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Listen to part of a lecture in an environmental science class.
聽下面一段環境科學課程
Ok, so we have been talking about theories that deal with the effects of human activity on the climate.
好吧，我們前面討論了一些關于研究人類活動對氣候影響的理論。
But today I'd like to talk a little bit about other theories that can explain variations in climate.
不過今天我想講一講其他解釋氣候變化的理論，
And one of the best-known is called the Milankovitch Hypothesis.
其中最著名的理論當屬米蘭科維奇假說。
Now what the Milankovitch Hypothesis is about?
嗯，那么米蘭科維奇假說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，假設認為，地球運行的變化，特別是繞太陽軌道的變化，會導致太陽到達地球能量的不同。
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，正是這些抵達地球能量的差異導致了地球氣候的變化。
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，許多人認為地球繞太陽的軌道是正圓、平滑弧線形的，地球圍繞太陽的運動就像表針在高質量的手表上運動一樣。但是事實并不如此。
You are probably aware that the earth's orbit around the sun, it is not shaped like a perfect circle.
可能你們也知道了，地球繞太陽的軌道實際上不是一個標準的圓形，
It is more of an oval, it is elliptical.
而更像一個橢圓。
But the shape of this orbit isn't consistent; it varies over time, over a period of about a thousand years.
不過，軌道的形狀并不是始終如一的；它會隨時間變化，變化周期為一百年左右。
Sometimes it is a little more circular, sometimes it is more elliptical.
有時它更圓，有時它更扁。
And when earth's orbit is more elliptical, earth is actually closer to the sun during part of the year.
并且，當地球軌道更加橢圓的時候，地球事實上在一年的某個時候距離太陽更近。
Which makes earth, and in particular, the northern hemisphere, warmer.
這使得地球，特別是北半球更加溫熱。
And why is that important?
那么，為什么這很重要呢？
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，因為行星大多數的冰川都在北半球，如果那里溫度太高，冰川就無法維持了。
And we've already talked about how that affects earth's overall temperature.
前面我們業已講述過這種效應會如何影響地球整體的溫度了。
The second movement involved in the hypothesis has to do with axial tilt.
假說的第二種運動與地軸傾斜有關，
The tilt of earth's axis, that imaginary pole that runs through the center of the earth.
地軸是我們虛構的穿過地心的一個傾斜的直線軸。
And depending on the angle it tilts at, the seasons can be more or less severe.
根據它傾斜的角度，季節可能更加極端或者不極端。
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.
傾斜角的變化，會使冬天更冷，夏天更熱，或者與之相反，就像一些人認為現在的狀況一樣，它使夏季不熱，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.
正如我前面講述的那樣，使得冰川不再形成，甚至融化。
There is a third movement the hypothesis covers called precession.
假說還包括了第三種名為“旋式推進”運動。
Precession basically is the change in the direction of earth's axis of rotation.
旋進基本上是地軸旋轉方向的變化。
It will take me a million years to explain even just the basics of this movement as precession is quite complex.
因為它特別復雜，即便花費了一個世紀我們也沒弄清楚。
And all these details are way beyond our scope.
此外，其運動細節也超出了我們的知識范圍。
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.
你們特別需要了解的是，這三種運動都是周期性的，他們共同導致了復雜但有規律的地球氣候變化，并且使得冰川增長或衰退。
Now, when Milankovitch first proposed this theory in the 1920s, many of his colleagues were skeptical.
20 世紀 20 年代米蘭科維奇假說剛發表的時候，在許多同行都持懷疑態度。
Milankovitch didn't have any proof.
米蘭科維奇假說確實沒有任何的證據的支持。
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.
事實上，直到 20 世紀 70 年代之前，不能有任何支持它的證據，因為直到此后海洋學家才能夠潛入海底鉆孔并且取樣，然后將樣本交給地質學家分析。
And from these samples they were able to put together a history of ocean temperatures going back hundreds of thousands of years,
由此，我們能夠得出幾十萬年的關于海洋氣候的歷史狀況。
and this showed that earth's climate had changed pretty much the way Milankovitch's hypothesis suggested it would.
這些證明了地球氣候的變化恰恰與米蘭科維假說所描述的一樣。
So this evidence was pretty strong support for the Milankovitch Hypothesis.
這樣的話，這些證據很有力地證實了對米蘭科維奇假說。
And by the 1980s, most people accepted this theory.
到了 20 世紀 80 年代，多數人接受了這一理論。
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 世紀 80 年代后期，一些科學家探索位了于圣海倫娜的充滿了水的大洞穴，其地理位置在美國西部的，遠離內華達的海洋。
Over millions of years, groundwater left deposits of a mineral called calcite, on the rock within Devil's Hole.
在過去的幾百萬年，在圣海倫娜地下水沉淀了一種叫做方解石的礦物質。
And by studying these calcite deposits, we can determine the climate conditions, the temperatures over the last half million years.
通過對這些方解石的研究，我們能夠判定氣候的情況，了解過去 50 萬年氣溫的變化。
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.
不過，圣海倫娜的發現與在 20 世紀 70 年代的觀點是相互矛盾的，這樣的話，一個根本的問題產生了:到底是其中一個樣本的年代有問題，還是兩個樣本的年代都有問題？抑或科學家誤解了證據的意義？
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 世紀 90 年代，科學家們對這兩個樣本重新進行了的研究，并且發現海底獲取的樣本是正確的，在圣海倫娜發現的也是正確的。
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.
而現在，一般都認為圣海倫娜發現的樣本僅僅體現了美國西部當地的氣候，不能代表全球的氣候變化。
