August 9, 2016
During recent years, as human fossil-fuel emissions have forced the Earth to warm,observations of Greenland’s surface has indicated a rising rate of melt. What has been less well-observed is melt rates beneath the ice and near the ice base. This is important because the pooling of water beneath the great ice sheet can help speed its movement toward ocean outlets, along with accumulating heat at the base of the ice — which can also quicken the pace of overall melt.
A new scientific study headed by NASA researchers has developed one of the first comprehensive maps of melt along Greenland’s basal zone, where the ice contacts the ground surface. What they have found is that large portions of Greenland are melting from below:
(New, first-of-its-kind map shows extensive melt along the Greenland ice sheet base. Melt in this region is a sign that heat is building up beneath the ice as well as on top. Image source:NASA.)
This mapping study found that wide expanses of northern Greenland and pretty much all of southern Greenland are now experiencing melt at the ice sheet base. As the interior of Greenland has a cracked-bowl topography — with land bowing down into a central trough and numerous furrows connecting the ice sheet with the ocean — understanding where liquid water and heat are pooling at the bottom of the ice sheet will help scientists to get a better idea of how Greenland’s glaciers will respond to human-forced warming.
Joe MacGregor, lead study author and glaciologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland recently noted:
“We’re ultimately interested in understanding how the ice sheet flows and how it will behave in the future. If the ice at its bottom is at the melting-point temperature, or thawed, then there could be enough liquid water there for the ice to flow faster and affect how quickly it responds to climate change.”
Geothermal Melt, Ice Sheet Heat Accumulation, and Climate Change
Melt along the base of the Greenland ice sheet has long been influenced by heat welling up from or trapped near the Earth’s surface. The heavy, thick ice sheet densely packs the ground and rocks under it, which generates and amplifies geothermal hot-spots beneath Greenland. In addition, the ice creates a kind of insulating layer which locks that ground heat in. As a result, the bottom of the ice sheet is often tens of degrees warmer than its top.
Alone, this blanketing effect is enough to generate some melt along the bottom of Greenland. But now that the surface is melting more and more, heat transport from the ice surface to the bottom via liquid water funneling down to pool below is a more common occurrence.
(Recharge of subglacial lake by surface melt near the Flade Isblink ice cap is an example of how surface melt can interact with basal melt, driving the formation of water at the ice sheet base. Image source: Nature.)
The way this heat transfer works is that rising temperatures over Greenland form more extensive surface lakes and melt ponds during the increasingly warm summers (and sometimes briefly during other periods). Often, the meltwater will find a crack in the ice and flow down to the ice interior. Sometimes the water remains suspended in the middle layers between the surface and the ice sheet base as a kind of heat bubble. At other times, the water will bore all the way down to the ground where it can form into pools or subglacial lakes.
At Flade Isblink in northeastern Greenland, such a filling of a subglacial lake was observed during the 2011 and 2012 melt years. As Greenland warms, such instances are likely to become more common. In this way, melt at the surface can add to the amount of heat trapped below the ice sheet — forming a kind of synergistic melt process.
The new NASA study helps our understanding of how such a process might unfold by showing the current extent of subsurface melt. The study combined physical models with observations to create this larger picture of bottom melt, telling a dramatic story of the opening period of human-forced Greenland melt, in which sub-surface melt is already very extensive.
Conditions in Context — The Level of Atmospheric Greenhouse Gasses is Now About Equal to Where They Were When the Greenland Ice Sheet First Formed
In context, the Greenland ice sheet is the largest repository of land ice remaining in the Northern Hemisphere. Covering a vast region of 1,710,000 square kilometers and rising up to 3 kilometers high at its tallest point, this ice sheet contains fully 2,850,000 cubic kilometers of ice. If all this ice melted, it would raise the world’s sea levels by around 7.2 meters (nearly 24 feet).
This enormous mountain of ice astride Greenland began to form about 11 to 18 million years ago during the Middle Miocene climate epoch. Back then, atmospheric carbon dioxide ranged from 405 to 500 parts per million. This decline from earlier, higher CO2 concentrations was allowing the world to cool enough to begin to support glacial ice in this region (around 4 C warmer than 1880s values).
(Losses of Greenland mass from the surface zone have been accelerating during recent years. This loss has primarily been driven by human-forced warming of the Arctic. Though the North Atlantic Oscillation can generate melt variability by driving warm air flows toward or away from Greenland, the overall long-term driver has been a rapid warming of the Arctic region due to fossil-fuel emissions. Though we have a pretty good understanding of surface melt, our understanding of melt at the base of the ice sheet and heat accumulation there is less complete. Such an understanding may help us to predict future ice sheet behavior. Image source: Skeptical Science.)
Back then, Greenland’s ice was far smaller, far less extensive. It was a baby ice sheet that would grow into a behemoth as the Miocene cooled into the Pliocene — when CO2 levels fell to around 390 to 405 ppm — and then into the various ice ages and interglacials that followed (featuring atmospheric CO2 in the range of around 180 ppm during ice ages and around 275 ppm during interglacials).
Now, human fossil-fuel burning has put the ice sheet in a great global-warming time machine. With atmospheric CO2 levels hitting Middle Miocene ranges of 407.5 ppm at Mauna Loa this year, an accumulation of enough heat to significantly melt large portions of Greenland’s ice is a very real and growing concern. Exactly how that melt may unfold is still a big scientific mystery, but the risks are growing along with the heat and the new NASA basal melt study helps to shed a little light.
First Map of Thawed Areas Under Greenland Ice Sheet
NASA Maps Thawed Areas Under the Greenland Ice Sheet
Recharge of Subglacial Lake by Surface Melt Water in Northeast Greenland
Greenland Ice Sheet