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HOME > PAST ISSUE > July-August 1999 > Article Detail


Rapid Climate Change

New evidence shows that earth's climate can change dramatically in only a decade. Could greenhouse gases flip that switch?

Kendrick Taylor

Ice as Thermometer

Air temperature is naturally of primary interest when we talk about climate, and fortunately we have three ways to determine what it was in the past. First, we can measure the isotopic composition of the oxygen and hydrogen in the ice. When water vapor in clouds condenses, the ratio of oxygen-18 to oxygen-16 and the hydrogen-2/hydrogen-1 ratio are affected by the ambient temperature; the colder the cloud, the lower the ratio. Measuring how the ratios of these isotopes changes along an ice core gives us a good idea how the air temperature changed over time.

Figure 2. Different equipment is used . . .Click to Enlarge Image

The second way to determine prehistoric temperatures is to measure the isotopic composition of the nitrogen gas trapped in the ice. At depths between about 5 and 50 meters in an ice sheet, air can move in interconnected pores but is sheltered from mixing by the wind. Nitrogen-15 slowly moves toward colder locations, and nitrogen-14 slowly moves toward warmer locations. This process creates a near-surface gradient in the nitrogen-15/nitrogen-14 ratio that depends on the near-surface temperature gradient. The resulting isotopic composition of the nitrogen trapped in the ice depends on the difference between the surface temperature and the temperature at depth at the time when the ice overburden pressure closes the pores and traps the nitrogen gas in the ice. Variations in the isotopic composition of the nitrogen along a core show when and by how much the surface temperature changed.

Figure 3. Richard Alley of Pennsylvania State . . .Click to Enlarge Image

Finally, because of the large thermal inertia of an ice sheet, the current temperature distribution in an ice sheet is strongly influenced by what the surface temperature was in the past. The physics is similar to cooking a large frozen turkey. If we move the turkey directly from the freezer into the oven, the outside of the turkey will be done before the inside even defrosts. By modeling the current thermal state of the turkey, or an ice sheet, we can determine the history of the turkey's, or ice sheet's, surface temperature. The physics of these three approaches is well understood; together they allow us to reconstruct how the surface temperature changed during the past several hundred thousand years.

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