Three
Cabot Institute researchers provide their own insights on the highly publicised
news story about the extent of melting observed on the Greenland Ice Sheet.
Chris Vernon, Ph.D
student in the Bristol Glaciology Centre, studying the mass balance of the
Greenland Ice Sheet
Last
week NASA released new images of the Greenland ice sheet generated from
satellite data showing that between the 8th and 12th of July 2012 the area of
the ice sheet’s surface that was melting had increased from about 40 percent to
an estimated 97 percent. On average
during the summer approximately half of the ice sheet experiences such surface
melting and this expansion of the melt area to include the highest altitude and
coldest regions was described as “unprecedented” by the scientists at
NASA. Such widespread melting has not
been seen before during the past 34 years of satellite observations and melting
at Summit Station, near the highest point on the ice sheet, has not occurred
since 1889 based on ice core records.
The
Greenland ice sheet gains mass from rain and snowfall and loses mass by solid
ice discharge to the ocean (iceberg calving) and runoff of surface melt
water. During the period 1961-1990 these
processes are thought to have been in balance with the ice sheet’s mass stable
(Rignot et al., 2008). During the last
two decades, however, both ice discharge and liquid runoff have increased
resulting in the ice sheet losing mass over this period at an accelerating rate
(Velicogna, 2009, Rignot et al., 2011). Changes to these two processes have
contributed approximately equally to recent mass loss (van den Broeke et al.,
2009). Whilst these NASA images do not provide
data about how much snow and ice have melted or the direct effect on mass
balance, they do indicate a significantly larger area of the ice sheet has been
melting.
While
this melting is an extreme weather event, associated with a series of unusually
warm fronts passing over Greenland this summer, new research on the ice sheet’s
albedo from Jason Box, a researcher with Ohio State University’s Byrd Polar
Research Center, shows summer albedo has been decreasing over the last
decade. This reduced reflectivity,
particularly at high elevations, is associated with warming related feedbacks
and means more energy is absorbed at the surface for melting leading Box to
suggest earlier this year that it is reasonable to expect 100% melt extent
within another decade of warming (Box et al., 2012). His latest albedo data are available here: http://bprc.osu.edu/wiki/Latest_Greenland_ice_sheet_albedo.
References (some
behind paywall)
BOX,
J. E., FETTWEIS, X., STROEVE, J. C., TEDESCO, M., HALL, D. K. & STEFFEN, K.
2012. Greenland ice sheet albedo feedback: thermodynamics and atmospheric
drivers. The Cryosphere Discuss, 6, 593-634.
RIGNOT,
E., BOX, J. E., BURGESS, E. & HANNA, E. 2008. Mass balance of the Greenland
ice sheet from 1958 to 2007. Geophysical Research Letters, 35.
RIGNOT,
E., VELICOGNA, I., VAN DEN BROEKE, M. R., MONAGHAN, A. & LENAERTS, J. 2011.
Acceleration of the contribution of the Greenland and Antarctic ice sheets to
sea level rise. Geophysical Research Letters, 38.
VAN
DEN BROEKE, M., BAMBER, J., ETTEMA, J., RIGNOT, E., SCHRAMA, E., VAN DE BERG,
W. J., VAN MEIJGAARD, E., VELICOGNA, I. & WOUTERS, B. 2009. Partitioning
Recent Greenland Mass Loss. Science, 326, 984-986.
VELICOGNA,
I. 2009. Increasing rates of ice mass loss from the Greenland and Antarctic ice
sheets revealed by GRACE. Geophysical Research Letters, 36.
Liz Stephens,
Research Assistant in flood risk and co-author of article: 'Communicating probabilistic information from
climate model ensembles-lessons from numerical weather prediction’ soon to
be published in WIRES Climate Change
The
story of the unprecedented extent of melting of the Greenland ice sheet no
doubt forms an important discussion point amongst scientists and those
concerned about future climate change in the Arctic. However, for me it
demonstrated the problems of clumsy communication; causing confusion that led
some to think that the entire ice sheet had melted, and accusations of
sensationalism from climate change sceptics (see http://sfy.co/a1AS for examples).
My
main grievance is in the use of colour in the images. This may be the standard
colour bar used by the NASA scientists, but it is too emotive for those not
used to what is being referred to. At first glance the white area suggests
‘this is ice’, and the red, ‘we should be really scared that this is no longer
white’. In my opinion the colour white
should not be used because it is evocative of what is ice rather than what is
freezing ice, and so more neutral colours should be used to distinguish areas
of melting from areas of freezing ice.
Additionally,
I think that some of the language used is problematic; scientists need to be
careful not to assume that people understand what is meant by the terms ‘ice
sheet’, ‘area’, ‘surface’ etc., so that people don’t think that the entire
volume of the ice sheet has disappeared.
