Listen to part of a lecture in an environmental science class.
Ok, so we have been talking about theories that deal with the effects of human activity on the climate.
But today I'd like to talk a little bit about other theories that can explain variations in climate.
And one of the best-known is called the Milankovitch Hypothesis.
Now what the Milankovitch Hypothesis is about?
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.
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.
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.
You are probably aware that the earth's orbit around the sun, it is not shaped like a perfect circle.
It is more of an oval, it is elliptical.
But the shape of this orbit isn't consistent; it varies over time, over a period of about a thousand years.
Sometimes it is a little more circular, sometimes it is more elliptical.
And when earth's orbit is more elliptical, earth is actually closer to the sun during part of the year.
Which makes earth, and in particular, the northern hemisphere, warmer.
And why is that important?
Well, because most of the planet's glaciers are in the northern hemisphere, and if it gets too warm, then glaciers will stop forming.
And we've already talked about how that affects earth's overall temperature.
The second movement involved in the hypothesis has to do with axial tilt.
The tilt of earth's axis, that imaginary pole that runs through the center of the earth.
And depending on the angle it tilts at, the seasons can be more or less severe.
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.
Which just like what I mentioned before, can also stop, prevent glaciers from forming, or cause them to melt.
There is a third movement the hypothesis covers called precession.
Precession basically is the change in the direction of earth's axis of rotation.
It will take me a million years to explain even just the basics of this movement as precession is quite complex.
And all these details are way beyond our scope.
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.
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.
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,
and this showed that earth's climate had changed pretty much the way Milankovitch's hypothesis suggested it would.
So this evidence was pretty strong support for the Milankovitch Hypothesis.
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.
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.