Although almost all climate scientists agree that the Earth is gradually warming, they have long been of two minds about the process of rapid climate shifts within larger periods of change. Some have speculated that the process works like a giant oven or freezer, warming or cooling the whole planet at the same time. Others think that shifts occur on opposing schedules in the Northern and Southern Hemispheres, like exaggerated seasons. Recent research by Frank Lamy of the University of Bremen in Germany examining climate patterns in the Southern Hemisphere at the end of the last Ice Age strengthens the idea that warming and cooling occurs at alternate times in the two hemispheres. A more definitive answer to this debate will allow scientists to better predict when and how quickly the next climate shift will happen.
Few climate researchers, most of whom are from North America and Europe, have carefully examined climate patterns in much of the Southern Hemisphere. Lamy and his colleagues, however, collected sediments from the Pacific Ocean off the coast of Chile and other sediment samples from locations directly inland and used them to examine the time period between 10,000 and 50,000 years ago. The chemical content of these samples establishes temperature fluctuations over time periods of hundreds to thousands of years. The 40,000-year period Lamy focused on was marked by large climate fluctuations in both hemispheres.
The ocean sediment samples revealed a southern climate that was starkly out of sync with that in the north. Warm periods in the Chilean waters matched up to times of cold weather in North America and Europe. Previous work had revealed the same pattern in Antarctica’s climate, but until Lamy’s work it wasn’t clear if the icy continent’s climate history was an oddity or reflected trends present throughout the Southern Hemisphere.
The results strengthen the argument that ocean currents, not atmospheric changes, dominate the movement of heat between the Northern and Southern Hemispheres. Ocean currents redistribute heat so that one hemisphere may remain warm while the other experiences major cooling. If temperature changes primarily resulted from some global aspect of the atmosphere—an increase or decrease in the amount of heat trapped by greenhouse gases, for example—then Earth should show simultaneous warming (or cooling) both north and south of the equator.
At first glance, the South American glaciers appear to tell a different story. Earlier studies had shown that inland glacier melting in South America was not timed with warming in Antarctica. Looking at the iron content of sediments on land, where higher iron content corresponded to larger glaciers, Lamy also noticed that the melting of the Patagonian ice sheet, a large Ice Age glacier that once covered modern-day Chile and Argentina, showed peak melting in the same pattern as the ocean sediments. The melting, however, did not occur when the nearby oceans were warmest. He attributes this seemingly paradoxical result to “glacial inertia.” The idea is that temperatures had to warm substantially before large sheets of ice at high altitudes would begin to melt. In this case, Lamy estimates that the melting of the Patagonian Ice Sheet would have lagged up to 1,000 years behind the surface warming, enough to explain the apparent anomaly.
Until now climate researchers have focused primarily on the Atlantic Ocean and Antarctica, with the bulk of the Southern Hemisphere left out of the picture. Lamy’s work suggests that the Southern Hemisphere could help provide a much clearer portrait of historical warming patterns and improve predictions of future rapid climate change. Lamy also thinks these changes are possible—but in his world rapid means at least 100 years.