) is well established, we do not know with complete certainty why glaciation takes place. For over a century, geologists and climatologists have struggled with this problem, but it remains unsolved.
It is long known that Earth’s orbit around the Sun changes periodically, cyclically affecting the way solar radiation strikes the Earth, but the idea that these changes affect climate was first advanced by James Croll in the late 1800s. Later, Milutin Milankovitch elaborated the theory with calculations that convincingly argued that the cycles, now known as Milankovitch cycles, could cause climatic variations.
The Milankovitch cycles emerge from the way three cyclic changes in Earth’s orbit combine. One characteristic of Earth’s orbit is its eccentricity, the degree to which the orbit is an ellipse rather than a circle. Changes in the eccentricity of Earth’s orbit occur in a cycle of about 96,000 years. The inclination, or tilt, of Earth’s axis also varies periodically, moving between 22 degrees and 24.5 degrees. The tilt of Earth’s axis, toward the Sun at some times of the year and away from the Sun at other times, is responsible for the annual cycle of seasons. The greater the tilt, the greater the contrast between summer and winter temperatures. Changes in the tilt occur in a cycle 41,000 years long. Also, Earth wobbles as it spins, like a slightly unsteady top. The wobble cycle is completed once every 21,700 years. Changes in eccentricity, tilt and wobble do not affect the total amount of solar radiation Earth receives in a year, but they do affect how evenly or unevenly this radiation is disturbed over the course of a year. According to the Milankovitch theory, about every 40,000 years the three separate cycles combine in such a way that the difference between summer and winter temperatures is at a minimum. At this point winter temperatures are milder but so too are summer temperatures. As a result, less ice is melted in the summer than is formed in the winter, so glaciers build up and a period of glaciation results.
Milankovitch worked out the ideas of climatic cycles in the 1920s and 1930s, but it was not until the 1970s that a detailed chronology of the Pleistocene temperature changes was determined that could test the predictions of this theory. A correspondence between Milankovitch cycles and climate fluctuations of the last 65 million years seems clear. Furthermore, studies or rock samples drilled from the deep-sea floor and the fossils contained in them indicate that the fluctuation of climate during the past few hundred thousand years is remarkably close to that predicted by Milankovitch.
A problem with Milankovitch’s explanation of glaciation arises from the fact that the variations in Earth’s orbit, and hence the Milankovitch cycles, have existed for billions of years. Thus we might expect that glaciation would have been a cyclic event throughout geologic time. In fact, periods of glaciation are rare. So there must be another factor acting together with the Milankovitch cycles that causes periods of glaciation. Once this additional factor makes the temperature low enough, the cyclic variations of the Milankovitch cycles will force the planet into and out of glacial epochs with a fixed regularity.
Many hypotheses have been proposed for the additional cooling factor. Some suggest that variations in the Sun’s energy output could account for the ice ages. However, our present understanding of the Sun’s luminosity holds that it should have progressively increased, not decreased, over the course of Earth’s history. Still others argue that volcanic dust injected into the atmosphere shields Earth from the Sun’s rays and initiates an ice age. However, no correlation has been found between volcanic activity and the start of the last ice age. An increasingly attractive theory holds that decreases in atmospheric carbon dioxide starts the cooling trend that leads to glaciation. Carbon dioxide traps solar energy reflected from the Earth’s surface. If carbon dioxide levels decrease, less heat is trapped and Earth’s surface cools. Recent studies of the carbon dioxide content of gas bubbles preserved in the Greenland ice cap do in fact show that high carbon dioxide levels are associated with warm interglacial periods, and low levels h8pgs;with cold glacial periods.
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