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John Wheeler, WDAY, Published December 02 2013

Weather Talk: Variety of methods used to reconstruct past climate

We often hear that Earth’s climate has warmed, but most people probably have not considered how past climate is reconstructed.

After all, the thermometer record only dates back about 150 years. In order to determine the temperatures of the past, temperatures are inferred from other sources. Tree rings can be helpful but only for places on Earth with trees and only for the past few hundred years. Ice and lake sediment cores can reveal clues going back thousands of years through chemical analysis, including ratios of isotopes of oxygen and carbon dioxide as well as fossil pollen. There is geological evidence that comes from estimates of the extent of glaciers around the world.

Much of what we know about the really distant past (millions of years) comes from ocean sediment cores. A very general estimate can be derived from the rate of accumulation of shellfish within these cores. None of these methods are particularly accurate. But because they often reveal similar, overlapping patterns, confidence in the conclusions are reasonably high.

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