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CS_Ch12_ClimateChange 3/1/2005 4:56 PM Page 757 Activity 2 Paleoclimates temperature (°C) 2 Geo Words 0 isotope: one of two or more kinds of atoms of a given chemical element that differ in mass because of different numbers of neutrons in the nucleus of the atoms. –2 –4 –6 0 50,000 100,000 150,000 200,000 250,000 300,000 350,000 400,000 –8 years before present Figure 3 Temperature variation over the past 420,000 years, relative to the modern surface temperature at Vostok (–55.5°C). Bubbles of air trapped in the ice contain samples of the atmosphere from the time when the snow fell. Paleoclimatologists study the oxygen in the water molecules in the ice.Almost all of the oxygen atoms in the atmosphere are in two forms, called isotopes.The two isotopes are oxygen-16 (written 16O) and oxygen-18 (written 18O).They are the same chemically, but they have slightly different weights. 18O is slightly heavier than 16O.The proportion of these two isotopes in snow depends on average global temperatures. Snow that falls during periods of warmer global climate contains a greater proportion of 18O, and snow that falls during periods of colder global climate contains a smaller proportion.The ratio of 18O to 16O can be measured very accurately with special instruments.Another important way of using the glacier ice to estimate global temperature is to measure the proportions of the two naturally occurring isotopes of hydrogen: 1H, and 2H (which is called deuterium). The air bubbles in the ice contain carbon dioxide.The amount of carbon dioxide in the glacier ice air bubbles depends on the amount of carbon dioxide in the air at that time.The amount of carbon dioxide in the atmosphere can be correlated to global temperatures. During times when the paleoclimate is thought to have been warm, the ice core record shows relatively higher levels of atmospheric carbon dioxide compared to times of interpreted colder climate. Measurements of carbon dioxide taken from the cores give a global picture, because carbon dioxide is uniformly distributed in the global atmosphere. A third component of the ice that yields clues to paleoclimates is dust. During colder climates, winds tend to be stronger.The stronger winds erode more dust, and the dust is deposited in small quantities over large areas of the Earth. 757 Coordinated Science for the 21st Century