Calendar in Ice

How useful would a calendar be if it didn't have months or dates on each page? So just like a calendar records history by days, weeks and months. A glacier records history over years. Hundreds of years, or even hundreds of thousands of years. Perhaps it's easier to understand how glaciers tell time if we look at something more familiar, like tree rings. How do tree rings measure time? If you imagine cutting down a tree and then looking at the stump of that tree, you'll notice that there are rings. These rings help us to measure time. The older rings of the tree are going to be in the center, because they were formed when the tree first started to grow. On the outside, the rings are going to be younger. Year after year, the tree will continue to add rings to it as it grows. So, like a calendar, tree rings also measure time. Let's take a closer look. In the stump, we can see the tree rings. Now notice that there are two kinds of rings, thick, light colored rings, and thin, dark colored rings. The thick light colored ring show more growth in the spring and summer when the weather becomes warm and there's plenty of rain. The dark colored rings show less growth in the autumn, and the winter when it starts to get cold and there's less rain together, one light ring, and one dark ring tell the story of one year. By counting the rings from the center to the outside, we can tell the age of the tree. What do tree rings record? Each ring has its own story to tell. By looking at the different rings in the tree, we can actually get an idea of what's happened in that area in the past. Tree rings give us clues to help understand how the environment has changed from season to season. Let's take a look at this tree stump. Can you see how some of the light colored rings are thicker than other light colored rings? The thicker rings mark the seasons when there was more rain. While the thinner rings marked the seasons where there was less rain or a drought, tree rings can also tell us about other things, like when there was a forest fire in the area, where it harmful insects attacked to the tree. By looking at all the information recorded in the rings, we can begin to see how the environment around that tree has changed over time. How do glaciers measure time? Much like the rings of a tree helped to tell the story of the environment around that tree, the layers of a glacier helped to tell the story of the climate around that glacier. Let's see if we can figure out a way to tell the difference from a wet season to a dry season in a glacier. A thicker, cleaner layer of snow is accumulated during a wet season, where a thinner dirty layer of snow is accumulated during a dry season. Look at this picture of the edge of a glacier. What's your estimate of how many years of recorded between the ice at the bottom of the picture and the ice at the top of the picture? Do you see only one or two layers? 5 or ten layers. No, there are many more layers than that. So this glacier carries a record that's many decades long. Like a calendar, the layers of the glacier also measure time. Sometimes other materials in the glacier can also help scientists tell time. Well, one kind of material that helps with telling time is the product of volcanic gases. Let's use Mount Vesuvius as an example. History tells us that in 79 AD, Mount Vesuvius erupted and spewed ash and gases containing sulfur into the atmosphere. Scientists can look at ice cores collected from glaciers to find this layer containing products of the volcanic sulfur gas. Because we know that Mount Vesuvius erupted in 79 AD, they know that the layer containing the products was formed in 79 AD. Scientists can use this layer to help them measure time. The layers below the layer containing volcanic products are older than 79 AD and the layers above are younger. The information that we get from the ice cores helps scientists to start putting together this calendar. From there, scientists are working to fill in the details to understand earth's climate history.