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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.