Most of us are used to seasons. Each year, spring follows winter, which follows autumn, which follows summer, which follows spring, with winters that are colder than summers. But Earth can go through much larger temperature cycles over longer times than those that we normally experience.
Between 65,000 and 35,000 years ago, for instance, the planet was much colder than it is now. The temperature also changed a lot during that time, with periods of warming and cooling. Ice melted during the warm periods, which made sea levels rise. Water refroze during the cold times.
Sea ice in Alaska.
|Jo Goldmann, U.S. Fish and Wildlife Service|
A new study sheds light on where ice sheets melted during the ice age's warm periods. It now seems that the ice melted at both ends of Earth, rather than just in northern or southern regions. This result surprised the researchers, who are from the University of Bern in Switzerland.
Scientists have long assumed that most of the ice melting occurred in the Northern Hemisphere during the 30,000-year period of the ice age. That's because the North Pole is surrounded by land, while the South Pole is surrounded by the Antarctic Ocean. It's easier for ice sheets to flow and grow on land. Otherwise, the ice can just slip into the ocean instead of building up.
The researchers used a computer model to look at different types of melting that might affect sea level. They compared these results to evidence of how temperatures and currents actually changed during that time.
The model showed that melting just in the Northern Hemisphere would have shut down ocean currents and lowered sea temperatures much more than actually happened in the North Atlantic. Studies suggest that melting just in the Southern Hemisphere would have been impossible, too.
The only reasonable conclusion, the scientists decided, was that ice melted equally in the North and the South. The rest of the story about why this would occur, however, remains a mystery.—E. Sohn
Perkins, Sid. 2004. North and south: Equal melting from each hemisphere raised ice age sea levels.  Science News 166(Aug. 28):133. Available at http://www.sciencenews.org/articles/20040828/fob5.asp  .
You can learn more about ice ages at www.pbs.org/wgbh/nova/ice/  (Nova, PBS), www.dmns.org/main/minisites/iceage/ia_indepth/index.html  (Denver Museum of Nature & Science), and www.museum.state.il.us/exhibits/ice_ages/index.html  (Illinois State Museum).