Paleoclimatology? What is that?

Friday, February 20, 2015 by Christina Hernandez

These days, we hear a lot about climate change and global warming. The Earth has been around a lot longer than humans, and we only started using thermometers a couple hundred years ago. So how do scientists know what the temperature or sea level were like in the far distant past? They use a number of different proxies to help draw conclusions about past climate. A proxy is a measurement that we can know for sure, and represents something that we can’t measure directly. An example are tree rings. Trees grow at different rates in the summer and winter, leading to a pattern that is visible when the tree has been cut down (there are also ways to drill out a thin stick from a tree to look at its rings without killing it). If there was a drought or some other condition that prevents trees from growing well, the ring for that year would be skinnier, leaving a clue behind for scientists. The proxy is the pattern of tree ring widths, which scientists then use as evidence of droughts in the past.

There are few trees that are older than 500 years, but luckily there are a number of other proxies to choose. One that is used very often is mud! Particularly for people studying paleoceanography, the study of the ancient ocean, sediment records can provide information about the environment for hundreds of thousands of years into the past. At the very bottom of the ocean, in places where the water is more than a mile and a half deep, sediment accumulates slowly, about 1cm every thousand years. In amongst this mud, we can find tiny shells, about the size of a grain of sand, that come from a group of animals called foraminifera. Their shells are made of calcium carbonate, just like clam shells or coral skeletons. By studying these animals while they are alive, we have learned how the chemistry of their shell changes based on the temperature and salinity of the water that they live in. Then, if we study the chemistry of shells found 1 meter below the sea floor, we can gain clues about what the temperature of the ocean was 100 thousand years ago.

     

                  Ammonia beccarii a type of Foraminifera                                                                                           Foraminifera tests (shells)

It’s important to remember that, because these measurements are done through proxies instead of directly measuring the temperature of the water, it takes a lot of evidence in order for scientists to feel confident about the conclusions they draw. If we use many different types of proxies from many different locations and we keep getting the same answer, then we can start to believe that our answer is correct.



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