‘Hidden plumbing’ helps slow Greenland ice flow
Hotter summers may not be as catastrophic for the Greenland ice sheet as previously feared and may actually slow down the flow of glaciers, according to new research.
A letter published in Nature on 27 January explains how increased melting in warmer years causes the internal drainage system of the ice sheet to ‘adapt’ and accommodate more melt-water, without speeding up the flow of ice toward the oceans. The findings have important implications for future assessments of global sea level rise.
The Greenland ice sheet covers roughly 80% of the surface of the island and contains enough water to raise sea levels by 7 metres if it were to melt completely. Rising temperatures in the Arctic in recent years have caused the ice sheet to shrink, prompting fears that it may be close to a ‘tipping point’ of no return.
Some of the ice loss has been attributed to the speed-up of glaciers due to increased surface melting. Each summer, warmer temperatures cause ice at the surface of the sheet to melt. This water then runs down a series of channels to the base of the glacier where it acts as a lubricant, allowing the ice sheet to flow rapidly across the bedrock toward the sea.
Summertime acceleration of ice flow has proved difficult for scientists to model, leading to uncertainties in projections of future sea level rise.
“It had been thought that more surface melting would cause the ice sheet to speed up and retreat faster, but our study suggests that the opposite could in fact be true,” said Professor Andrew Shepherd from the University of Leeds School of Earth and Environment, who led the study.
“If that’s the case, increases in surface melting expected over the 21st century may have no affect on the rate of ice loss through flow. However, this doesn’t mean that the ice sheet is safe from climate change, because the impact of ocean-driven melting remains uncertain.”
The researchers used satellite observations of six landlocked glaciers in south-west Greenland, acquired by the European Space Agency, to study how ice flow develops in years of markedly different melting.
Although the initial speed-up of ice was similar in all years, slowdown occurred sooner in the warmest ones. The authors suggest that in these years the abundance of melt-water triggers an early switch in the plumbing at the base of the ice, causing a pressure drop that leads to reduced ice speeds.
This behaviour is similar to that of mountain glaciers, where the summertime speed-up of ice reduces once melt-water can drain efficiently.
Study co-author Dr Edward Hanna from the University of Sheffield added: “This work also underlines the usefulness of modern gridded climate datasets and melt-model simulations for exploring seasonal and year-to-year variations in Greenland ice sheet dynamics and their relationship with the global climate system.”
The study was funded by the Natural Environment Research Council’s National Centre for Earth Observation, the Philip Leverhulme Trust, and by the European Commission Ice2Sea project.
For more information
The Letter entitled ‘Melt-induced speed-up of Greenland ice-sheet offset by efficient subglacial drainage’ by Aud Venke Sundal, Andrew Shepherd, Peter Nienow, Edward Hanna, Steven Palmer & Philippe Huybrechts is published in Nature on 27 January 2011 [doi:10.1038/nature09740].
Contact Hannah Isom in the University of Leeds press office on 0113 343 5764 or email h.isom@leeds.ac.uk.
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See also:
Upcoming paper in Nature – Greenland ice sheet melt: “it’s weather, not climate”
Greenland Ground Zero for Global Soot Warming
h/t to Steve Milloy
While the Greenland icecap is situated in a depression surrounded by highlands or mountains (which is the reason it is so extremely stable) relatively little of it is based below sea level. If it melted completely central Greenland would becom a large rather shallow freshwater lake draining westwards. There is good map on page 10 of this:
http://www.geus.dk/publications/bull/nr14/nr14_p01-13_A1b.pdf
Sorry – fat finger on the exponent above: correctly “1.71E+6 sq km area (the full sheet)”. The rise figure is correct.
That is probably quite true, but I think we should be open to consider an alternative:
As the CAGW bunch looses its grip, dissenting views are easier to publish. This is not new, there were always critical views of catastrophic hypothesis, but they became more silent during the political dominance of climate science. See for example an excelent article about the stability of Antarctica during the last 14 million years by D. Sugden and its relation to how hypothesis work in earth sciences:
The East Antarctic Ice Sheet: unstable ice or unstable ideas?
