NASA reported yesterday that satellite measurements have shown an unprecedented extensive surface melting of Greenland's glaciers. From the press release:

For several days this month, Greenland's surface ice cover melted over a larger area than at any time in more than 30 years of satellite observations. Nearly the entire ice cover of Greenland, from its thin, low-lying coastal edges to its two-mile-thick center, experienced some degree of melting at its surface, according to measurements from three independent satellites analyzed by NASA and university scientists.

On average in the summer, about half of the surface of Greenland's ice sheet naturally melts. At high elevations, most of that melt water quickly refreezes in place. Near the coast, some of the melt water is retained by the ice sheet and the rest is lost to the ocean. But this year the extent of ice melting at or near the surface jumped dramatically. According to satellite data, an estimated 97 percent of the ice sheet surface thawed at some point in mid-July.

Researchers have not yet determined whether this extensive melt event will affect the overall volume of ice loss this summer and contribute to sea level rise.

So how much might melting glaciers in Greenland contribute to future sea level rise? A modeling study published by researchers at the Potsdam Institute for Climate Impact Research in Germany in the March issue of Nature Climate Change estimated that an increase in average global temperature in the range of 0.8–3.2 °C, with a best estimate of 1.6 °C, could lead to an ice-free Greenland. If all the ice in Greenland were to melt sea level would rise by about 24 feet.

Regarding that study, Live Science further reported:

If humanity managed to limit global warming to 3.6 degrees F (2 degrees C) — a goal embraced by climate negotiators but one that looks increasingly unlikely — the Greenland ice sheet would disappear in 50,000 years, according to the study.

However, the greater the warming, the more rapid the melt; 14.4 degrees F (8 degrees C), which the researchers say equates to a "business-as-usual scenario of greenhouse gas emissions," would result in a complete loss in 2,000 years.

"This is not what one would call a rapid collapse," said lead study researcher Alexander Robinson with the Potsdam Institute for Climate Impact Research in Germany in a press release issued by the Institute. "However, compared to what has happened in our planet's history, it is fast. And we might already be approaching the critical threshold."

Assuming that somehow global temperatures were to reach and stay at 8 degrees C (14.4 degrees F) warmer than pre-industrial temperatures for two millennia, that would imply an additional 0.14 inches of sea level rise per year (24 feet divided 2,000 years). Adding this to the background rate of 7 inches of sea level rise per century suggests sea level could rise by 21 inches in the next century. That's not nothing, but an immediate evacuation of the coasts would not seem to be called for. And with regard to the hypothesized 8 degree C increase, keep in mind that man-made global warming is thought to have increased global average temperatures by about 0.7 degrees C so far.

Laying aside modeling results for the moment, what is the current rate of melting? A study published in Nature in February looked at data from the GRACE satellites which measure changes in the earth's gravity to uncover trends in the globe's ice mass balances. Using data from 2003 to 2010 that study reports:

...we show that GICs [glaciers and ice caps], excluding the Greenland and Antarctic peripheral GICs, lost mass at a rate of 148 ± 30 Gt yr−1 from January 2003 to December 2010, contributing 0.41 ± 0.08 mm yr−1 to sea level rise...

The high mountains of Asia, in particular, show a mass loss of only 4 ± 20 Gt yr−1 for 2003–2010, compared with 47–55 Gt yr−1 in previously published estimates. For completeness, we also estimate that the Greenland and Antarctic ice sheets, including their peripheral GICs, contributed 1.06 ± 0.19 mm yr−1 to sea level rise over the same time period. The total contribution to sea level rise from all ice-covered regions is thus 1.48 ± 0.26 mm −1, which agrees well with independent estimates of sea level rise originating from land ice loss and other terrestrial sources.

If the rate of melting in this study were to remain unchanged for the next century, that would imply that all glacial run off would add about 6 inches to global sea level rise contributing to a rise of 13 inches over the next century. Again, no reason to run for the hills immediately.