3. Greenland Largely Ice Free
If the world warms by 4 degrees Celsius (7.2 degrees Fahrenheit), the Greenland ice sheet (above, meltwater lakes and streams near Kangerlussuag) will almost unavoidably melt away--leading to potentially dangerous global warming, a majority of experts said in a March 2009 study.
An ice-free Greenland would cause up to about 20 to 23 feet (6 to 7 meters) of sea-level rise, threatening up to 300 million people with harmful floods, experts say.
(Related: "Photos: Huge Greenland Glacier Disintegrating."
Of the five tipping points assessed in a recent survey of climate experts, the scientists showed the "highest concern" about the Greenland ice sheet, according to lead study author Elmar Kriegler of Germany's Potsdam Institute for Climate Impact Research.
The survey participants also judged the likelihood of these tipping points being achieved if the climate achieves low, medium, and high levels of future global warming.
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Photograph by James Balog/NGS