An analysis of prehistoric discharges of icebergs in the North Atlantic shows even slight warming can trigger a collapse of ice shelves, U.S. researcher say.Study lead author Shaun Marcott of the University of Oregon says the analysis provides historical evidence that warming of water by just 3 to 4 degrees was enough to trigger massive episodic discharges of ice from the Laurentide Ice Sheet in what is now Canada.The results are important because of concerns warmer water from global warming could cause a comparatively fast collapse of ice shelves in Antarctica or Greenland, increasing the flow of ice into the ocean and raising sea levels, an OSU release said Monday.If the West Antarctic Ice sheet, one of the areas said to be most vulnerable to rising temperatures, were all to melt it would raise global sea level by about 11 feet, the researchers said."We don't know whether or not water will warm enough to cause this type of phenomenon," Marcott said. "But it would be a serious concern if it did, and this demonstrates that melting of this type has occurred before."An increase in temperatures of just 2 degrees could increase the rate of melting of the ice sheets to 30 feet a year, which could cause many of them to completely melt in less than a century, researchers said.