? ??傳統(tǒng)上，氣候科學(xué)家認(rèn)為地球經(jīng)歷了四個(gè)主要的冰川期（被冰蓋覆蓋），中間有溫暖的間冰期。但是氣候e research has now shown that over the past 800,000 years Earth has seen a complex pattern of some twenty major climatic shifts, with temperatures alternating between very warm and intensely cold, featuring warm seas in northern Europe at one extreme and ice sheets covering vast areas of the globe at the other. The cause of these fluctuations lies in the complex relationship between Earth and the Sun. We are all familiar with the fact that Earth orbits the Sun and that it spins around its own axis, which is at an angle to the plane of its orbit. This causes some parts of Earth's surface to be nearer the Sun for periods of time, accounting for the differences between summer and winter. If all parts of this system were stable. Earth's climate would remain constant, but this is not so. First, Earth's or bit is not perfectly circular but is slightly elliptical, causing a variation on a 100,000-year cycle. Secondly, the axis of Earth hanges its tilt (angle by a fraction over a .41,000-year cycle; and thirdly, the planet . has a slight wobble (shaking movement) about its axis as it spins, setting up changes over a cycle of 23,000 years. The combination of all these factors (known as the Milankovitch cycles) creates very small changes in the Sun-Earth relationship that determine the expansion or contraction of the polar ice cap and thus the sequence of fluctuating ice ages.

???????The complexity of these changes has taken some time to unravel. The earliest work was done on glaciers in the lpine region, and it was here that the four major stages of glaciation were identified, providing a useful regional sequence. More recently, over the last fifty years or so, work on deep-sea cores has pioneered a new way to study the phenomenon globally. The principle is quite simple: eep-sea cores provide stratified sequences of accumulations of calcium carbonate derived from the shells of dead organisms. The calcium carbonate in these layers contains two different oxygen isotopes (oxygen of different atomic weights), 16o and 180, which the organisms extract from the atmosphere. Since it can be shown that cold conditions favor 180 over 160, by assessing the ratio of the two it is possible to arrive at a direct measurement relating to the temperature of the ocean at the time of deposition. From these measurements a system of twenty phases, known as marine isotope stages, can be distinguished, reflecting changes in Earth's climate. The method was extremely valuable in providing a relative sequence, but it needed to be calibrated to give approximate dates for the phases.

???????The breakthrough came when, in one of the cores, it was possible to identify a point at which a major reversal had occurred in Earth's magnetic field, the time when the North and South Poles took up their present positions. This same reversal has been recorded in rocks, where it could be dated to around 736.000 years ago using an absolute dating method. With this one point securely established, and assuming that the deep- sea sediments had accumulated at a standard rate, it has been possible to assign dates to the entire sequence. nother valuable dating method, which has been developed over the last thirty years, is based on cores bored out of the Greenland ice cap. The largest core is 3,000 meters deep and represents the buildup of the ice over a 110.000- year period. From the cores it is possible to measure annual increments, one year's winter ice being separated from the next year's by a fine dust layer formed during the summer melt. The temperatures prevailing at the time are estimated from the (16)0 to (18)0 ratios and from proportions of the windblown chemical particles The cores provide a very precise dated sequence of climatic events extending back from the present.

1.Traditionally, climate scientists believed that Earth underwent four major periods of glaciation (being covered by ice sheets) with warm interglacials in between.ut climate research has now shown that over the past 800,000 years Earth has seen a complex pattern of some twenty major climatic shifts, with temperatures alternating between very warm and intensely cold, featuring warm seas in northern Europe at one extreme and ice sheets covering vast areas of the globe at the other. The cause of these fluctuations lies in the complex relationship between Earth and the Sun. We are all familiar with the fact that Earth orbits the Sun and that it spins around its own axis, which is at an angle to the plane of its orbit. This causes some parts of Earth's surface to be nearer the Sun for periods of time, accounting for the differences between summer and winter. If all parts of this system were stable. Earth's climate would remain constant, but this is not so. First, Earth's or bit is not perfectly circular but is slightly elliptical, causing a variation on a 100,000-year cycle. Secondly, the axis of Earth hanges its tilt (angle by a fraction over a .41,000-year cycle; and thirdly, the planet . has a slight wobble (shaking movement) about its axis as it spins, setting up changes over a cycle of 23,000 years. The combination of all these factors (known as the Milankovitch cycles) creates very small changes in the Sun-Earth relationship that determine the expansion or contraction of the polar ice cap and thus the sequence of fluctuating ice ages.