Climate Science

April 5, 2010

Ice ages: They come and go - but why?

We are told by geologists and other scientists that a massive ice sheet, more than a mile thick, covered a large portion of North America.

It almost seems like a fairy tale. But a walk along the Lake Champlain shore, or in the low lands of Vermont and New York, reveals traces of this spectacular event.

It is hard to imagine today when standing on Rattlesnake Mountain near Willsboro, or Pok-O-Moonshine near Keeseville, on a lovely warm day, that ice covered this area as far as the eye could see. All of the peaks would have been covered (a few tips may have poked through) and all one would see was white: ice. That ice sheet grew over tens of thousands of years, spreading down from northern Canada and reaching as far south as New York City. Then it all melted and nature was primed to repeat itself with another 100,000-year-or-so cycle.

Let's see what science tells us and look at the first chart. It is a bit busy but full of interesting data. The figure is taken from the Intergovernmental Panel on Climate Change Report AR4 "Climate Change 2007" (page 444). It shows four complete tracings, and a partial one, with information on ice volume (bottom line), temperature (second line from bottom), and the levels of three greenhouse gases, methane (third line from the bottom) and carbon dioxide (fourth line from the bottom). The incomplete top line is the available data on nitrous oxide. All of this data was obtained from ice core samples from Antarctica.

The interesting aspect here is that we now have information on at least six different ice ages over the past 650,000 years. The time axis along the bottom is in hundreds of thousands of years. The five vertical shaded areas are the warm periods, in between the glacial periods, such as the interval we are in now (vertically shaded area at the far right). These warm periods are when the glaciers melted and the climate was much as we know it today.

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