托福阅读背景知识(181)：冰河时期形成原因

　　在2014年5月24日的托福阅读考试中有这样一道题：冰河时期形成原因。针对这道托福考题，新东方富亦聪老师来为大家普及一下关于冰河时期形成原因的背景知识，这样有助于考生在面对这类题目时方便作答，新东方富亦聪老师指出：冰期地球表面覆盖有大规模冰川的地质时期。又称为冰川时期。两次冰期之间唯一相对温暖时期，称为间冰期。地球历史上曾发生过多次冰期，最近一次是第四纪冰期。

　　托福阅读真题再现：

　　冰河时期形成原因

　　第一段：地球周期一直被人们观测。但直到科学家M，才提出是地球的orbit三个因素共同发生造成的。Eccentric, tilt and orbit.。

　　第二段：三个理论。【好长一段】

　　第三段：三个角度变化要好多年。周期不能解释。

　　第四段：还有好多其他解释，有人说火山，有人说…有人说…

　　新东方老师解析：

　　冰期地球表面覆盖有大规模冰川的地质时期。又称为冰川时期。两次冰期之间唯一相对温暖时期，称为间冰期。地球历史上曾发生过多次冰期，最近一次是第四纪冰期。 地球在40多亿年的历史中，曾出现过多次显著降温变冷，形成冰期。特别是在前寒武纪晚期、石炭纪至二叠纪和新生代的冰期都是持续时间很长的地质事件，通常称为大冰期。大冰期的时间尺度至少数百万年。大冰期内又有多次大幅度的气候冷暖交替和冰盖规模的扩展或退缩时期，这种扩展和退缩时期即为冰期和间冰期。

　　学者们提出过种种解释，但至今没有得到令人感到满意的答案。归纳起来，主要有天文学和地球物理学成因说。

　　天文学成因说

　　天文学成因说主要考虑太阳、其他行星与地球间的相互关系。①太阳光度的周期变化影响地球的气候。太阳光度处于弱变化时，辐射量减少，地球变冷，乃至出现冰期气候。米兰科维奇认为，夏半年太阳辐射量的减少是导致冰期发生的可能因素。②地球黄赤交角的周期变化导致气温的变化。黄赤交角指黄道与天赤道的交角，它的变化主要受行星摄动的影响。当黄赤交角大时，冬夏差别增大，年平均日射率最小，使低纬地区处于寒冷时期，有利于冰川生成。

　　地球物理学成因说

　　地球物理学成因说影响因素较多，有大气物理方面的，也有地理地质方面的。①大气透明度的影响。频繁的火山活动等使大气层饱含着火山灰，透明度低，减少了太阳辐射量，导致地球变冷。②构造运动的影响。构造运动造成陆地升降、陆块位移、视极移动，改变了海陆分布和环流型式，可使地球变冷。云量、蒸发和冰雪反射的反馈作用，进一步使地球变冷，促使冰期来临。③大气中CO2的屏蔽作用。CO2能阻止或减低地表热量的损失。如果大气中CO2含量增加到今天的2~3倍，则极地气温将上升8~9℃;如果今日大气中的CO2含量减少55~60%，则中纬地带气温将下降4~5℃。在地质时期火山活动和生物活动使大气圈中CO2含量有很大变化，当CO2屏蔽作用减少到一定程度，则可能出现冰期。

　　托福阅读相关背景：

　　An ice age is a period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Within a long-term ice age, individual pulses of cold climate are termed "glacial periods" (or alternatively "glacials" or "glaciations" or colloquially as "ice age"), and intermittent warm periods are called "interglacials".Glaciologically, ice age implies the presence of extensive ice sheets in the northern and southern hemispheres. By this definition, we are in an interglacial period - the holocene, of the ice age that began 2.6 million years ago at the start of the Pleistocene epoch, because the Greenland, Arctic, and Antarctic ice sheets still exist.

　　Variations in Earth's orbit (Milankovitch cycles)

　　The Milankovitch cycles are a set of cyclic variations in characteristics of the Earth's orbit around the Sun. Each cycle has a different length, so at some times their effects reinforce each other and at other times they (partially) cancel each other.

　　Past and future of daily average insolation at top of the atmosphere on the day of the summer solstice, at 65 N latitude.

　　There is strong evidence that the Milankovitch cycles affect the occurrence of glacial and interglacial periods within an ice age. The present ice age is the most studied and best understood, particularly the last 400,000 years, since this is the period covered by ice cores that record atmospheric composition and proxies for temperature and ice volume. Within this period, the match of glacial/interglacial frequencies to the Milankovi? orbital forcing periods is so close that orbital forcing is generally accepted. The combined effects of the changing distance to the Sun, the precession of the Earth's axis, and the changing tilt of the Earth's axis redistribute the sunlight received by the Earth. Of particular importance are changes in the tilt of the Earth's axis, which affect the intensity of seasons. For example, the amount of solar influx in July at 65 degrees north latitude varies by as much as 22% (from 450 W/m2 to 550 W/m2). It is widely believed that ice sheets advance when summers become too cool to melt all of the accumulated snowfall from the previous winter. Some workers believe that the strength of the orbital forcing is too small to trigger glaciations, but feedback mechanisms like CO2 may explain this mismatch.

