The Heat Is Online

Rapid Climate Change: A Primer

The American Scientist

July-August, Volume 87, No. 4

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

Abstract:


Much to the surprise of investigators, evidence is mounting that major changes in the earth’s climate can take place in a very short time. Data from ice cores and ocean sediments suggest, for example, that 11,650 years ago the climate in Greenland switched from ice-age conditions to the current relatively warm conditions (a warming of 5 to 10 degrees Celsius on average) in only 40 years. The author describes the oceanic currents that influence climate and establish its stability, as well as "triggers" that may perturb changes -- including the possibility that "greenhouse" warming could invoke a rapid switch.

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.

The Greenland Weather Report

In Greenland, annual ice layers are stacked up like thousands of annual weather reports. In 1982, a European and American team made the first attempt to read that record, by recovering an ice core from southern Greenland. Measurements on the ice core indicated that about 11,700 years ago the climate of the North Atlantic region changed from a dry and cold ice age to the current warmer and wetter Holo cene. Altogether it took 1,500 years for the climate transition to be complete and a few thousand more years to melt most of the ice, but the surprise was that most of the transition occurred in only 40 years. This was only one record, and it came from a single 10-centimeter-diameter ice core. Still, this finding was impossible to ignore and too puzzling to comprehend.

In 1993, Americans and Europeans led by Paul Mayewski of the University of New Hampshire and Bernhard Stauffer of the University of Bern in Switzerland finished recovering two new ice cores from the summit of the Greenland ice sheet. More than 40 university and national laboratories participated in the projects. We shared samples, spent time in one another’s labs, replicated one another’s results, proposed ideas, tore them apart and then jointly proposed better ones.

One of the justifications for these new cores, located 30 kilometers apart, was to verify and learn more about the 40-year change in climate, an event observed in both cores. The records stored in these cores were more detailed than before and showed that within a 20-year period at the summit of Greenland, where ice is thickest, the amount of snow deposited each year doubled, average annual surface temperature increased by 5 to 10 degrees Celsius and wind speeds increased. The same ice cores also showed that the spatial extent of sea ice decreased, atmospheric-circulation patterns changed, and the size of the world’s wetlands increased. Many of these shifts in parameters, including at least a 4-degree Celsius increase in the average annual air temperature, happened in less than 10 years. These changes were not restricted to Greenland; the global nature of many of these ice-core records showed that low-latitude, continental-scale regions rapidly got warmer and wetter. The most dramatic change occurred 11,700 years ago. But we also found comparable anomalies every several thousand years during the Wisconsin ice age (see Figure 6). Further, Antarctic ice cores also show comparable climate transitions at these times.

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