Melt water, Greenland summer 2010
Photo: Sarah Das, WHOI
Record high temperatures over Greenland in spring and summer brought record melting of Greenland's ice cap in 2010.
Cumulative net annual area changes for the 35 widest marine-terminating glacier outlets
Increasing flow of warm melt waters into cracks and crevasses may lead to ice cap destabilization, rapid outlet glacier flow, rapid ice cap decline and rapid sea level rise.
Warm air above Greenland at 500mb (the level of approximately half the thickness of the atmosphere) led to record ice cap melting this summer.
Figure GL1. The geopotential height and wind anomalies for JJA 2010 (referenced to the 1971–2000 mean) at 500 hPa from the NCEP/NCAR Reanalysis. Areas where geopotential height anomalies were at least twice the 1971–2000 standard deviation are hatched. The blue arrows represent wind vector anomalies, with scale indicated by the blue arrow below the plot.
Greenland climate in 2010 is marked by record-setting high air temperatures, ice loss by melting, and marine-terminating glacier area loss. Summer seasonal average (June-August) air temperatures around Greenland were 0.6 to 2.4°C above the 1971-2000 baseline and were highest in the west. A combination of a warm and dry 2009-2010 winter and the very warm summer resulted in the highest melt rate since at least 1958 and an area and duration of ice sheet melting that was above any previous year on record since at least 1978. The largest recorded glacier area loss observed in Greenland occurred this summer at Petermann Glacier, where 290 km2 of ice broke away.
The rate of area loss in marine-terminating glaciers this year (419 km2) was 3.4 times that of the previous 8 years, when regular observations are available. There is now clear evidence that the ice area loss rate of the past decade (averaging 120 km2/year) is greater than loss rates pre-2000.
Weather patterns in 2010 favored rapid melting in the Arctic ocean in early summer with slow melting over Greenland. Late summer weather brought rapid melting over Greenland and slow melting in the Arctic ocean. Overall, the melt season has grown much longer since 1979, leading to much greater melt volumes. The large increase in the rate of melting of Greenland's ice cap is happening because the Arctic is the fastest warming region on earth.
The melt duration was as much as 50 days greater than average in areas of west Greenland that had an elevation between 1200 and 2400 meters above sea level.
Figure 2. Difference (days) in summer 2010 melt duration compared to the 1979-2007 average.
Greenland's record level of total melting in 2010 was much greater than the 1979-2007 average.
Figure GL2. Time series of Greenland melt extent derived from passive microwave remote sensing from 2010 (red), 2007 (blue) and the 1979-2007 average (green), after Mote (2007).
The calving of the larger-than-Manhattan sized iceberg from the Petermann glacier has profound consequences because the mass of the marine end of the glacier helps to hold back the inland part of the glacier. Glaciologist Jason Box was studying the Petermann glacier when it calved the huge iceberg.
Box says a large calving event, such as what occurred with the Petermann Glacier, can speed up a glacier’s slide towards the sea:
"The big question I think everyone wants to know," Box says, is whether global warming is driving the ice loss such as the calving event seen last summer.
"[The] Balance of evidence is strongly suggesting" a relationship to warming temperatures, Box says, but exactly how such warming is driving ice loss is unclear. He notes two warming-related processes that are probably playing a role: surface melt and "hydrofracture," a process by which water fills crevices in the ice and forces them apart.
It is of concern because there is no expected or known mechanism to reverse the accelerated loss. It’s like removing a cork from a bottle. It will take an extended cold period to grow back the glacier ice on the front of a glacier like Petermann.
NASA's MODIS satellites recorded the breakup of the leading edge of the Petermann glacier
above, acquired August 5, 2010; below acquired July 28, 2010
Melt water on the surface of Greenland's ice sheet can hydrofracture the ice, causing sudden failure of melt ponds. Warm melt water may penetrate deep into Greenland's glaciers, warming and lubricating them.
According to research by glaciologists Sarah Das of WHOI and Ian Joughin of UW, the lubricating effect of the meltwater can accelerate ice flow 50- to 100 percent in some of the broad, slow-moving areas of the ice sheet.
We found clear evidence that supraglacial lakes—the pools of meltwater that form on the surface in summer—can actually drive a crack through the ice sheet in a process called hydrofracture," said Das, an assistant scientist in the WHOI Department of Geology and Geophysics. "If there is a crack or defect in the surface that is large enough, and a sufficient reservoir of water to keep that crack filled, it can create a conduit all the way down to the bed of the ice sheet."
Researchers at the University of Colorado, Bolder found that melt water may be warming Greenland's ice sheet much faster than scientists previously thought possible. Because the ice sheet has a network of fairly closely spaced fractures, melt water can carry heat rapidly into the body of the ice cap. Warmer ice is less viscous than colder ice. Because it is less viscous, warmer ice flows faster than colder ice. Therefore, penetration by melt water could rapidly warm and destabilize Greenland's ice cap.
The presence of liquid water in the cryo-hydrologic system (CHS) due to surface melt leads to warming of the ice. The magnitude and time-scale of CH warming is controlled by the average spacing between elements of the CHS, which is often of the order of just 10's of meters. The corresponding time-scale of thermal response is of the order of years-decades, in contrast to conventional estimates of thermal response time-scales based on vertical conduction through ice (~102–3 m thick), which are of the order of centuries to millennia. We show that CH warming is already occurring along the west coast of Greenland. Increased temperatures resulting from CH warming will reduce ice viscosity and thus contribute to faster ice flow.
Borehole temperatures in an outlet glacier in western Greenland show that rapid glacial warming is already occurring and their new cryo-hydrologic model fits the data well. Warm temperatures penetrate over 300m (about 1000 feet) into the glacier, far deeper than can be explained by the standard heat flow model.
A western Greenland glacier has warmed rapidly through the top 1000 feet.
Measured (symbols) and simulated (lines) temperatures for borehole TD3 on Sermeq Avannarleq, outlet glacier northeast of Ilulissat, Greenland. The two simulations with cryo-hydrologic warming (CHW on) used a specified temperature gradient of 0.0227 K/m ("gradient") and a fixed temperature equal to the measured temperature ("fixed") at the bed. Without cryo-hydrologic warming (CHW off), the base-case simulation (1991) used a fixed temperature equal to the measured temperature at the bed, the "min" and "max" curves were obtained by multiplying the u(z)*α*λ term by factors of about 2 and inline equation.
Current models of the dynamics of the ice caps of Greenland and Antarctica do not consider the effects of melt water influx on warming and flow. The new data and models indicate that the ice caps may be much less stable than older data and models indicated. The ice caps may respond much more rapidly to Arctic and Antarctic warming than previously recognized.
Global sea levels will rise more rapidly than previously predicted if the ice caps melt more rapidly and flow faster than forecast by old models. The most recent IPPC report may have substantially underestimated the rate of sea level rise. Sea level rise may have much larger human, economic and political consequences than planners and politicians expect.