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What Causes Ice Ages
10/31/2002
Listen in RealAudio 
Over the last million years, our planet has mostly been covered in ice. But every 100,000
years a remarkable transformation takes place, as the mile-thick ice sheets retreat northward
and, for about ten thousand years, the Earth bursts into the warm, green place that we know
and love. But why?
Hi, I'm Bryan Yeaton, and this is The Weather Notebook.
Surprisingly, scientists still aren't certain what causes the remarkable cycle of the Ice
Ages. One of the most popular ideas is the Milankovitch Theory, which proposes that variations
in the earth's orbit around the sun causes the changes. But the cycles astronomers see in such
things as the 23,000-year wobble of the Earth's axis either don't match up to the longer
cycles seen on Earth or are calculated to be too small to make up much of a
difference.
But now a Dartmouth College scientist has a new idea. Looking at data that captures variations
in the sun, Mukul Sharma found an unexpected correlation between the 100,000-year variation
in the sun's magnetic activity and our Ice Age cycle.
When the sun's magnetic activity is high; it blocks many cosmic rays from striking earth.
These cosmic rays usually produce an isotope of Beryllium in the atmosphere, which falls out
into rock layers. So when the sun's magnetic activity is higher, less Beryllium is found in
our rocks, and vice-versa.
Sharma found that for the last 200,000 years the amount of deposited Beryllium closely matched
the 100,000 oscillation of the Ice Ages, suggesting that the sun's activity may be responsible
for these dramatic shifts in climate.
It's not clear how this variation in magnetic activity would translate into large temperature
swings here on Earth, and Sharma says more study is needed.
Today's show was written by David Appell. The Weather Notebook is supported by Subaru and the
National Science Foundation.
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