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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.

Climate, from the Bottom DownOne can also learn a lot about what controls climate by studying sediments on the ocean floor. These sediments contain the decayed remains of ocean organisms and inorganic material from the erosion of rocks. Ocean organisms assimilate chemical compounds from the water as they grow, and the compounds they incorporate are partially determined by the environment in which they live. Thus the decayed remains of the organisms that fall to the ocean floor contain a record of what chemical compounds were available and the temperature of the water in which they lived.

For example, consider an ocean-sediment core collected at Bermuda Rise, a place where ocean currents deposit a lot of sediment. The oxygen-18/oxygen-16 ratio of seawater varies through time depending on how much water is locked in ice sheets and how much water is in the ocean (see Figure 7). The near surface–dwelling foraminiferan Globigerinoides ruber uses seawater to make its shell. By measuring the oxygen isotopic composition of the shells recovered from an ocean core, we can determine how much water was locked up in ice sheets when the foraminiferan was living. Likewise, the bottom-dwelling foraminiferan Nutallides umbonifera incorporates cadmium and calcium in its shell. By measuring the ratio of cadmium to calcium in the shells recovered from an ocean core, we can tell where the bottom water came from when the foraminiferan was living. High values of the cadmium-to-calcium ratio indicate that the water near the bottom came to the Bermuda Rise from the south, whereas a low ratio indicates that the bottom water came from the north.

Ocean sediments also contain ground-up rock, which is transported and deposited by ocean currents, just as wind carries airborne dust to be deposited on ice sheets. The mineralogy of the ground-up rock can be used to identify where it came from. For example, a layer of hematite-rich sediments in ocean cores near Bermuda indicates that ocean currents were transporting material from the east coast of Canada to Bermuda when the sediments in the layer were deposited.

To determine what the temperature of the ocean surface was in the past we can use organic compounds made by phytoplankton. Phytoplankton live near the ocean surface where there is light for photosynthesis. Some phytoplankton produce compounds know as alkenones which are straight chains of carbon atoms. Along these chains of carbon there can be two or three double bonds. The number of double bonds depends on the water temperature. The double bonds are thought to keep the cell membrane pliable in cold water. When the phytoplankton die, the alkenones fall to the bottom and become incorporated into the sediment. By measuring the ratio of different types of alkenones we can determine what the surface water temperature was when the phytoplankton were living.

By collecting cores of the ocean sediments at different locations, we can determine a lot about how the ocean circulated water and heat in the past. The rapid climate changes recorded in the ice cores encouraged a search for ocean sediment records with high time resolution. In the past few years locations have been identified in the ocean where sediment accumulates rapidly, and the sediment cores from these locations have comparable time resolution to the ice cores. Coring projects off the coast of Bermuda by Konrad Hughen, Julian Sachs and Scott Lehman with the University of Colorado, in conjunction with Lloyd Keigwin of Woods Hole Oceanographic Institution and Ed Boyle of the Massachusetts Institute of Technology, found the same rapid changes in climate as were recorded in the ice cores. Other groups have found similar records near Santa Barbara, California and off the coast of India.

Paleoclimatic evidence worldwide shows that a global change in climate took place 11,700 years ago, and in the North Atlantic a large part of the change took less than 20 years. It was a few thousand years before the completion of the transition from ice age to warm period; still, in just a 20-year period the climate of a large part of the earth changed significantly. There was no warning. A threshold was crossed, and the climate in much of the world shifted abruptly from cold to warm. This was not a small perturbation; our civilization has never experienced a climate change of this magnitude or speed. To get an idea of what happened, imagine that over a 20-year period the weather at your home became that typical of a place 400 to 600 miles farther south. What might be the mechanism for so rapid and large a climate change?

Climate’s Control Mechanism

Like the atmosphere, the oceans are far from static. Currents, of which the Caribbean-Atlantic Gulf Stream is just a small part, continually exchange water among all the oceans and between the surface and the depths. For the sake of convenience, we shall start this journey in the Gulf Stream, where water moves northward along the East Coast of the U.S. toward Iceland. Along the way, the water exchanges heat with the air, warming the air and cooling the water in the process. Water evaporates from the surface and leaves behind dissolved salt. The combination of chilling and evaporation makes surface water denser as it moves north. In the vicinity of Iceland, the surface water becomes denser than the water below it and sinks. This dense, cold water then moves south along the bottom of the Atlantic, around the Horn of Africa and, still near the bottom, continues to the North Pacific, where it upwells to the surface. Surface water in the North Pacific makes room for the upwelling bottom water by moving south, passing between Asia and Australia and finally catching the tail of the circulation pattern at the beginning of the Gulf Stream in the Atlantic off Central America (see Figure 8). For most of its journey, the surface water collects heat and freshwater, which makes the surface water more buoyant than the water underneath it. But in the North Atlantic, the combination of cold temperatures and evaporation makes the water dense again and it sinks.

Wally Broecker of Columbia University likens this circulation pattern to a long conveyor belt that moves water, salt and heat. He was among the first to recognize that alterations in the path of the ocean conveyor belt would change climate in much the same way that turning off the furnace fan changes the temperature distribution in a house. He proposed that the large oscillations in climate observed in the geologic record were caused by different patterns of ocean circulation.

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© American Scientist 1999