March 12, 2012. Greenland's ice sheet is more sensitive to global warming than previously thought and may already be approaching a critical threshold, researchers in Spain and Germany found. The ice sheet may lose its ability to grow once warming reaches 1.6 degrees Celsius (2.9 degrees Fahrenheit) since pre- industrial times, according to a study published in Nature Climate Change. That's below the previous best estimate of 3.1 degrees, the scientists at Madrid’s Complutense University and Germany’s Potsdam Institute for Climate Impact Research found. The United Nations estimates the Greenland ice sheet contains enough water to raise global sea levels by about seven meters (23 feet), threatening coastal cities from New York to London and Bangkok. Even so, the researchers said it could take thousands of years for the entire sheet to melt. Temperatures have warmed 0.8 degree since industrialization began in the 18th century, an increase that marks the lower limit of the 0.8 degree to 3.2 degree range of uncertainty within which the melt threshold could be reached, according to the paper. With 2 degrees of warming, it would take 50,000 years to melt the ice sheet, the scientists said. At 4 degrees, the time frame decreases to 8,000 years, and at 8 degrees, it's 2,000 years. The researchers used computer models that were able to replicate past growth and shrinkage of the Greenland ice sheet to project what might happen in the future. They factored in the effects of melting of the 3,000-meter thick ice sheet below certain altitude thresholds, which makes refreezing less likely, even with a return to previous temperatures. They also included the warming effect caused by exposure of dark land and sea areas that absorb more light rather than reflect it, as ice does.