Temperature has yo-yoed over the ages as wildly as it does through any single year. Like natural thermometers, ice cores have recorded these fluctuations, which scientists can "read" by examining isotopes of oxygen and hydrogen in water trapped in the ice. These isotopes come in two forms--"light" and "heavy." Light isotopes have regular hydrogen and oxygen, while heavy isotopes have either hydrogen with an extra neutron or oxygen with one or two additional neutrons. Since heavy isotopes precipitate out of the atmosphere more quickly than light ones, scientists can measure the ratio between the two isotopes to estimate the temperature at any given time. The data in this graph, gleaned from a core drilled in central Greenland, shows how temperatures have risen by more than 20°C (36°F) since the height of the Ice Age 25,000 years ago.
Cuffey, K.M., Clow, G.D., Alley, R.B., Stuiver, M., Waddington, E.D. and Saltus, R.W., 1995, Large Arctic-temperature change at the Wisconsin-Holocene transition, Science 270, 455-458.
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