Toefl – causes of ice ages – reading from diagnostic test

Causes of Ice Ages

Geologists have shown that for about 80 percent of the past 2.5 million years, ice-age conditions have prevailed on the Earth’s surface. During the past one million years, increased glacial conditions have run in cycles of approximately 100,000 years.
Many different factors may contribute to these increases in glaciation at regular intervals throughout Earth’s more geologically recent history. The three most prominent factors probably relate to the amount of sunlight that reaches the Earth. This varies over time for three main reasons. First, the planet wobbles* as it spins, due to the pull of the sun and moon. Furthermore, the Earth tilts* on its axis and the degree of tilt changes over time. Finally, the orbit of the Earth around the sun is elliptical and the length of the major axis of the ellipse changes over a period of about 100,000 years. A mathematician named Milutin Milankovitch discovered in the 1930s that the pattern of insolation, or sunlight, predicted by these eccentricities in the Earth’s movement matched the period of the last several eras of intense glaciation.
These Milankovitch insolation cycles were the dominant theory in ice-age research for much of the twentieth century despite the fact that the match between periods of peak insolation and most intense glaciation were not exact. For example, a cycle of 400,000 years predicted by the Milankovitch theory has never shown up in the climate records obtained through the study of microfossils deposited on the sea floor. Also, recent analysis has shown that the insolation theory predicts peaks of sunlight at intervals of 95,000 and 125,000 years. Climatological data does not support this predicted sunlight peaking. Other damaging evidence was the indication of a precisely measured sudden rise in temperature at a water-filled cave in Nevada, which preceded the increase in solar radiation that was supposed to cause it.
These and other problems with the Milankovitch cycles led some researchers to seek alternative explanations for the cyclic arrival of extended ice ages. In the 1990s, it was discovered that the orbital inclination of the Earth to the sun and planets could also be responsible for climate changes. If we imagine a flat plane with the sun in the center and the planets revolving around it, the Earth slowly moves in and out of the flat plane by a few degrees, repeating the cycle every 100,000 years. Two scientists, Muller and MacDonald, have proposed that it is this orbital inclination which is ultimately responsible for the periods of glaciation and warming. They argue that because of the oscillation, the Earth periodically travels through clouds of debris, in the form of dust and meteoroids. Such debris could reduce the amount of solar energy reaching the surface of our planet, thus plunging it into regular cold periods.
The advantage of this theory is that it is not confronted with several of the problems associated with the Milankovitch theory. In particular, the new theory fits well with the analysis of ocean sediments taken from eight locations around the world. This analysis yielded data clearly showing the peak of the last several ice ages with a period of 100,000 years and corresponding to the periods when the Earth’s oscillating inclination takes it through clouds of extraterrestrial debris.
However, many researchers in this field are not yet persuaded by the inclination hypothesis.

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Toefl – causes of ice ages – reading from diagnostic test