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Slow
Feedbacks Faster Than Expected: New Study Finds Greenland Ice Sheet
Softening Up Like Hot Butter
Melt pools form on Sermeq Avannarleq Glacier, in a region about 16 kilometers (10 miles) from the ice edge.
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
Melt pools form on Sermeq Avannarleq Glacier, in a region about 16 kilometers (10 miles) from the ice edge.
16
July, 2013
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.
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 interglacial. 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.
Links:
Evaluation
of cryo-hydrolic warming as an explanation for increase ice
velocities in the wet snow zone
FRISCO — Greenland’s melting ice cap will continue to contribute to sea level rise, but iceberg calving will become less of a factor as glaciers retreat inland. Instead, surface melting and runoff will account for more than 80 percent of ice cap’s contribution to sea level rise, according to new research from the Vrije Universiteit Brussel.
Study
foresees big changes in Greenland ice melt
By
Summit Voice
16
July, 2013
FRISCO — Greenland’s melting ice cap will continue to contribute to sea level rise, but iceberg calving will become less of a factor as glaciers retreat inland. Instead, surface melting and runoff will account for more than 80 percent of ice cap’s contribution to sea level rise, according to new research from the Vrije Universiteit Brussel.
Changes
in its total mass are governed by two main processes — fluctuations
in melting and snowfall on its surface, and changes to the number of
icebergs released from a large number of outlet glaciers into the
ocean. The ice loss from the ice sheet has been increasing during the
last decade, with half of it attributed to changes in surface
conditions with the remainder due to increased iceberg calving.
The
research team from the Vrije Universiteit Brussel, funded
by ice2sea, a European Union project,
tackled the question of how both processes will evolve and interact
in the future with a new computer model to help accurately project
future ice sheet evolution. The study was published last week in the
Journal
of Glaciology.
“Our
research has shown that the balance between the two most important
mass loss processes will change considerably in the future so that
changes in iceberg calving only account for a small percentage of the
sea-level contribution after 200 years with the large remainder due
to changes in surface conditions,” said lead author Dr. Heiko
Goelzer.
“This
scenario is no reason to be complacent,” said British
Antarctic Survey
Professor David Vaughan, coordinator of the ice2sea program. “The
reason the significance of calving glaciers reduces compared to
surface melting is, so much ice will be lost in coming decades that
many glaciers currently sitting in fjords will retreat inland to
where they are no longer affected by warming seas around Greenland.”
If
Greenland’s ice melted all at once, it would raise sea level by
more than 20 feet, but the study calculated that, under an average
global warming scenario, enough ice will melt to raise sea level by
about 7 centimeters in the next 100 years, and by 21 centimeters
after 200 years.
But
when glaciers retreat farther inland, they won’t be able to
discharge icebergs into the sea. In a couple of hundred years,
calving may only account for between 6 percent and 18 percent
of the Greenland ice cap’s sea-level contribution. This is
important, because variations in outlet glacier dynamics have often
been suspected to have the potential for very large sea-level
contributions.
“Our
research has shown that the balance between the two most important
mass loss processes will change considerably in the future so that
changes in iceberg calving only account for a small percentage of the
sea-level contribution after 200 years with the large remainder due
to changes in surface conditions,” said lead author Dr. Heiko
Goelzer.
“This
scenario is no reason to be complacent,” said British Antarctic
Survey Professor David Vaughan, coordinator of the ice2sea program.
“The reason the significance of calving glaciers reduces compared
to surface melting is, so much ice will be lost in coming decades
that many glaciers currently sitting in fjords will retreat inland to
where they are no longer affected by warming seas around Greenland
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