Milankovitch Cycles Theory背景知识~~~

shihui0012

Most scientists believe that the ice ages are the result of subtle changes in Earth's orbit, known as the Milankovitch cycles. One such cycle describes the way Earth's orbit gradually changes shape from a circle to a slight ellipse and back again roughly every 100,000 years. The theory says this alters the amount of solar radiation that Earth receives, triggering the ice ages. However, a persistent problem with this theory has been its inability to explain why the ice ages changed frequency a million years ago.

7   "In Milankovitch, there is certainly no good idea why the frequency should change from one to another," says Neil Edwards, a climatologist at the Open University in Milton Keynes, UK. Nor is the transition problem the only one the Milankovitch theory faces. Ehrlich and other critics claim that the temperature variations caused by Milankovitch cycles are simply not big enough to drive ice ages.

8   However, Edwards believes the small changes in solar heating produced by Milankovitch cycles are then amplified by feedback mechanisms on Earth. For example, if sea ice begins to form because of a slight cooling, carbon dioxide that would otherwise have found its way into the atmosphere as part of the carbon cycle is locked into the ice. That weakens the greenhouse effect and Earth grows even colder.

9   According to Edwards, there is no lack of such mechanisms. "If you add their effects together, there is more than enough feedback to make Milankovitch work," he says. "The problem now is identifying which mechanisms are at work." This is why scientists like Edwards are not yet ready to give up on the current theory. "Milankovitch cycles give us ice ages roughly when we observe them to happen. We can calculate where we are in the cycle and compare it with observation," he says. "I can't see any way of testing [Ehrlich's] idea to see where we are in the temperature oscillation."

10  Ehrlich concedes this. "If there is a way to test this theory on the sun, I can't think of one that is practical," he says. That's because variation over 41,000 to 100,000 years is too gradual to be observed. However, there may be a way to test it in other stars: red dwarfs. Their cores are much smaller than that of the sun, and so Ehrlich believes that the oscillation periods could be short enough to be observed. He has yet to calculate the precise period or the extent of variation in brightness to be expected.

11  Nigel Weiss, a solar physicist at the University of Cambridge, is far from convinced. He describes Ehrlich's claims as "utterly implausible". Ehrlich counters that Weiss's opinion is based on the standard solar model, which fails to take into account the magnetic instabilities that cause the temperature fluctuations.
米兰科维奇理论认为，北半球高纬夏季太阳辐射变化是驱动第四纪冰期旋回的主因。这个理论的核心是单一敏感区的触发驱动机制，即北半球高纬气候变化信号被放大、传输进而影响全球。最近，由于大量高分辨率及精确定年的气候变化记录的获得，从以下4个方面构成了对米氏理论的挑战：1）一些低纬地区并没有明显的10万年冰量周期，而是以2万年岁差周期为主，表明北半球冰盖的扩张、收缩变化并没有完全控制低纬区的气候变化；2）在最近几次冰消期时，南半球和低纬区的温度增高，要早于北半球冰盖的融化，表明冰消期的触发机制并非是北半球高纬夏季太阳辐射；3）大气CO2浓度在第2冰消期的增加同南极升温相一致，表明该时大气CO2浓度增加亦有可能早于北半球冰盖消融；4）南半球的末次冰盛期有可能早于北半球。这就说明单一敏感区触发驱动机制已难以圆满解释所有观察事实，天文因素控制下轨道尺度气候变化机制研究正面临理论突破的新需求和新机遇

当背景看看吧~~

seleneli

感谢分享！               

ifly

顶

精灵宝宝

应该是类似 不是原题

小耗子ss

对呀 75的答案错了 应该是E  楼主更正

cissy0010

到底是不是直接考OG 里的啊 不会吧

joyce22

顶起来~ 难道考试都直接拿OG的题了？？！！ 有刚考过的同学希望来确认下。。。 另外75题OG的答案是E吧

gwendolinecn

thanks

Arielbao

http://www.litesee.com/verbal/verbal/reading/reading-passage18.htm

-- by 会员 hanmeng (2009/12/14 16:12:30)

It's fun, thax~

hanmeng

http://www.litesee.com/verbal/verbal/reading/reading-passage18.htm