Climatologists have long puzzled over whether ice ages hit the Northern and Southern hemispheres at the same time. Now researchers report evidence that they do and suggest that the atmosphere plays a key role in quickly conveying cold across the equator.
Until recently, scientists didn't expect ice ages in the Northern and Southern hemispheres to occur simultaneously. That's because changes in Earth's orbit and tilt tend to expose one hemisphere to more of the sun's warmth. So it was a surprise in the 1990s when a research team using radiocarbon dating suggested that glaciers in west-central Chile advanced and retreated to a Northern Hemisphere beat during the last northern ice age.
It was conceivable, however, that these glaciers were reacting to quirky local conditions. To rule out that possibility, a team of geologists--led by Brad Singer of the University of Wisconsin, Madison, and Michael Kaplan, now at the University of Edinburgh--ventured into the arid deserts of Argentina. Here, past glaciers had left piles of boulders behind. To determine when the glaciers retreated, the team extracted quartz from the boulders and counted atoms that had been transformed by cosmic radiation--an indicator of how long the boulder had been exposed to the sky (cosmic rays don't penetrate far into ice).
The results were striking. In the March/April issue of the Geological Society of America Bulletin the team reports that the most recent southern ice age was at its maximum about 23,000 years ago and ended about 16,000 years ago, dates indistinguishable from the estimates from the Rocky Mountains and the Sierra Nevada in North America. Moreover, the period when ice extended furthest in the Southern Hemisphere, prior to 22,000 years ago, is just when Greenland was at its coldest. That suggests to the researchers that some rapid process--such as a redistribution of warmth-capturing water vapor around the globe--brought cold weather to the Southern Hemisphere. This flies in the face of the popular notion that the ocean does that job. If that were the case, the team argues, there would have to be at least a thousand-year lag.
The apparent precision in the new study makes it difficult to ignore the role of the atmosphere in linking hemispheres, says glaciologist Eric Steig of the University of Washington, Seattle. But he notes that many lines of evidence suggest that the ocean is a major influence.