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Heavy
summer rain in Greenland speeds up ice melt
Rapid
ice melt in Greenland is driven by intense late summer rainfall,
making the vast ice sheet even more vulnerable as the earth warms,
new research shows.
Formation
of lakes on the Greenland Ice Sheet after the main melt-season due to
warm wet weather and heavy rain. Later this water drains to the
bottom of the ice sheet enabling it to move and melt faster. This
makes the ice sheet more vulnerable to such rapid melt events as the
climate warms, scientists say in a new study. (Photo: Sam Doyle)
13
July, 2015
You
may think that warmer temperatures alone were enough to drive the
melting of the Greenland Ice Sheet, and you would not be alone --
Many glaciologists thought roughly the same.
But
a new study shows for the first time that the Greenland Ice Sheet
melts rapidly, not just with warm summer temperatures, but also after
intense late-summer rain.
The
new study shows that these heavy rain events have occurred frequently
due to the warmer, wetter weather of the last 20 years, penetrating
deeper into the ice sheet, making it move and melt faster.
“We
saw 10 to 15 % of the total annual surface melt occur in this unusual
week of warm, wet weather in late summer 2011. When this water
reached the bed, the ice sheet lifted up and moved faster,” says
Sam Doyle, from Aberystwyth University, UK, lead author of the new
study, which has just been published in Nature Geoscience.
According
to his colleague and co-author, professor Jason Box of the Geological
Survey of Denmark and Greenland, their results add more weight to
scientists’ concerns of the sensitivity of the entire Greenland Ice
Sheet to melting.
“We’re
seeing that warm wet weather, that is increasing with climate change,
is driving more melt of the Greenland Ice Sheet than we thought. And
worryingly, this melt is reaching further into the ice sheet,“ says
Box, speaking via satellite phone, along with Doyle, as they work
together on another fieldtrip in Greenland.
Scientist:
An interesting, thorough, and novel study
Edward
Hanna, professor in climate change with the University of Sheffield,
UK, is impressed with the new study.
“This
is an interesting, thorough and novel study based on a wealth of
robust meteorological and glaciological datasets, and is based on
meteorological models as well as observations,” says Hanna, who was
not involved in the research.
“The
study adds to an already complex story of how Greenland Ice Sheet
dynamics appear to be influenced by changes in weather conditions,
sometimes on a daily timescale,” he says.
Heavy
rain makes the Greenland Ice Sheet move
A
monster moulin on the Russell Glacier, Greenland. You can see a
helicopter for scale. These large gaping holes allow rain and melt
water to drain quickly into the ice sheet. (Photo: Alun Hubbard)
The
study began after observing exceptionally warm wet weather in late
summer 2011, causing huge amounts of melt at the ice surface. At this
time of the year, there was no snow on the surface of the ice to
absorb and act as a buffer for all this rain and melt water, which
then moved very quickly through the ice sheet.
“At
first our observations of this late-August intense rainfall and melt
event were met with the attitude of “hey big deal, it’s summer,
it melts”,” says professor Alun Hubbard from the Center for
Arctic Gas Hydrate, Environment and Climate at The Arctic University
of Norway, and principle investigator of the project that led to the
study.
“But
this new data, shows these periods of rapid movement of the ice are
in fact tied to a particular type of rainfall event, the kind of
storm you would expect to see in the mid-latitudes -- UK or
Scandinavia -- and not so much in Greenland where high pressure
systems are more common.”
“We
can now reinterpret these big late summer melt events to get a much
better understanding of what is going on -- what is affecting the ice
sheet system and how it is responding” he says.
Imagine
a storm drainage system after heavy rain
Hubbard
uses the analogy of a storm drainage system, to explain how the rain
and melted ice moves through the ice sheet so quickly.
“Imagine
a big downpour in a city. You get so much rain so quickly, on to an
impermeable concrete surface that the water is immediately shunted
into the city's drainage system -- which due to the huge volumes of
water -- can't cope. Drains back up and it floods. Basically, the
same thing happens on the bare ice sheet surface, which like the city
is literally comprised of pipes, conduits and cavities,” he says.
According
to him, when these pipes back up the hydraulic pressure lifts the ice
sheet up, like a gigantic iceberg. There is less friction at the bed
and so it moves faster.
Melting
extends far into glacier interior
But
observing one event was not enough to convince some glaciologists
that this process is important over longer periods of time, or that
rain could cause melting over such a wide area.
Doyle
and his team analysed the strange event in more detail.
They
monitored the ice-movement by GPS, set up their own weather stations
on the ice, and measured how much water was being discharged from the
ice sheet.
According
to Hubbard, the rain event covered a huge area, and melting extended
deep into the interior of the ice sheet, by about 140 km. This is
where you would expect it to be frozen, and for rain to fall as snow.
They
also gathered rainfall data for the last 20 years and compared it
with other known melt events across Greenland.
“We
saw these August rainfall events were in fact quite common, a major
one occurring once every couple of years, and hence could explain
some of the other rapid flow acceleration events recorded by other
research groups working on the ice sheet,” says Hubbard.
Timely
rainfall catches the ice sheet off guard
A
scientist watches melt water drain from the Russell glacier catchment
in Greenland. Scientists do not yet know just how often these rapid
melt events may occur in the future as the earth warms and Greenland
receives more rain. (Photo: Sam Doyle)
According
to Doyle, it is not only the amount of rainfall that is important,
but also the precise timing of this rain.
