Friday, 22 July 2016

Ice melt in Greenland

Large Sections of Greenland Covered in Melt Ponds, Dark Snow


21 July, 2016

Over the past couple of days, temperatures across the Greenland Ice Sheet have really ramped up. The result has been a pretty significant mid-to-late season melt pulse. According to NSIDC, nearly 40 percent of the ice sheet surface has been affected by surface melt during recent days. And Greenland ice mass balance appears to have also taken a hit.

This surface melt pulse is, arguably, best portrayed in the satellite imagery:
Greenland Melt July 20
(Large section of Western Greenland near the Jackobshavn Glacier experiencing significant surface melt on July 20, 2016. Image source: LANCE MODIS.)

On July 20th, this approximate 300 x 70 mile swath of Western Greenland shows a number of distinct strong melt features. Near the interior edge of the melt zone we notice the light blue coloration indicative of widespread and general surface melt. From the satellite, this bluing gives the impression of a thin layer of surface water covering a widespread area of the ice sheet. But it is more likely that the blue tint comes from a plethora of small melt ponds and rivers that blend together in the lower resolution satellite shot to lend the impression of ubiquitous water coverage.
Large Melt Ponds, Dark Snow Over Western Greenland

Further in, we notice the darker blue swatches that indicate large melt ponds. Some of these ponds are quite extensive — measuring 1/4 to up to 1 mile in length. Ponds of this size tend to put a lot of pressure on the Greenland surface and can pretty quickly bore down into the ice sheet’s depths and interior. The water then either becomes locked in the ice — forming a kind of subglacial lake — or flows to base regions of the glacier where it can lubricate the ice — causing it to speed up.

Large Melt Ponds Dark Snow Western Greenland
(Close up satellite shot shows 1/4 to 1 mile long melt ponds, general melt ponding and a darkened Greenland Ice Sheet. Image source: LANCE MODIS.)

Still closer to the ice edge we find greatly darkened patches of ice. Darkening occurs when ice melt reveals and thickens past layers of ice sheet dust and soot accumulation. Each year, winds carry dust from land masses and soot from fires — which now, due to rapid Earth warming, burn more frequently over the Arctic and near-Arctic — to the ice sheet where it accumulates. This darker material is then covered by the annual layers of snowfall. If enough snow and ice melts, the yearly layers of dust and soot accumulation can concentrate into a gray-black covering. Such a covering is clearly visible in the July 20 satellite imagery above.
According to Dr. Jason Box, as much as 5.6 percent of the Greenland Ice Sheet was covered by this darkening, which he calls Dark Snow, as recently as 2014. Darkening of the Greenland ice sheet can accelerate melt as it reduces the ice sheet’s ability to reflect the sun’s rays — resulting in more overall heat absorption.
Substantial Northeastern Greenland Melt Also Visible

Zachariae Surface Melt Darkening
(Zacharie Isstrom Glacier in Northeastern Greenland shows significant melt in July 20 satellite shot. Image source: LANCE MODIS.)

Though surface melt and darkening is quite extensive along the southwestern flank of Greenland, toward the north and east, widespread surface melt, ponding and ice darkening is also visible over sections of the Zachariae Glacier. Here, in a far northern section of Greenland that borders the Arctic Ocean, we find an approximate 100 x 20 mile region of melting and darkening ice. Note the tell-tale bluing and dark gray patches visible in the above image.
For this region, ice has tended to experience more melt during recent years as sea ice within the Fram Strait and Greenland Sea has receded. This has revealed more darker ocean surfaces which, in turn, has absorbed more incoming solar radiation resulting in increased warming for this section of Greenland.
Conditions in Context — Human-Forced Warming Pushing Greenland to Melt Faster

Overall, Greenland melt is this year less extensive than the record 2012 melt season. However, the current mid-to-late season pulse has forced a big melt acceleration that may result in melt that exceeds 250 billion tons of ice loss for 2016 (or the average over recent years). In the pretty near future, continued high global temperatures and additional warming due to human fossil fuel emissions will almost certainly push Greenland to melt at a faster pace.
To this point, the Earth has now warmed by more than 1 C above Preindustrial temperatures. And a range of 1-2 C warming from this baseline in past climate eras such as the Eemian resulted in a 10-20 foot rise in world ocean levels. We’re in this temperature range now. So that’s pretty bad news for sea level rise — to which Greenland now contributes enough melt to lift seas by about 0.75 mm every year. The only real questions at this point are how fast will that already substantial melt accelerate, and will we halt fossil fuel burning swiftly enough to slow it down.

Links/Attribution/Statements

The Dark Snow Project (please support)
Hat tip to Andy in San Diego
Hat tip to DT Lange
Scribbler-sponsored note on Trump:



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