Slow Feedbacks Faster Than Expected: New Study Finds Greenland Ice Sheet Softening Up Like Hot Butter

Melt Pools

Melt pools form on Sermeq Avannarleq Glacier, in a region about 16 kilometers (10 miles) from the ice edge.

(Image source: AGU)

A new study produced by the American Geophysical Union (AGU) has found that the Greenland Ice Sheet is softening up faster than expected. The study shows that surface melt water absorbs heat and sunlight then transfers that energy into the heart of Greenland’s ice sheets resulting in sagging and more rapid movement, not just at ice sheet edges, but deep within interior glaciers.

Over the past decade, researchers found that the speed of ice motion at the edge of Greenland’s vast ice sheets had increased resulting in larger flows into the ocean. Now, ice motion deep within Greenland’s interior is also found to have sped up. The study compared the rates of ice flow during 2000 to 2001 with a period from 2005 to 2008. The results were alarming:

“Through satellite observations, we determined that an inland region of the Sermeq Avannarleq Glacier, 40 to 60 miles from the coast, is flowing about 1.5 times faster than it was about a decade ago,” said Thomas Phillips, lead author of the new paper and a research associate at the time of the study with the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado, Boulder.

In 2000-2001, the inland segment was flowing at about 40 meters (130 feet) per year; in 2007-2008, that speed was closer to 60 meters (200 feet) per year.

Accelerated Ice Movement -- Greenland

Satellite observations indicate acceleration of an interior region of the southwestern Greenland Ice Sheet. In this map showing a portion of Southwest Greenland, reds and yellows mark areas where ice sheet velocity increased substantially between 2005 and 2007. CREDIT: CIRES image courtesy of AGU

At first, researchers were at a loss as to what had caused this extra ice motion. So Phillips and his team developed an advanced model to help determine how energy was being transported into the deep ice at Greenland’s heart. What they found was that melt water from the surface transfers heat energy deep into the ice sheet causing it to deform and flow faster like melting butter.

As is usual with past science on ice sheets, early models and studies concluded that it would take as long as centuries to millennia for the ice sheets to respond to surface warming. The CIRES study discovered rapid ice sheet response mere decades after the initial forcing — a blink of an eye in geological time. The AGU has accepted the CIRES study and published it for review here.

Researchers were troubled by the amount of melt water they were observing on the ice sheet’s surface. Much of this water later disappeared through great holes and chasms tunneling deep into the ice sheet. Researchers suspected this mechanism was transferring solar energy beyond the ice sheet surface and was likely affecting melt and rate of motion. The new model produced by the CIRES study provides confirmation to this observation.

Implications for Global Climate Models, Weather Stability, Speed of Sea Level Rise

Ice sheet rate of response is a key aspect of climate sensitivity. Current estimates for Earth surface temperature change assume a slow rate of ice sheet response. Rapid ice sheet response, as hinted at in this study and as observed during the past two decades, would result in far more unstable weather and climate conditions during rapid ice sheet melt (with greater swings between hot and cold in regions that may be far removed from the ice sheet) and a more rapid increase in global temperatures once compounding albedo loss occurred.

Current Equilibrium Climate Sensitivity (ECS) models account for only half of total long term warming due to an assumption that ice sheets will be slow to respond. If ice sheets respond faster, as indicated by this study and by recent observations, then the total Earth Systems Sensitivity temperature may be reached more rapidly. An Earth Systems Sensitivity for current levels of greenhouse gasses, at around 400 ppm, is probably about 3 degrees Celsius long-term. Such warming is enough to melt both Greenland and West Antarctica and probably a portion of Antarctica before Earth Systems Equilibrium is achieved. Total sea level rise in such a scenario is likely to approach as much as a 75 foot height at termination. The possibility that this may happen faster than previously expected is cause for serious concern.

Based on observations of increasing ice sheet melt and motion, I have estimated that sea level will increase by between 5 and 15 feet this century (depending on rate of greenhouse gas accumulation). This observation is faster than the IPCC case which estimates about 3 feet and James Hansen who estimates between 6 and 10 feet. My rationale for this rate of rise is based on a meta-analysis that includes the assumption that the human forcing is far faster than rates of forcing increase in the geological past. It is also based on an observation that sea level increased by as much as 10 feet per century at the end of the last glacial period. Hansen’s estimates are consistent with rates of melt observed at the end of the last ice age and mine assume that the speed of human forcing will result in added effects.

The CIRES study provides yet one more observation and related modeling consistent with a far faster than expected rate of ice sheet response. It is likely that we will know within the next couple of decades how this accelerated response translates to rates of ice sheet discharge and related sea level rise. Lastly, it is important to note that geological evidence is not consistent with steady rates of discharge and sea level rise. Unfortunately, major melt events have happened in great pulses that are consistent with catastrophic out-flow events. Such large events would result in serious risks for communities in wide areas surrounding the ice sheets. Tsunami-like melt pulses, therefore, cannot be ruled out. And the high volume of cold water such outbursts deliver to surrounding oceans and environments is also likely to be highly disruptive to established weather and ocean circulation patterns.

It is important to bring these observations to light as the more rapid ice sheet response rates indicated in the CIRES study and by observation heighten the risk for such events.


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