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 ver the course of geologic history, in climate happened slowly and would The dust, chemicals and gases in the Othe earth’s environment has been never affect me. After all, a single cli- ice reflect the environment along the far from static. Indeed, 600 million mate cycle that includes an ice age and water’s journey from distant sources to years ago the atmosphere lacked suffi- a warm period lasts 150,000 years and the glacier. They record how cold it cient oxygen to support animal life. is controlled by gradually changing or- was, how much snow fell in a year, More recently, as shown by sediments bital parameters of the earth. It did not what the concentration of atmospheric recording conditions over the past seem possible that climate cycles that gases was and what the atmospheric 500,000 years, the planet’s climate var- lasted so long could change perceptibly circulation patterns were. ied between at least two different states. during my lifetime. Even greenhouse- We can identify annual layers in the The record from the past 150,000 induced climate changes are normally ice because the concentration of sea years is particularly well preserved, of- predicted to happen gradually over salts, nitrate and mineral dust and the fering details about these repeated cli- several generations, allowing an oppor- gas content in winter snow are differ- mate changes. Between about 131,000 tunity for society to adapt. ent than in summer snow. We count and 114,000 years ago there was a My attitude changed profoundly the annual layers to determine the age warm period like today’s climate, re- while I was working on a project fund- of the ice, and by measuring the thick- ferred to in Europe as the Eemian or ed by the National Science Foundation ness of the annual layers we can deter- globally as Marine Isotope Stage 5e. to develop a climate record for the past mine how much snow fell each year. This was followed by the Wisconsin ice 110,000 years. By examining ice cores The gas trapped between ice crystals age, which ended about 12,000 years from Greenland, my colleagues and I offers a sample of the ancient atmos- ago when the current relatively warm determined that climate changes large phere, and we can use it to determine Holocene period began. enough to have extensive impacts on what the concentrations of greenhouse Although the past half-million years our society have occurred in less than gases such as carbon dioxide and constitutes the current-events period in 10 years. Now I know that our climate methane were long before human be- geologic time, on a human time scale could change significantly in my life- ings measured the atmosphere directly. the events I just described are in the dis- time. We are still a long way from be- General patterns of atmospheric circu- tant past. Because their time scales are ing able to predict such a change, but lation can be reconstructed by using so long, I used to believe that changes we are getting closer to understanding tracers such as soluble chemicals (for how it might take place. A pressing example, nitrate, ammonium, sodium concern is whether anthropogenic and calcium) and rare earth elements Kendrick Taylor is a research professor with the changes to our planet’s atmosphere in insoluble dust particles to determine Water Resources Center of the Desert Research might perturb the climate’s stability. how wind moved air and dust from Institute. He received his B.S. in geophysics from the Colorado School of Mines, his M.S. in geo- the source regions for these com- physics from the University of Wisconsin, Ice, the Museum of Climate pounds to the drilling site. Madison, and his Ph.D. in hydrogeology/hydrolo- One can learn a lot about what controls gy from the University of Nevada, Reno. Taylor climate by studying glacial ice. When Ice as Thermometer currently is the Chief Scientist for the National snow falls, it collects insoluble dust Air temperature is naturally of primary Science Foundation’s WAISCORES program, particles, soluble compounds and the interest when we talk about climate, which is recovering the first of two deep Antarctic water in the snow itself. In some places and fortunately we have three ways to ice cores. Although his major recent research inter- more snow falls in a year than melts or determine what it was in the past. est has been in the use of polar ice cores to develop sublimates away. Annual layers of First, we can measure the isotopic com- paleoclimatic records for use in improving under- snow pile up, with atmospheric gases position of the oxygen and hydrogen standing of the earth’s climate system, he has also led projects assessing groundwater supplies in the filling the open pores between snow in the ice. When water vapor in clouds U.S. and abroad. Address: Water Resources crystals. The weight of accumulating condenses, the ratio of oxygen-18 to Center of the Desert Research Institute, 2215 snow compresses the pores in the oxygen-16 and the hydrogen-2/hydro- Raggio Parkway, Reno, NV 89512-1095. Internet: snow below, turning the snow into ice gen-1 ratio are affected by the ambient [email protected] and trapping the atmospheric gases. temperature; the colder the cloud, the © 1999 Sigma Xi, The Scientific Research Society. Reproduction with permission only. Contact [email protected]. 320 American Scientist, Volume 87 Figure 1. Although earth’s climate has long been known to experience warm and cold phases, earth scientists have assumed that changes in climate take place only very slowly. Surprisingly, however, recent ice-core studies in Greenland and Antarctica, along with corroborating evi- dence from ocean-sediment cores, have established that as recently as 11,650 years ago, a change in average temperature as great as 10 degrees Celsius may have occurred within 20 years. This realization has led to a new understanding of earth’s climate as existing in three phases, which are relatively stable until perturbed to the point of a precipitous change. The author and other paleoclimate investigators are con- cerned that anthropogenic additions of carbon dioxide to earth’s atmosphere may alter the hydrologic cycle of the North Atlantic sufficiently to trigger a rapid switch from the present warm climate to a cold one. In this snow pit at Greenland’s summit, light from an uncovered, adja- cent snow pit shines through the layers of snow. The thick, dark bands of snow were deposited during the winter; the sequences of thin, lighter bands were deposited during the summer. These visual differences occur because the summer and winter snow have a different den- sity and crystal shape. (Unless otherwise noted, photographs by the author.) lower the ratio. Measuring how the ra- at the time when the ice overburden surface temperature changed during the tios of these isotopes changes along an pressure closes the pores and traps the past several hundred thousand years. ice core gives us a good idea how the nitrogen gas in the ice. Variations in the air temperature changed over time. isotopic composition of the nitrogen The Greenland Weather Report The second way to determine prehis- along a core show when and by how In Greenland, annual ice layers are toric temperatures is to measure the iso- much the surface temperature changed. stacked up like thousands of annual topic composition of the nitrogen gas Finally, because of the large thermal weather reports. In 1982, a European trapped in the ice. At depths between inertia of an ice sheet, the current tem- and American team made the first at- about 5 and 50 meters in an ice sheet, perature distribution in an ice sheet is tempt to read that record, by recover- air can move in interconnected pores strongly influenced by what the surface ing an ice core from southern Green- but is sheltered from mixing by the temperature was in the past. The land. Measurements on the ice core wind. Nitrogen-15 slowly moves to- physics is similar to cooking a large indicated that about 11,700 years ago ward colder locations, and nitrogen-14 frozen turkey. If we move the turkey di- the climate of the North Atlantic region slowly moves toward warmer loca- rectly from the freezer into the oven, the changed from a dry and cold ice age to tions. This process creates a near-sur- outside of the turkey will be done before the current warmer and wetter Holo- face gradient in the nitrogen-15/nitro- the inside even defrosts. By modeling cene. Altogether it took 1,500 years for gen-14 ratio that depends on the the current thermal state of the turkey, or the climate transition to be complete near-surface temperature gradient. The an ice sheet, we can determine the histo- and a few thousand more years to melt resulting isotopic composition of the ni- ry of the turkey’s, or ice sheet’s, surface most of the ice, but the surprise was trogen trapped in the ice depends on temperature. The physics of these three that most of the transition occurred in the difference between the surface tem- approaches is well understood; together only 40 years. This was only one perature and the temperature at depth they allow us to reconstruct how the record, and it came from a single 10- © 1999 Sigma Xi, The Scientific Research Society. Reproduction with permission only.