Even as the sea ice on the Arctic Ocean looks to be headed for a possible record meltback this year, scientists reported Wednesday that the land-based ice sitting atop Greenland has already melted more than any time in the past 30 years — and that’s with another four weeks left in the melting season. The new study, based on satellite observations, expands on a report in July that showed a record 97 percent of Greenland’s icy surface had undergone at least some melting during the summer season.
Supraglacial lake on Greenland ice.
Credit: Woods Hole Oceanographic Institute.
But that research only showed how widespread the melting was, not how long it went on in any given area — and in fact, some of that melted snow and ice almost certainly re-froze, meaning it won’t end up flowing to the ocean to raise sea level (total sea level rise is expected to reach 3 feet or more by 2100, and scientists expect Greenland to contribute a significant part of that increase).
The new study, by contrast, looks at both the extent and the duration of melting to put together something called the “cumulative melting index,” which is a better gauge of how much water is likely to drain permanently from the frozen continent. The actual number won’t be compiled until the end of the season, since some of the meltwater will be trapped in surface lakes, sitting atop the ice, that will eventually freeze back when winter comes.
Plenty of it, however, will drain into crevasses, and ultimately into the sea. Along the way, that drainage water can lubricate the undersides of glaciers, allowing them to flow more quickly, and dump more icebergs, into the ocean — another cause of sea level rise. “Over the past few years,” said Marco Tedesco of the City College of New York, who issued the new report, in an interview, “about half of Greenland’s contribution to sea level rise has come from meltwater, and half from ice going directly into the sea.”
But surface melting has accelerated over the past decade, he said, for a number of reasons. The most straightforward is that air temperatures in the Arctic have been rising. Even at the highest altitudes on Greenland, where melting normally happens only for a few days each summer, it now lasts for weeks. Another factor is that when snow melts and refreezes, it becomes darker overall, so it absorbs sunlight more efficiently. When it melts completely, the ice underneath is darker still.
“Snow absorbs about 20 percent of the energy that hits it and reflects the rest,” Tedesco said. “Ice absorbs more like 60 or 70 percent.” And in fact, a study that came out in June showed that Greenland’s albedo, or reflectivity, is lower than normal this year.
The ice melt on in the Arctic Ocean (which doesn’t contribute to sea-level rise) is different in many ways. Storms that roil the area can break up the ice and speed its melting, and strong winds can literally push ice out of the Arctic ocean altogether. Neither of these happens to the thick ice on Greenland. The sea ice, moreover, is melting not just from the warmer air temperatures above, but also from the warmer ocean below.
Still, the two are clearly related, Tedesco said. “The record for Greenland surface melt came in 2010, while the record melting for sea ice came in 2007. But 2007 was also a huge melting year for Greenland.” Combined with the faster slippage of glaciers toward the sea over the past decade or so, the prospects for rising seas over the next century and beyond are not encouraging.