Further, the subheading of the Guardian article - 97% surface melt over four days - is misleading, because the
images refer to the area of the ice sheet that is undergoing melting and not
the rate of melting itself, and so is not a direct indication of any volume of
ice lost.
I
also don't like some of the phrasing used, particularly, 'had thawed'. This is
perhaps misleading, because if 97% of the ice sheet surface 'had thawed', then perhaps
some might think that only 3% of the ice sheet surface would be left. I would
probably go for an image caption of:
"The
area of the Greenland ice sheet surface that was melting on July 8, left,
compared to July 12th on the right."
Subtle
changes to the language can make it clear that this is an unusual weather event
that could be indicative of climate change, rather than the ice sheet starting
to disappear for good.
Jon Hawkings, Ph.D
student in the Bristol Glaciology Centre, studies the chemistry of glacial
meltwaters
During
the course of my stay at the University of Bristol-led field site near Leverett
glacier in south-west Greenland, I witnessed the start of what has since been
identified as one of the most significant Greenland melt years over the past
century. Over that time Leverett glacier’s subglacial drainage river, fed by the
melting ice sheet surface together with stored meltwater from the bed, had altered
from a small stream to a raging torrent. Although this is usual for a glacial
river during a melt season in Greenland, the scale of change was unprecedented.
Temperatures around camp far exceeded my expectations. I packed expedition gear
expecting Arctic summer temperatures of around 10°C – a little higher than I had
previously experienced in the northerly island archipelago of Svalbard. What I
experienced were temperatures sometimes reaching 20°C. In our camp mess tent
where we cooked and ate our meals the temperature would sometimes exceed 30°C –
shorts and t-shirt weather - were it not for the thousands of mosquitoes that
were thriving in the warmer weather. In June I often found myself processing
samples in the science tent with beads of sweat on my brow.
When
the discharge of the river exiting the margin of Leverett glacier hit around
500 m3/s in late June (over six times that of the average River
Thames discharge when flowing through London), it was evident to all of the
camp that the 2012 melt season was going to be much larger than in previous
years. Over the period that Leverett catchment has been studied (2009-), river discharge
usually reaches a high of 405 m3/s, and that was in early August -
more than a month after this high (and therefore after a month's more melt). At
that time a bridge crossing the glacial meltwater river in the nearest town,
Kangerlussuaq, approximately 25 km downstream (Watson River, fed by Leverett
glacier and two other large glaciers in the area), had to be closed as the
amount of water deemed it unsafe. I’ve recently been informed that discharge of
Leverett river has subsequently hit more than 800 m3/s since I left
camp – nearly twice that of the previous high. At the same time Watson River
discharge at Kangerlussuaq was nearly double its previous high (3500 m3/s
– more than the average discharge of the Nile), and in dramatic fashion has
washed away the same bridge that was closed in 2010 (see http://www.guardian.co.uk/environment/picture/2012/jul/27/glaciers-flooding?newsfeed=true# and http://www.guardian.co.uk/environment/2012/jul/25/greenland-glacier-bridge-destroyed-video?newsfeed=true). Although a trend
for higher melt season discharge has been observed, locals and scientists in
the Kangerlussuaq area have all been taken aback by the magnitude of change experienced
this year (http://www.ouramazingplanet.com/3254-greenland-flooding.html).
As
this was my first field season in Greenland it was difficult for me to grasp
the scale of change from previous years. Ben Linhoff, an isoptope geochemist
from Woods Hole Oceanographic Institute in Massachusetts, USA, has been in camp
during the 2011 and 2012 melt seasons, and was surprised by the difference in temperature
and river size between the two years. He has documented the scale of change on
his Scientific American blog (http://blogs.scientificamerican.com/expeditions/tag/following-the-ice/), and in a short
video with Andrew Tedstone of the University of Edinburgh (http://www.whoi.edu/page.do?pid=80757&cl=82073&tid=5122). Ben comments that air
temperatures in camp are substantially warmer than in 2011 and that glacial
moraine deposited by Leverett glacial hundreds of years ago (possibly during
the Little Ice Age) was being eroded by Leverett river – likely for the first
time in decades. Dave Chandler, a University of Bristol researcher, camped within
the ice sheet interior, 40km from the margin, has also been surprised by the
warm temperatures. During the 2011 melt season he found that the temperature on
the ice very rarely exceeded freezing at night. In contrast, the temperature has stayed above freezing throughout most of June and
July at a similar point on the ice this year. Higher temperatures and the lack
of freezing conditions on the ice sheet interior mean that more glacial ice has
melted on the surface. This water is then thought to be routed to the bed
through conduits know as moulins. At the bed the meltwater joins a large subglacial
channel that flows under the ice and exits the glacier via a portal such as
that which exists on Leverett glacier. The discharge of these subglacial rivers
is thus indicative of the amount of ice melt.
