气象学在托福听力科技类文章常出现的主题，而且往往国内学生在这方面的托福听力背景知识知之甚少，今天我们就让大家感受一下托福考试中的气象知识。

　　比如有一篇难度颇大的文章，提到了全球气候在冰期和暖期间的切换。

　　Now, the typical pattern for an interglacial period, and we have studied several, is that the concentration of carbon dioxide and methane gas actually reaches it... its peak, that is, there is the most carbon dioxide and methane gas, uh, greenhouse gases in the atmosphere just after the beginning of the interglacial period. And then, for reasons which are not entirely clear, the concentration of greenhouse gases gradually goes down. Now, the climate continues to warm for a while because there is a lag effect. But uh, gradually as the concentration of greenhouse gases goes down, earth starts to cool again, and eventually you slip back into an Ice Age.

　　原来这大气中温室气体(主要是二氧化碳和甲烷)有关，当温室气体含量下降，温度就下降，慢慢的地球就进入了冰期。之后，因为某些尚不明确的原因，温室气体的含量有开始回升，地球又逐渐变暖。这就是地球在冰期和暖期切换的原因。

　　In fact, back in the 1970s, a lot of theorists were predicting that, you know, the climate would start to cool and we’d slowly enter into the new Ice Age. And then they were puzzled as to why it didn’t seem to be happening.

　　但是人类的活动正在改变这一循环，事实上，有一批科学家根据规律推测地球即将进入下一个冰期，但是这个冰期至今还没有到来。现在我们已经知道，这是因为人类排放了太多的二氧化碳到了大气中。

　　那未来究竟会怎样呢?教授是这样说的：

　　Now, what are the implications for the future? Well, um... it is a little tricky. I mean, you could say, well, here is an example of ... um ... human activity, the agricultural revolution which actually was beneficial; we altered the climate for the better, perhaps, by preventing an Ice Age. But then industrialization, of course, has drastically increased the amount of carbon dioxide that humans are putting into the atmosphere, the burning of fossil fuels tends to put a lot of CO2 into the atmosphere. Um... so we are entering into uncharted territory now, in terms of the amount of carbon dioxide, the concentrations of carbon dioxide that are now being put into the atmosphere as a result of industrialization and the use of fossil fuels.

　　目前的研究也没有得出明确的结论。好的方面是我们避开了一次本该到来的冰期。但是另一方面来看，人类燃烧化石能源，排放大量的二氧化碳有让地球的气候开始了史无前例的变化，使我们进入了未知的领域。

　　那未来究竟会怎样呢，后天电影中告诉我们地球可能很快进入冰期，迎来大冰冻，事实上，这个观点还是有一定科学依据的，这一事件在历史上发生过三次，最近的一次称为“新仙女木事件(Younger Dryas)”

　　The Younger Dryas, from c. 12,900 to c. 11,700 years ago (BP), was a sharp decline in temperature over most of the northern hemisphere, at the end of the Pleistocene epoch, immediately preceding the current warmer Holocene. It was the most recent and longest of several interruptions to the gradual warming of the Earth's climate since the severe Last Glacial Maximum, c. 27,000 to 24,000 BP. The change was relatively sudden, taking place in decades, and resulted in a decline of 2 to 6 degrees Celsius, advances of glaciers and drier conditions, over much the temperate northern hemisphere. It is thought to have been caused by a decline in the strength of the Atlantic meridional overturning circulation, which transports warm water from the equator towards the North Pole, and which in turn is thought to have been caused by an influx of fresh cold water from North America into the Atlantic. The Younger Dryas was a period of climatic change, but the effects were complex and variable. In the southern hemisphere, and some areas of the north such as the south-eastern United States, there was a slight warming.

　　上次冰期结束时，大约1.7万年前地球开始变暖，气温逐渐地回升。两极、北美和北欧的冰川开始消融，海平面逐渐上升，渤海、黄海、挪威海的草原被水淹没。到了1.3万年前，北美和北欧的冰雪已经融化了相当大一部分，南北半球春暖花开，一片繁荣景象。但就在此时北美巨大的冰川融化，和现在北极冰盖的融化相似，巨量的淡水冲入北大西洋，导致北大西洋暖流中断，在短短十年内，地球平均气温下降了大约7、8℃。这次降温持续了上千年，直到11500年前，气温才又突然回升。

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