On July 8, about 40% of the ice sheet had undergone thawing at or near the surface. In just a few days, the melting had dramatically accelerated and some 97% of the ice sheet surface had thawed by July 12.
|In the image, the areas classified as “probable melt” (light pink) correspond to those sites where at least one satellite detected surface melting. The areas classified as “melt” (dark pink) correspond to sites where two or three satellites detected surface melting. The satellites are measuring different physical properties at different scales and are passing over Greenland at different times. Credit: Nicolo E. DiGirolamo, SSAI/NASA GSFC, and Jesse Allen, NASA Earth Observatory.|
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% of the ice sheet surface thawed at some point in mid-July.
This extreme melt event coincided with an unusually strong ridge of warm air, or a heat dome, over Greenland. The ridge was one of a series that has dominated Greenland's weather since the end of May. "Each successive ridge has been stronger than the previous one," said Mote. This latest heat dome started to move over Greenland on July 8, and then parked itself over the ice sheet about three days later. By July 16, it had begun to dissipate.
As the ice warms, it loses albedo, i.e. less sunlight is reflected back into space. Darker surface absorbs more sunlight, accelerating the melting. The image below shows the Greenland ice sheet albedo from 2000 to 2011.
|Credit: NOAA Arctic Report Card 2011.|
The image below, from the meltfactor blog and by Jason Box and David Decker, shows the steep fall in reflectivity for altitudes up to 3200 meters in July 2012.
The image below, from the meltfactor blog, shows how the year 2012 compares with the situation at approximately the same time in previous years, 2011 and 2010, which are recognized as being record melt years.
|Photo shot by Jason Box on August 12, 2005|
Loss of albedo occurs as the darker bare ground becomes visible where the ice has melted away. Darkening of snow and ice can start even before melting takes place. Warming changes the shape and size of the ice crystals in the snowpack, as described at this NASA Earth Observatory page. As temperatures rise, snow grains clump together and reflect less light than the many-faceted, smaller crystals. Additional heat rounds the sharp edges of the crystals, and round particles absorb more sunlight than jagged ones.
|Dirty ice surrounds a meltwater stream near the margin of the ice sheet. Compared to fresh snow and clean ice, the dark surface absorbs more sunlight, accelerating melting. © Henrik Egede Lassen/Alpha Film, from the Snow, Water, Ice, and Permafrost in the Arctic report from the U.N. Arctic Monitoring and Assessment Programme. From NOAA Climatewatch.|
Another factor contributing to darkening is aerosols, in particular soot (i.e. black carbon) from fires and combustion of fuel, dust and organic compounds that enter the atmosphere and that can travel over long distances and settle on ice and snow in the Arctic.
The July data since 2000, from the meltfactor blog, suggest a exponential fall in reflectivity that, when projected into the future (red line, added by Sam Carana), looks set to go into freefall next year.
Is a similar thing happening all over the Arctic? Well, the map below, edited from a recent SSMIS Sea Ice Map, shows that sea ice concentration is highest around the North Pole.
So, can water be expected to show up at the North Pole? Well have a look at the photo from the North Pole webcam below.
|Photo from the North Pole webcam|
It's important to realize that surface albedo change is just one out of a number of feedbacks, each of which deserves a closer look.
As shown on the image below, the IPCC describes four types of feedbacks with a joint Radiative Forcing of about 2 W/sq m, i.e. water vapor, cloud, surface albedo and lapse rate.
The image below, from James Hansen et al., may at first glance give the impression that all aerosols have a cooling effect.
How much could Earth warm up due to decline of snow and ice? Professor Peter Wadhams estimates that the drop in albedo in case of total loss of Arctic sea ice would be a 1.3 W/sq m rise in radiative forcing globally, while additional decline of ice and snow on land could push the the combined impact well over 2 W/sq m.
Locally, the impact could be even more dramatic. The image below, from Flanner et al., shows how much the snow and ice is cooling the Arctic.
|Image, edited by Sam Carana, from Mark Flanner et al. (2011).|
The big danger is methane. Drew Shindell et al. show in Improved Attribution of Climate Forcing to Emissions that inclusion of aerosol responses will give methane a much higher global warming potential (GWP) than the IPCC gave methane in AR4, adding that methane's GWP would likely be further increased by including ecosystem responses. Indeed, as pictured in the image below, accelerated warming in the Arctic could trigger methane releases which could cause further methane releases, escalating into runaway global warming.