It would take 7400 years to melt Greenland at currently observed ice melting rates, and many hundreds of years even with a non-linear increase in melting rates.
Numerous studies and direct observation show that Greenland’s ice sheet is melting, and the latest studies show that snowfall on Greenland is not sufficient to offset this loss. If the entirety of Greenland’s ice sheet were to melt, sea levels worldwide would rise by roughly 20 feet. This rise would severely damage or wipe out many of the world’s major cities, and accordingly the threat of catastrophic sea level increase is listed among the primary threats of global warming.
This brings to light an important question: how long do we have until Greenland’s ice sheet melts, or until its melting causes a significant amount of sea level rise?
Greenland’s ice sheet has a total volume of 2.6 million kilometers cubed, and according to a recent study, lost an average of 222 km cubed of ice in the last couple of years. In 2007, the year that Arctic ice cover set a new summertime minimum, 350 km cubed of ice was lost. Assuming this extreme rate continues, Greenland will be fully melted in 7400 years. Of course, ice melt rates are non-linear, and so this rate could increase considerably as temperatures rise.
Assume for a moment that the rate of ice melt rises by an order of magnitude (10x) beyond the 2007 record. 740 years of melting would still be required to melt all of Greenland, and even at that rate sea levels would rise only 2 feet by 2100! If the ice immediately begins melting at 100 times the record rate, then a real catastrophe would ensue, as sea levels would rise by the full twenty feet before the end of the century.
What would it take to trigger a catastrophic increase in Greenland’s ice melt rate? Unfortunately, this isn’t yet well understood, with some research showing that Greenland’s ice sheet could tolerate significantly higher temperatures. This is based in part on research showing that during the last interglacial period, 125,000 years ago, about half of the Greenland ice sheet persisted despite temperatures 5C higher than today. It’s a good idea to keep this and the melt rate calculations in mind when weighing the threat of AGW-induced sea level rise against potential solutions.