“The
late-summer timing was critical. The event occurred after the end of
the melt season and the ice sheet's drainage system had started to
close down,” he says.
“In
this closed-down state the ice sheet's drainage system just couldn’t
cope with the exceptional water inputs from melt and rain, and this
is why the ice sheet reacted so dramatically, lifting off its bed and
accelerating the flow of the ice towards the sea,” he says.
Scientists
expect more melt due to rain
A
scientist watches melt water drain from the Russell glacier catchment
in Greenland. Scientists do not yet know just how often these rapid
melt events may occur in the future as the earth warms and Greenland
receives more rain. (Photo: Sam Doyle)
Box
emphasises how climate change will continue to drive this process in
the future.
“It
is rain and warmer temperatures that cause melt, and we show that
this melting trend has been underway for the last two decades as the
atmosphere has warmed,” he says.
According
to Hubbard a succession of these rain events would have a big impact
on the ice sheet -- allowing it to melt more and move faster.
“By
speeding up the ice sheet, we are effectively turning up the tap --
opening the sluice gate and hastening the speed at which the ice
moves and melts and hence ends up in the ocean. But we don’t yet
know for sure how frequent or how intense these late-season rainfall
events will be in the future,” he says.
Hanna
echoes this caution.
“The
trouble is we don't really have a clear prediction of how such events
will change in frequency and intensity over Greenland with on-going
climate change,” he says.
“But
certainly this is another, now well-evidenced potential feedback that
should be included in studies of ice sheet change during the rest of
this century,” says Hanna.
New
paper in Nature Geoscience, Jason Box and Alun Hubbard are
co-authors.
Above,
I got a quick rundown from Dr. Box, a few days ago, via (sorry)
herky-jerky skype from Northern Iceland.
UPDATE:
Below, I finally found the clip I was looking for, Jason talking to
the camera in August of last year, in the midst of a 30 hour rain
event that engulfed our camp in southwest Greenland.
While
on the ice for Dark Snow 2014, we experienced a 30 hour precip event
that featured high wind and horizontal driving rain. Afterward,
ablation stakes on-site indicated that a very large amount of surface
melting had taken place during the previous day and a half.
The
new study finds that increased incidences of rain on the Greenland
sheet may indeed be a contributing factor to more mass loss, by
sending large volumes of warm water deep into the ice, now and even
more so as the arctic continues to warm.
One
of the key factors that accelerates Greenland mass loss is the
so-called “Zwally effect”, wherein warm melt water “lubricates”
the underbelly of the ice, where it meets bedrock, and cause
acceleration toward the sea.
Scientists
now know that this effect is moderated over time, as the glacial
drainage system opens up during the summer melt and becomes more
efficient, meaning more water delivered to the sea with less uplift
to the overlying ice.
However, in later summer, that drainage system begins to close down as temperatures drop, – making it vulnerable to increasing numbers of late season warm and wet events, which can cause more upheaval at the base of the ice, and could be responsible for substantial increases in late season movement.
Nature
Geoscience: Amplified melt and flow of the Greenland ice sheet driven
by late-summer cyclonic rainfall
For eight days during
late August and early September 2011, reanalysis data27 indicate that
a cyclone (minimum surface pressure of 992hPa) centred on Baffin Bay
off the west coast of Greenland advected warm, southwesterly airflow
over the GIS, bringing extensive precipitation, which was especially
heavy in southeast Greenland.
–
We
find that a concomitant flow response is evident in all available
velocity records from these regions, including three major
marine-terminating glaciers located up to 370km north of
Kangerlussuaq: ice flow increased by 9% and 95% above the preceding
week at GPS sites on Store Glacier30 (S11) and Sermeq Avannarleq6
(A20) respectively.
Science
Nordic:
The
study began after observing exceptionally warm wet weather in late
summer 2011, causing huge amounts of melt at the ice surface. At this
time of the year, there was no snow on the surface of the ice to
absorb and act as a buffer for all this rain and melt water, which
then moved very quickly through the ice sheet.
“At
first our observations of this late-August intense rainfall and melt
event were met with the attitude of “hey big deal, it’s summer,
it melts”,” says professor Alun Hubbard from the Center for
Arctic Gas Hydrate, Environment and Climate at The Arctic University
of Norway, and principle investigator of the project that led to the
study.
“But
this new data, shows these periods of rapid movement of the ice are
in fact tied to a particular type of rainfall event, the kind of
storm you would expect to see in the mid-latitudes — UK or
Scandinavia — and not so much in Greenland where high pressure
systems are more common.”
“We
can now reinterpret these big late summer melt events to get a much
better understanding of what is going on — what is affecting the
ice sheet system and how it is responding” he says.
Imagine
a storm drainage system after heavy rain
Hubbard
uses the analogy of a storm drainage system, to explain how the rain
and melted ice moves through the ice sheet so quickly.
“Imagine
a big downpour in a city. You get so much rain so quickly, on to an
impermeable concrete surface that the water is immediately shunted
into the city’s drainage system — which due to the huge volumes
of water — can’t cope. Drains back up and it floods. Basically,
the same thing happens on the bare ice sheet surface, which like the
city is literally comprised of pipes, conduits and cavities,” he
says.
According
to him, when these pipes back up the hydraulic pressure lifts the ice
sheet up, like a gigantic iceberg. There is less friction at the bed
and so it moves faster.
Below,
my 2013 interview with Alun Hubbard
even in breaking ice you need to use hydraulic lift
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