TORONTO - The massive glacial sheet that blanketed most of Canada during the last Ice Age may be long gone, but the footprint it left on the Earth's crust continues to influence global climate patterns.
Now scientists have a pretty clear picture of what our deeply frozen country looked like up to 10,000 years ago -- and it was 21st-century technology that helped them map that image.
Geophysicists from the University of Toronto and Harvard analyzed data from a pair of satellites designed to measure the Earth's gravitational field, zeroing in on Canada where the pull of gravity is abnormally low.
"What these satellites allow us to do is measure tiny, minuscule ripples or changes in that gravity field, and that has never been possible before,'' said Jerry Mitrovica of the University of Toronto and co-author of the research appearing Friday in the journal Science.
"Now we have whole maps over Canada that say here's how the gravity field over Canada is changing,'' he said. "I think we've now nailed the Ice Age, at least what the Ice Age looked like over Canada.''
Co-author Mark Tamisiea said the ice field that began creeping down from the Arctic over the Canadian landscape 20,000 years ago was so thick and crushing that it left a depression in the Earth's crust. Called the Laurentide Ice Sheet, it was more than three kilometres thick in some spots.
"The ice sheet is now melted but the crust still hasn't rebounded, it hasn't completely recovered to its original position,'' Tamisiea said from Liverpool, England, where he recently joined the Proudman Oceanographic Laboratory after leaving the Harvard-Smithsonian Center for Astrophysics.
"What that means is there is missing mass . . . so that causes a gravity low.''
Mitrovica said gravitational signatures picked up by the satellites show that the Laurentide ice sheet didn't spread out over Canada with a smooth, even surface, but had peaks and valleys -- in particular two huge domes to the east and west of Hudson's Bay that were maybe half a kilometre thicker than the surrounding area.
"That's really remarkable because the ice has been gone for 10,000 years,'' he said. "But because of the way it deformed the Earth underneath it, it was not completely flat, it had little bumps and it was deeper where it was thicker ice.''
"We're still seeing the final last gasp of those bumps.''
So why is knowing what the Ice Age looked like millennia ago important today?
"Because the Ice Age is still doing things on this Earth of ours,'' stressed Mitrovica. "You know how big that ice sheet was? That ice sheet was so big over Canada that when it melted, the sea level rose 60 metres. So if you don't understand what that ice sheet looked like, it's hopeless.''
What he means is that any scientist trying to calculate global climate change _ including whether ice fields over Greenland and Antarctica are melting and at what rate _ will be able to more accurately take into account the effects of the Ice Age on the Earth's crust, which continues ever so slowly to rebound.
"The ripples of the gravity field tell you,'' Mitrovica said. "So now we have a handle on the Ice Age that is so much better, when we start looking at modern climate records, our estimates of how modern ice sheets are changing will be more accurate.''
