Let's Talk About: Earth's past climate

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What do detectives and climate scientists have in common? They both follow clues to learn what happened in the past. A detective looks for clues like fingerprints, receipts or DNA to figure out who committed a crime. How do we know what we know about Earth’s past climate? Climate scientists have had to come up with creative solutions to obtain data from thousands (or millions) of years ago.

We can understand Earth’s historical climate using proxies — something that tells us about past climate indirectly. One common proxy is pollen. You may have seen a layer of pollen dust coating your car or home. Pollen also coats the surface of lakes. Once pollen falls on the lake, it slowly sinks to the bottom where it forms a layer. This happens year after year, and we can use the pollen in lake sediment to learn what kind of plants lived in the area at particular times. If we know the kind of climate favored by those plants, we can infer the climate of that region.

Most trees produce one growth ring every year. The size of each ring depends on the amount of rainfall received that year. The white pine, which is common in Pennsylvania, can live for 400 years. The bristlecone pine, found in the Western United States, can live for 5,000 years. Studying tree rings all over the world helps us learn about rainfall throughout history.

We also can use proxies to learn about the ancient atmosphere. Air bubbles can be trapped in glacial ice. This air from long ago can tell us how much carbon dioxide was in the atmosphere. Carbon dioxide is important because it’s one of the gases that affects Earth’s temperature.

Climate scientists use these clues and more to solve the mystery of Earth’s past climate. Knowing how Earth’s climate has changed in the past helps us prepare for the future.

— By Brad E. Peroney Jr., program development coordinator, Carnegie Science Center

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