本帖最后由 菱形兽 于 2011-10-9 21:25 编辑
Okay, so we've been talking about theories that deal with the effects of human activity on the climate. Buttoday I'd like to talk a little bit about other theories that can explainvariations in climate. And one of the best known is called the Milankovitch Hypothesis. Um, now what the Milankovitch Hypothesis isabout? Um, it says that variations in Earth's movements, specifically in its orbit around the Sun, thesevariations lead to differences in the amount of solar energy that reaches Earth. And it's these differences inthe amount of energy that's reaching Earth from the Sun; it's what causesvariations in Earth's climate. Okay, a lot of people think of Earth's orbit around the sun as being perfectly circular, as smooth and as regular as um, say the way the hands move on a well-made watch. But just doesn'twork that way. You're probablyaware that the Earth's orbit around the Sun is not shaped like a perfectcircle. It's more of an oval, it's elliptical. But the shape of this orbit isn't consistent; it varies over time over a period ofabout a hundred thousand years. Sometimes it's a little more circular,sometimes it's more elliptical. And when Earth's orbit is moreelliptical, Earth is actually closer to the Sun during part of the year, whichmakes Earth, and in particular, the Northern Hemisphere warmer. Andwhy is that important? Well, because most of the planet's glaciers are in the Northern Hemisphere, and ifit gets too warm then glaciers will stop forming. And we already talked about how that affects Earth's overall temperature. The secondmovement involved in the Hypothesis has to do with axial tilt, the tilt of Earth's axis, thatimaginary pole that runs through the center of the Earth. And depending on the angle it tilts at, the seasons can be more or lesssevere. It makes winters cooler and summers warmer, or, what some might say isdoing now, it makes summers less hot and, more importantly, the winter lesscold, which, just like what I mentioned before, can also stop, um, prevent glaciers from forming, or cause themto melt. There's a thirdmovement the Hypothesis covers called Precession. Precession basically is thechange in the direction of Earth's axis of rotation. It would take me a million years to explain even just the basics of this movement asPrecession is quite complex. And all these details are way beyond our scope. What's important for you tounderstand is that these three movements, well they are cyclical and they work together to from, ah, toproduce complex but regular variations in Earth'sclimate and lead to the growth or decline of glaciers. Now when Milankovitchfirst proposed this theory in the 1920s, many of this colleagues were skeptical. Milankovitchdidn't have any proof. Actually there wouldn't be any evidence to support hishypothesis until the 1970s, when oceanographers were able to drill deep intothe seafloor and collect samples, samples which were then analyzed bygeologists. And from these samples they were able to put together a history ofocean temperatures going back hundreds of thousands of years. And this showedthat Earth's climate had changed pretty much the way Milankovitch's hypothesissuggested it would. So this evidence was pretty strong support for theMilankovitch hypothesis and by the 1980s most people accepted this theory. However in thelate 1980s, some scientists were exploring Devil's Hole, which is basically anextensive water-filled cave, far from the ocean in Nevada, in the western United States. Overmillions of years, ground water left deposits of a mineral called calcite on the rock within Devil's Hole. And bystudying these calcite deposits, we could determine the climate conditions, thetemperatures over the last half million years. Well, the Devil's Hole findings contradicted the one obtained during the 1970s, sobasically the questions was: were the ages of one or both of the samples wrong?Or were scientists misunderstanding the significance of the evidence? Well, um, in the1990s a new study was done on the two samples and the oceanfloor samples were found to be correct, as were the samples from Devil's Hole.And now it's generally believed that the samples from Devil's Hole correspondto variations in local climate in the western United's States rather than global climate changes.
hypothesis
[haiˈpɔθisis]
n.假说,假设,前提 variation
[ˌveəriˈeiʃən]
n.变化,变动;变体,变种;变奏(曲) oval
[ˈəuvəl]
n.椭圆形 a.椭圆形的 elliptical [i'liptikəl] adj. 椭圆的, 省略的, 简要的 pole
[pəul]
n.柱,杆;地极,磁极,电极 tilt
[tilt]
v.(使)倾斜,(使)倾侧 n.倾斜,倾侧 severe
[siˈviə]
a.严重的;严厉的;严峻的;朴素的 melt
[melt]
v.(使)融化;(使)消散,(使)逐渐消失 precession [pri'seʃən] n. 先行, [物]进动, (分点)岁差
skeptical
[ˈskeptikəl]
a.表示怀疑的 Nevada
[neˈvɑ:də]
n.内华达[美国州名] deposit
[diˈpɔzit]
n.定金;存款;矿藏 vt.使沉淀;寄存;储蓄 contradict
[ˌkɔntrəˈdikt]
vt.反驳,否认…的真实性;与…发生矛盾 correspond
[ˌkɔriˈspɔnd]
vi.(with)相符合;(to)相类似;通信 |