在以往的托福阅读中考过关于冰河时期形成原因的话题。针对这个话题，前程百利小编为大家普及一下相关背景知识，这样有助于考生在面对这类题目时的作答。
托福阅读真题: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。
第二段：三个理论。
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。
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
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 400000 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 800000 years the dominant period of glacial–interglacial oscillation has been 100000 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 41000-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 100000-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 100000-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 400000 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 100000-year cycle by the modulating effect of eccentricity (weak 100000-year cycle) on precession (26000-year cycle) combined with greenhouse gas feedbacks in the 41000- and 26000-year cycles. Yet another theory has been advanced by Peter Huybers who argued that the 41000-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 100000-year periodicity is really an illusion created by averaging together cycles lasting 80000 and 120000 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 41000-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 100000-year eccentricity cycle but a non-linear response to mainly the 41000-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 8000 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 1000 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.
以上就是今天的托福阅读材料，大家可以在练习托福阅读材料的同时，积累一些相关词汇及句型，以便考试的时候更好地把握文意，夺得托福阅读的高分。前程百利小编预祝大家托福考试取得优异成绩!