　　While Milankovitch forcing predicts that cyclic changes in the Earth's orbital elements can be expressed in the glaciation record, additional explanations are necessary to explain which cycles are observed to be most important in the timing of glacial–interglacial periods. In particular, during the last 800,000 years, the dominant period of glacial–interglacial oscillation has been 100,000 years, which corresponds to changes in Earth's orbital eccentricity and orbitalinclination. Yet this is by far the weakest of the three frequencies predicted by Milankovitch. During the period 3.0–0.8 million years ago, the dominant pattern of glaciation corresponded to the 41,000-year period of changes in Earth's obliquity (tilt of the axis). The reasons for dominance of one frequency versus another are poorly understood and an active area of current research, but the answer probably relates to some form of resonance in the Earth's climate system.

　　The "traditional" Milankovitch explanation struggles to explain the dominance of the 100,000-year cycle over the last 8 cycles. Richard A. Muller, Gordon J. F. MacDonald, and others have pointed out that those calculations are for a two-dimensional orbit of Earth but the three-dimensional orbit also has a 100,000-year cycle of orbital inclination. They proposed that these variations in orbital inclination lead to variations in insolation, as the Earth moves in and out of known dust bands in the solar system. Although this is a different mechanism to the traditional view, the "predicted" periods over the last 400,000 years are nearly the same. The Muller and MacDonald theory, in turn, has been challenged by Jose Antonio Rial.

　　Another worker, William Ruddiman, has suggested a model that explains the 100,000-year cycle by the modulating effect of eccentricity (weak 100,000-year cycle) on precession (26,000-year cycle) combined with greenhouse gas feedbacks in the 41,000- and 26,000-year cycles. Yet another theory has been advanced by Peter Huybers who argued that the 41,000-year cycle has always been dominant, but that the Earth has entered a mode of climate behavior where only the second or third cycle triggers an ice age. This would imply that the 100,000-year periodicity is really an illusion created by averaging together cycles lasting 80,000 and 120,000 years. This theory is consistent with a simple empirical multi-state model proposed by Didier Paillard. Paillard suggests that the late Pleistocene glacial cycles can be seen as jumps between three quasi-stable climate states. The jumps are induced by the orbital forcing, while in the early Pleistocene the 41,000-year glacial cycles resulted from jumps between only two climate states. A dynamical model explaining this behavior was proposed by Peter Ditlevsen. This is in support of the suggestion that the late Pleistocene glacial cycles are not due to the weak 100,000-year eccentricity cycle, but a non-linear response to mainly the 41,000-year obliquity cycle.

　　Changes in Earth's atmosphere

　　There is considerable evidence that over the very recent period of the last 100–1000 years, the sharp increases in human activity, especially the burning of fossil fuels, has caused the parallel sharp and accelerating increase in atmospheric greenhouse gases which trap the sun's heat. The consensus theory of the scientific community is that the resulting greenhouse effect is a principal cause of the increase in global warming which has occurred over the same period, and a chief contributor to the accelerated melting of the remaining glaciers and polar ice. A 2012 investigation finds that dinosaurs released methane through digestion in a similar amount to humanity's current methane release, which "could have been a key factor" to the very warm climate 150 million years ago.

　　There is evidence that greenhouse gas levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is difficult to establish cause and effect (see the notes above on the role of weathering). Greenhouse gas levels may also have been affected by other factors which have been proposed as causes of ice ages, such as the movement of continents and volcanism.

　　The Snowball Earth hypothesis maintains that the severe freezing in the late Proterozoic was ended by an increase in CO2 levels in the atmosphere, and some supporters of Snowball Earth argue that it was caused by a reduction in atmospheric CO2. The hypothesis also warns of future Snowball Earths.

　　In 2009, further evidence was provided that changes in solar insolation provide the initial trigger for the Earth to warm after an Ice Age, with secondary factors like increases in greenhouse gases accounting for the magnitude of the change.

　　William Ruddiman has proposed the early anthropocene hypothesis, according to which the anthropocene era, as some people call the most recent period in the Earth's history when the activities of the human species first began to have a significant global impact on the Earth's climate and ecosystems, did not begin in the 18th century with the advent of the Industrial Era, but dates back to 8,000 years ago, due to intense farming activities of our early agrarian ancestors. It was at that time that atmospheric greenhouse gas concentrations stopped following the periodic pattern of the Milankovitch cycles. In his overdue-glaciationhypothesis Ruddiman states that an incipient glacial would probably have begun several thousand years ago, but the arrival of that scheduled glacial was forestalled by the activities of early farmers.

　　At a meeting of the American Geophysical Union (December 17, 2008), scientists detailed evidence in support of the controversial idea that the introduction of large-scale rice agriculture in Asia, coupled with extensive deforestation in Europe began to alter world climate by pumping significant amounts of greenhouse gases into the atmosphere over the last 1,000 years. In turn, a warmer atmosphere heated the oceans making them much less efficient storehouses of carbon dioxide and reinforcing global warming, possibly forestalling the onset of a new glacial age.