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Listen to part of a lecture in an environmental science class.
听下面一段环境科学课程
Ok, so we have been talking about theories that deal with the effects of human activity on the climate.
好吧，我们前面讨论了一些关于研究人类活动对气候影响的理论。
But today I'd like to talk a little bit about other theories that can explain variations in climate.
不过今天我想讲一讲其他解释气候变化的理论，
And one of the best-known is called the Milankovitch Hypothesis.
其中最著名的理论当属米兰科维奇假说。
Now what the Milankovitch Hypothesis is about?
嗯，那么米兰科维奇假说是到底是怎么回事呢？
It says that variations in earth's movements, specifically in its orbit around the sun, these variations lead to differences in the amount of solar energy that reaches the earth.
嗯，假设认为，地球运行的变化，特别是绕太阳轨道的变化，会导致太阳到达地球能量的不同。
And it is these differences in the amount of energy that's reaching earth from the sun; it is what causes variations in earth's climate.
并且，正是这些抵达地球能量的差异导致了地球气候的变化。
Ok, a lot of people think of earth's orbit around the sun as being perfectly circular, as smooth and as regular as, say, the way that hands move on a well -made watch, but it just doesn't work that way.
好的，许多人认为地球绕太阳的轨道是正圆、平滑弧线形的，地球围绕太阳的运动就像表针在高质量的手表上运动一样。但是事实并不如此。
You are probably aware that the earth's orbit around the sun, it is not shaped like a perfect circle.
可能你们也知道了，地球绕太阳的轨道实际上不是一个标准的圆形，
It is more of an oval, it is elliptical.
而更像一个椭圆。
But the shape of this orbit isn't consistent; it varies over time, over a period of about a thousand years.
不过，轨道的形状并不是始终如一的；它会随时间变化，变化周期为一百年左右。
Sometimes it is a little more circular, sometimes it is more elliptical.
有时它更圆，有时它更扁。
And when earth's orbit is more elliptical, earth is actually closer to the sun during part of the year.
并且，当地球轨道更加椭圆的时候，地球事实上在一年的某个时候距离太阳更近。
Which makes earth, and in particular, the northern hemisphere, warmer.
这使得地球，特别是北半球更加温热。
And why is that important?
那么，为什么这很重要呢？
Well, because most of the planet's glaciers are in the northern hemisphere, and if it gets too warm, then glaciers will stop forming.
嗯，因为行星大多数的冰川都在北半球，如果那里温度太高，冰川就无法维持了。
And we've already talked about how that affects earth's overall temperature.
前面我们业已讲述过这种效应会如何影响地球整体的温度了。
The second movement involved in the hypothesis has to do with axial tilt.
假说的第二种运动与地轴倾斜有关，
The tilt of earth's axis, that imaginary pole that runs through the center of the earth.
地轴是我们虚构的穿过地心的一个倾斜的直线轴。
And depending on the angle it tilts at, the seasons can be more or less severe.
根据它倾斜的角度，季节可能更加极端或者不极端。
It makes winters cooler and summers warmer, or what some might say it is doing now; it makes summers less hot, and more importantly, the winters less cold.
倾斜角的变化，会使冬天更冷，夏天更热，或者与之相反，就像一些人认为现在的状况一样，它使夏季不热，冬季不冷。更重要的是，冬季不冷，
Which just like what I mentioned before, can also stop, prevent glaciers from forming, or cause them to melt.
正如我前面讲述的那样，使得冰川不再形成，甚至融化。
There is a third movement the hypothesis covers called precession.
假说还包括了第三种名为“旋式推进”运动。
Precession basically is the change in the direction of earth's axis of rotation.
旋进基本上是地轴旋转方向的变化。
It will take me a million years to explain even just the basics of this movement as precession is quite complex.
因为它特别复杂，即便花费了一个世纪我们也没弄清楚。
And all these details are way beyond our scope.
此外，其运动细节也超出了我们的知识范围。
What's important for you to understand is that these three movements, well, they are cyclical, and they work together to form, to produce complex but regular variations in earth's climate, and lead to the growth or decline of glaciers.
你们特别需要了解的是，这三种运动都是周期性的，他们共同导致了复杂但有规律的地球气候变化，并且使得冰川增长或衰退。
Now, when Milankovitch first proposed this theory in the 1920s, many of his colleagues were skeptical.
20 世纪 20 年代米兰科维奇假说刚发表的时候，在许多同行都持怀疑态度。
Milankovitch didn't have any proof.
米兰科维奇假说确实没有任何的证据的支持。
Actually there wouldn't be any evidence to support his hypothesis until the 1970s, when oceanographers were able to drill deep into the seafloor and collect samples, samples which were then analyzed by geologists.
事实上，直到 20 世纪 70 年代之前，不能有任何支持它的证据，因为直到此后海洋学家才能够潜入海底钻孔并且取样，然后将样本交给地质学家分析。
And from these samples they were able to put together a history of ocean temperatures going back hundreds of thousands of years,
由此，我们能够得出几十万年的关于海洋气候的历史状况。
and this showed that earth's climate had changed pretty much the way Milankovitch's hypothesis suggested it would.
这些证明了地球气候的变化恰恰与米兰科维假说所描述的一样。
So this evidence was pretty strong support for the Milankovitch Hypothesis.
这样的话，这些证据很有力地证实了对米兰科维奇假说。
And by the 1980s, most people accepted this theory.
到了 20 世纪 80 年代，多数人接受了这一理论。
However, in the late 1980s, some scientists were exploring Devil's Hole, which is basically an extensive water-filled cave, far from the ocean, in Nevada, in the western United States.
然而，在 20 世纪 80 年代后期，一些科学家探索位了于圣海伦娜的充满了水的大洞穴，其地理位置在美国西部的，远离内华达的海洋。
Over millions of years, groundwater left deposits of a mineral called calcite, on the rock within Devil's Hole.
在过去的几百万年，在圣海伦娜地下水沉淀了一种叫做方解石的矿物质。
And by studying these calcite deposits, we can determine the climate conditions, the temperatures over the last half million years.
通过对这些方解石的研究，我们能够判定气候的情况，了解过去 50 万年气温的变化。
Well, the Devil' s Hole findings contradicted the ones obtained during the 1970s, so basically the question was, were the ages of one or both the samples were wrong, or were scientists misunderstanding the significance of the evidence.
不过，圣海伦娜的发现与在 20 世纪 70 年代的观点是相互矛盾的，这样的话，一个根本的问题产生了:到底是其中一个样本的年代有问题，还是两个样本的年代都有问题？抑或科学家误解了证据的意义？
Well, in the 1990s, a new study was done on the two samples. And the ocean floor samples were found to be correct, as were the samples from Devil's Hole.
最后，到了 20 世纪 90 年代，科学家们对这两个样本重新进行了的研究，并且发现海底获取的样本是正确的，在圣海伦娜发现的也是正确的。
And now it is generally believed that the sample from Devil's Hole correspond to variations in local climate, in the western United States, rather than global climate changes.
而现在，一般都认为圣海伦娜发现的样本仅仅体现了美国西部当地的气候，不能代表全球的气候变化。
