考满分 toefl

1 .Listen to part of a lecture in an Earth Science class.

听一段地球科学课上的部分内容。

1 .Continuing with our discussion of meteorology. You know, today, we have weather stations around the world that provide information on climate variables such as temperature and rainfall. Records from these weather stations go back to the year 1850.

继续我们关于气象学的讨论。你们知道，如今，我们在世界各地都有气象站，它们提供诸如温度和降雨量等气候变量的信息。这些气象站的记录可以追溯到 1850 年。

1 .Now, that may seem like a considerable amount of time for most of us, but for climate scientists, it was only yesterday. Understanding what earth's climate was like in the distant past requires information going back thousands, even millions of years.

对于我们大多数人来说，这可能看起来是一段相当长的时间，但对于气候科学家来说，这就像是昨天一样。要了解地球远古时期的气候，需要追溯到数千甚至数百万年前的信息。

1 .One way to get that kind of information is by using something called isotopes. The building blocks for all matter in the universe are chemical elements like carbon and oxygen. A single element can have different forms called isotopes, which differ in weight depending on how many neutrons are in each atom of the element.

获取这类信息的一种方法是使用一种叫做同位素的东西。宇宙中所有物质的构成单元是像碳和氧这样的化学元素。单一元素可以有不同的形式，被称为同位素，它们的重量因该元素每个原子中的中子数量不同而有所差异。

1 .Oxygen, one of our most common elements, has three isotopes, two of which are very useful in reconstructing climate data, oxygen 16 and oxygen 18. The numbers here refer to the weight of an atom. An atom of oxygen 18 contains two more neutrons than an atom of oxygen 16, so it weighs more. Oxygen 18 is sometimes called heavy oxygen, while oxygen 16 is light oxygen.

氧，我们最常见的元素之一，有三种同位素，其中两种在重建气候数据方面非常有用，即氧 16 和氧 18。这里的数字指的是一个原子的重量。一个氧 18 原子比一个氧 16 原子多两个中子，所以它更重。氧 18 有时被称为重氧，而氧 16 是轻氧。

1 .So why are these oxygen isotopes particularly useful for reconstructing past temperatures? Well, first, they're very abundant on Earth. The large quantity means that measurements are easy to take, allowing us to ensure measurement accuracy. And also, these isotopes have been studied for a long time, so their relationship with climate variables like temperature is already well documented.

那么，为什么这些氧同位素对重建过去的温度特别有用呢？首先，它们在地球上非常丰富。数量众多意味着很容易进行测量，这使我们能够确保测量的准确性。而且，这些同位素已经被研究了很长时间，所以它们与温度等气候变量之间的关系已经有了充分的记录。

1 .Now. Water molecules contain oxygen, so rainwater and snow do too. But as we'll see, how much of each isotope is present within them depends on temperature. And there are other regional factors as well.

现在。水分子中含有氧，所以雨水和雪也含有氧。但正如我们将看到的，它们当中每种同位素的含量取决于温度。而且还有其他的区域因素。

1 .We've drilled deep into the ice at the North and South Poles, extracting cores of old ice. These cores were formed over centuries by snow that fell and accumulated and was compressed into layers. After we've dated each layer, we can measure the ratio of heavy oxygen to light oxygen in the ice. This lets us draw a conclusion about the global temperature when the ice in each layer formed.

我们已经深入钻探了南北极的冰层，提取了古老的冰芯。这些冰芯是经过几个世纪的降雪积累并被压缩成层而形成的。在确定了每一层的年代之后，我们可以测量冰中重氧与轻氧的比例。这使我们能够得出关于每一层冰形成时全球温度的结论。

1 .This relationship between temperature and isotope ratio depends on two things. The first is that air currents that carry water vapor typically move from tropical areas, warm areas near the equator, toward the north and south poles.

温度和同位素比例之间的这种关系取决于两件事。第一，携带水蒸气的气流通常从热带地区，即赤道附近的温暖地区，向南北极移动。

1 .Second, because it's heavier, oxygen 18 falls out of the air more easily than oxygen 16. So when rain falls in tropical areas, it preferentially removes oxygen 18 from the air, that leaves less oxygen 18 in the air as air currents move toward the poles. So precipitation at the poles contains less oxygen 18 than precipitation near the equator.

第二，因为氧 18 更重，所以它比氧 16 更容易从空气中沉降下来。所以当热带地区降雨时，它会优先从空气中去除氧 18，当气流向两极移动时，空气中的氧 18 就会更少。所以两极的降水比赤道附近的降水含有的氧 18 更少。

1 .Now, um when global temperatures are low, like during an ice age, even tropical areas experience drops in temperature. The cooler temperatures encourage more precipitation near the equator, and even more oxygen 18 is removed from the air, so there will be even less that falls at the poles. That's the reason why polar ice cores can tell us what global temperatures were like when each layer formed. The colder the global temperature, the less oxygen 18.

现在，嗯，当全球温度较低时，比如在冰河时代，即使是热带地区也会经历温度下降。较低的温度会促使赤道附近有更多的降水，而且更多的氧 18 会从空气中被去除，所以落到两极的氧 18 就会更少。这就是为什么极地冰芯能够告诉我们每一层冰形成时全球温度是什么样的。全球温度越低，氧 18 就越少。

1 .Interestingly, a somewhat similar phenomenon is exhibited by corals, colonies of invertebrates that grow in earth's oceans. Corals use oxygen to build their skeletons. Corals more easily use oxygen 18 to build because the heavier isotope is more stable. In addition, more oxygen 18 is available in seawater during cold periods, so more is absorbed by the coral.

有趣的是，在地球海洋中生长的无脊椎动物群体 —— 珊瑚，也表现出了一些类似的现象。珊瑚利用氧来构建它们的骨骼。珊瑚更容易利用氧 18 来构建骨骼，因为较重的同位素更稳定。此外，在寒冷时期，海水中有更多的氧 18，所以珊瑚会吸收更多的氧 18。

1 .And their skeletons can be read through time in a similar way as tree rings. You know the circular patterns you see in the stump after a tree is cut down? Those growth rings show the years in a tree's life. Well, in corals, there are highly dense bands in the skeleton that are um deposited in the winter, when the coral grows slowly, much less dense bands form in the summer, when it grows quickly. Each pair of bands represents one year. So coral records are very detailed.

而且它们的骨骼可以像树木的年轮一样随着时间被解读。你们知道一棵树被砍倒后树桩上看到的圆形图案吧？那些年轮显示了一棵树生命中的年份。嗯，在珊瑚中，骨骼中有非常密集的条带，这些条带是在冬天沉积的，那时珊瑚生长缓慢，而在夏天珊瑚生长迅速，就会形成密度小得多的条带。每一对条带代表一年。所以珊瑚的记录非常详细。

1 .These bands from which we measure isotope quantities can be dated almost to the month. That's one advantage this has over other methods, like analyzing ice cores. But on the other hand, the amount of information from corals is limited by their lifespan, about 1000 years at maximum.

我们测量同位素数量所依据的这些条带几乎可以精确到月份来确定年代。这是它相对于其他方法，比如分析冰芯的一个优势。但另一方面，来自珊瑚的信息数量受到它们寿命的限制，最长大约为 1000 年。