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Canadian researchers have found a gravitational shadow hanging over North America, left over from the last ice age. The work is published in the May 11 edition of the journal Science.

Using four years of data from the Gravity Recovery and Climate Experiment (GRACE) satellite mission – a pair of satellites that are measuring the Earth’s gravitational field to determine how mass is being redistributed on the planet - the scientists were able to construct an image of the long-vanished Laurentide ice sheet.

Professor Jerry Mitrovica, of the U of T and one of the paper's authors, describes the phenomenon as "the ghost of the ice age" that still hangs over the continent.

This was a truly massive ice sheet. It had two enormous domes one east and the other west of the Hudson Bay area. When it receded, global sea levels went up about sixty metres. Its immense weight actually compressed the Earth's crust.

"These are parts of the crust that haven’t completely rebounded from the giant depression caused by the ice," explains Dr. Mark Tamisiea, of the Harvard-Smithsonian Center for Astrophysics.

"The ongoing uplift of the land continues to have an effect on gravity. Constraining the geometry of the ice is important to understanding ice age climate, and it is also important for making accurate corrections before we interpret modern climate records." ®

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