To add to my previous comment, this paper from some of the same authors stresses the uncertainty of the models regarding the Greenland Ice Sheet. The degree of uncertainty makes any prediction basically useless, but we have hardly heard anything about those results, even if the paper is from 2007:
Impact of model physics on estimating the surface mass balance of the Greenland ice sheet
Abstract:
Long-term predictions of sea level rise from increased Greenland ice sheet melting have been derived using Positive Degree Day models only. It is, however, unknown precisely what uncertainties are associated with applying this simple surface melt parameterization for future climate. We compare the behavior of a Positive Degree Day and Energy Balance/Snowpack model for estimating the surface mass balance of the Greenland ice sheet under a warming climate. Both models were first tuned to give similar values for present-day mass balance using 10 years of ERA-40 climatology and were then run for 300 years, forced with the output of a GCM in which atmospheric CO2 increased to 4 times preindustrial levels. Results indicate that the Positive Degree Day model is more sensitive to climate warming than the Energy Balance model, generating annual runoff rates almost twice as large for a fixed ice sheet geometry. Roughly half of this difference was due to differences in the volume of melt generated and half was due to differences in refreezing rates in the snowpack. Our results indicate that the modeled snowpack properties evolve on a multidecadal timescale to changing climate, with a potentially large impact on the mass balance of the ice sheet; an evolution that was absent from the Positive Degree Day model.
It is interesting how a couple folks have commented on this quote…
“If that’s the case, increases in surface melting expected over the 21st century may have no affect on the rate of ice loss through flow. However, this doesn’t mean that the ice sheet is safe from climate change, because the impact of ocean-driven melting remains uncertain.”
And added in the notion that the authors are “grant trawling” since they mention ocean-driven melting. With no evidence to counter the authors’ statement, the commenters here resort to accusations. Way to go, very classy.
“With higher temps the glaciers will melt …”
Really? So if the average temperature goes from -10 F to -5 F, there will be melting?
tty says:
January 27, 2011 at 3:15 am
“While the Greenland icecap is situated in a depression surrounded by highlands or mountains (which is the reason it is so extremely stable) relatively little of it is based below sea level. If it melted completely central Greenland would becom a large rather shallow freshwater lake draining westwards. ”
Are you sure? The map seems to indicate several depressions below sea level, the largest about 500km wide by 750 km long. There are large parts that are between 100m – 200m below sea level with the deepest between 200m and 300m. Not as low as the Dead Sea but considerably larger in area. It would make it a bigger area below sea level than the Caspian sea. Were you being sarcastic?
And…what about colder summers?
@tty: thanks – very informative. On the hat-throw basis before, I eyeball the residual lake as about 1/3rd total Greenland land area and, say, 250m mean depth so 142,500 cu km volume. Allowing for that, sea level rise would be 6.77m – but this would effectively be at 0C temperature and there would be extra thermal expansion at lower latitudes.
I think it all points to 7m being about the correct figure.
This pic shows how radically changed the new glacial isostatic adjustment model (left) is versus the old model (right).
http://thegwpf.org/cache/multithumb_images/3399907772.png
from:
http://thegwpf.org/the-observatory/1438-ice-sheet-loss-cut-in-half.html
In other words it is believed that most of Greenland is sinking rather than rising as previously modeled and therefore throws previous ice loss and SLR estimates out the window.
Does the calculated increase in sea level take into account the fact that the sea/land boundary is not generally vertical, so as sea levels rise, the area of ocean over which the calculation is based, actually increases significantly. If the area increases, the resultant rise, for a given volume of additional water, must be less.
It is nowhere near a linear relationship
Tim C,
Actually, if you insert the area of the entire globe, 510072000 sq km (Wiki) the rise will be 5.6 meters.
It seems out of proportion, are we forgetting something?
@Dan: the rise will only affect the oceans and the newly inundated littoral areas, not the entire globe.
@Tony B: as the rise was only reduced by 11cm, allowing for immediate inundation of all the littoral areas in full, this didn’t seem to matter much. Perhaps halve the difference, as a reasonable estimate?
TimC,
My calculation lays out a 5.8 m layer of water across the entire globe. If only some parts are flooded, the rise will be greater and approach the 7.something figure.
This means that, contrary to my first posting, there is little alleviation from ocean area increase. Bugger.