OLD JJ

　　V1

　　是讲某人MM的理论，关于change in earth‘s orbit影响气候什么的，也没看懂……

　　V2

　　03/21 第二篇是change in earth orbit. 第一段讲M的理论多牛B，第二段讲50年代的什么试验证明M是错的，然后第二段的后半部分又说原来50年代的试验方法(还是数据?)是错误的，这样的话M的理论又被证明是对的了。

　　有益补充1：地球轨道根数变化与第四纪冰期 Changes of the Earth's Orbital Elements and the Quaternary Glacial Epoch

　　米兰柯维奇(Milankovitch)天文气候学理论和第四纪地质时期以来冰期的研究进展.研究结果表明,地球上的冰体积具有近10万yr的变化周期,并伴有近4万yr和2万yr的变化周期,它们是由于地球的轨道根数变化导致的气候变迁所致;不同的地球物理资料中均存在上述类似的变化周期,表明气候变迁所导致的变化是全球性效应,证实米兰柯维奇天文理论是基本正确的。这个像不像jj里讲的mm理论? 冰川变化 地球轨道

　　有益补充2 (补充1的英文)：

　　At the recent American Geophysical Union meeting in San Francisco, the 25th anniversary of one of the great

　　papers in paleoclimatology was celebrated. The paper, entitled “Variations in the Earth’s orbit: Pacemaker

　　of the Ice Ages,” presented important new evidence supporting the orbital theory of glaciation. Orbital theory goes back over a century but is most closely associated with Milankovitch, who calculated the effects of gravitational perturbations on the seasonal cycle of Earth’s insolation (the radiation incident at the top of the atmosphere). Insolation varies on several time scales, including ~20,000 years (termed precession), ~40,000.

　　参考阅读

　　Can We Date the Ice Ages?

　　Following improvements in the ability to measure isotope ratios which came about as a spin-off of the wartime Manhattan project, physical chemist Harold Urey began to examine the possibility that the ratio of the two principal isotopes of oxygen found in the atmosphere might provide a clue as to past temperatures. It was based on the idea that the ratio of the heavier isotope (oxygen-18) to the more prevalent isotope (oxygen-16) found at the sea surface would change depending on the temperature of the ocean water near the surface. Urey thought that a careful study of the oxygen isotope ratio in the shells of sea creatures, which build their calcium carbonate shells from oxygen available in the seawater, might serve to indicate the temperature of the water in which they formed. During warmer periods, it was thought, evaporation from the ocean surface would tend to enrich the sea surface water with the heavier isotope of oxygen.

　　Perhaps, Urey reasoned, the isotope ratios found in the layers of discarded shells of sea organism which form the ocean bottom could thus serve as a record for the past temperatures of the ocean.

　　The theory is fraught with many ifs, but it was pursued with persistence, starting in the 1950s, by Italian-educated micropaleontologist Cesare Emiliani, a one-time collaboator of Urey at the Argonne Laboratory then associated with University of Chicago. Emiliani identified certain species of small shell-forming sea organisms known as foraminifera, which he thought suitable for oxygen-isotope analysis to determine past climates. The conclusions he drew as to the dating of the ice ages were constantly challenged by leading oceanographers, who found them in contradiction with their studies of ocean bottom cores. The method was also attacked on the grounds that it wasn't clear that the creatures formed their shells, known as tests, near enough to the surface to reflect changes in isotope ratios.

　　About 1968, a somewhat new interpretation of the oxygen isotope record was proposed by a young oceanographer and climatologist, Nicholas Shackleton, a Cambridge graduate and great nephew of the famous British Antarctic explorer of the same name. Shackleton proposed that the oxygen-isotope ratio could serve as a proxy, not for temperature but for sea level--the idea being that during periods of glacial advance, when a large volume of ocean water had been taken up into the continental ice sheets, the oxygen-18 ratios of the remaining water would consequently be higher. These might be detected in the foraminifera layers found in the ocean bottom cores. Again there are many ifs, but Shackleton examined isotopic ratios of snows in Alpine and Arctic regions as well as many other factors to bolster his hypothesis. In the 1970s a National Science Foundation-funded program of oceanographic studies, known as CLIMAP, collected a large number of sediment cores from different parts of the world ocean. The program, known as the Decade of the Oceans, was run in conjunction with some flawed statistical approaches to modeling of global atmospheric circulation that had originated in efforts of John von Neumann to use computer modeling for studies of weather modification. However, analysis of the oxygen isotopic ratios of foraminifera found in the undersea cores suggested to a team working at the Lamont-Doherty Geological Laboratory that there was a definite signal of 100,000 year cyclicity. Dr. John Imbrie, who ran the computer programs analyzing the data, was the first to hypothesize that the periodicities were caused by the Milankovitch orbital cycles.

　　A landmark paper by Hays, Imbrie and Shackleton, published in the December 1976 issue of Science magazine ("Variations in the Earth's Orbit: Pacemaker of the Ice Ages"), argued that the advance and retreat of the ice sheets was triggered by the changes in the Earth's orbital parameters. Other factors might also be present to reinforce these relatively small changes in solar radiation, but these were the pacemaker. By the theory of the orbital cycles, the evidence from the undersea cores explained that a major glaciation would be set off about every 100,000 years, followed by a short period known as an interglacial, a melt back lasting about 10,000 to 12,000 years. By the calculations of astronomers, the present interglacial, which has lasted about 11,000 years, is due to end any time. Indeed we have been in a period of long-term cooling for more than 6,000 years. The maximum summer temperature experienced in Europe over the last 10,000 years occurred about 6000 B.C. Over North America, where the process of glacial retreat lagged somewhat, the maximum was reached by about 4000 B.C. These estimates based on a vast array of evidence from geology, botany, and many other fields are consistent with the orbital theory of climate, for the northern hemisphere Summer would have been occurring at a point in Earth's orbit much nearer to the Sun than presently.

　　"One of the fundamental tenets of palaeoclimate modeling, the Milankovitch theory, is called into doubt by isotope analysis of a calcite vein, just reported in Science by Winograd and colleagues. The theory, which is backed up by a compelling bank of evidence, suggests that the ice ages determined, with unprecedented accuracy, in the new record cannot be reconciled with the planetary cyclicity. . .

　　Winograd and colleagues' evidence also turns on oxygen isotope data, this time from vein calcite coating the hanging wall of an extensional fault at Devils Hole, an aquifer in southern Nevada. In 1988, the authors published a date, 145,000 years, based on 234U-230Th dating for the end of the penultimate ice age (Termination II), marked by an increase in the 18O to 16O ratio, a change taken to mirror an increase in local precipitation. Although the date was only 17,000 year earlier than the previously accepted date of 128,000 years, if correct, this change is enough to bring Milankovitch mechanism into serious doubt. . .

　　I remain confused. The geochemist in me says that Devils Hole chronology is the best we have. And the palaeoclimatologist in me says that correlation between accepted marine chronology and Milankovitch cycles is just too convincing to be put aside. . .

　　One side will have to give, and maybe - just to be safe - climate modellers should start preparing themselves for a world without Milankovitch."

　　以上就是关于米兰科维奇的冰期理论的GMAT阅读机经的全部内容，考生可以有选择的看看，机经虽好，但是也要适度哟。

       最后，小编祝所有考生顺利通过GMAT考试并申请到理想的学校。