Rapid Climate Change

Kendrick Taylor

Full Text Sections
Abstract
Introduction
Ice, the Museum of Climate
Ice as Thermometer
The Greenland Weather Report
Climate, from the Bottom Down
Climate’s Control Mechanism
Three Climate Modes
Tampering with Our Stable Mode?
Climate and Choices
Bibliography

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Rapid Climate Change

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

Over the course of geologic history, the earth’s environment has been far from static. Indeed, 600 million years ago the atmosphere lacked sufficient oxygen to support animal life. More recently, as shown by sediments recording conditions over the past 500,000 years, the planet’s climate varied between at least two different states.

The record from the past 150,000 years is particularly well preserved, offering details about these repeated climate changes. Between about 131,000 and 114,000 years ago there was a warm period like today’s climate, referred to in Europe as the Eemian or globally as Marine Isotope Stage 5e. This was followed by the Wisconsin ice age, which ended about 12,000 years ago when the current relatively warm Holocene period began.

Although the past half-million years constitutes the current-events period in geologic time, on a human time scale the events I just described are in the distant past. Because their time scales are so long, I used to believe that changes in climate happened slowly and would never affect me. After all, a single climate cycle that includes an ice age and a warm period lasts 150,000 years and is controlled by gradually changing orbital parameters of the earth. It did not seem possible that climate cycles that lasted so long could change perceptibly during my lifetime. Even greenhouse-induced climate changes are normally predicted to happen gradually over several generations, allowing an opportunity for society to adapt.

My attitude changed profoundly while I was working on a project funded by the National Science Foundation to develop a climate record for the past 110,000 years. By examining ice cores from Greenland, my colleagues and I determined that climate changes large enough to have extensive impacts on our society have occurred in less than 10 years. Now I know that our climate could change significantly in my lifetime. We are still a long way from being able to predict such a change, but we are getting closer to understanding how it might take place. A pressing concern is whether anthropogenic changes to our planet’s atmosphere might perturb the climate’s stability.

Ice, the Museum of Climate One can learn a lot about what controls climate by studying glacial ice. When snow falls, it collects insoluble dust particles, soluble compounds and the water in the snow itself. In some places more snow falls in a year than melts or sublimates away. Annual layers of snow pile up, with atmospheric gases filling the open pores between snow crystals. The weight of accumulating snow compresses the pores in the snow below, turning the snow into ice and trapping the atmospheric gases. The dust, chemicals and gases in the ice reflect the environment along the water’s journey from distant sources to the glacier. They record how cold it was, how much snow fell in a year, what the concentration of atmospheric gases was and what the atmospheric circulation patterns were.

We can identify annual layers in the ice because the concentration of sea salts, nitrate and mineral dust and the gas content in winter snow are different than in summer snow. We count the annual layers to determine the age of the ice, and by measuring the thickness of the annual layers we can determine how much snow fell each year. The gas trapped between ice crystals offers a sample of the ancient atmosphere, and we can use it to determine what the concentrations of greenhouse gases such as carbon dioxide and methane were long before human beings measured the atmosphere directly. General patterns of atmospheric circulation can be reconstructed by using tracers such as soluble chemicals (for example, nitrate, ammonium, sodium and calcium) and rare earth elements in insoluble dust particles to determine how wind moved air and dust from the source regions for these compounds to the drilling site.

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.

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.

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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© American Scientist 1999