Sunday, 18 November 2012

At last! Greenland research confirms common sense

We've been told for years that the Greenland ice-sheet is "melting faster than expected", so contributing more meltwater to sea-level rise. We've been told that meltwater percolates to the base of glaciers, providing lubrication that speeds up movement of the ice towards the sea. This last claim has always seemed to me to be counter-intuitive - glaciers may be melting at the surface in summer, but surely the ice temperature drops with depth? Surely the meltwater would freeze before it reached the rock at the base of the ice? It seems that simple common sense backed by actual research (none of the "experts" postulating meltwater lubrication had taken any measurements) has triumphed over scientific supposition. A paper published in Nature just over a week ago (9th. November 2012) is revealingly titled Greenland ice-sheet contribution to sea-level rise buffered by meltwater storage in firn (J. Harper, N. Humphrey, W. T. Pfeffer, J. Brown & X. Fettweis). The abstract reads
Surface melt on the Greenland ice sheet has shown increasing trends in areal extent and duration since the beginning of the satellite era. Records for melt were broken in 2005, 2007, 2010 and 2012. Much of the increased surface melt is occurring in the percolation zone, a region of the accumulation area that is perennially covered by snow and firn (partly compacted snow). The fate of melt water in the percolation zone is poorly constrained: some may travel away from its point of origin and eventually influence the ice sheet’s flow dynamics and mass balance and the global sea level, whereas some may simply infiltrate into cold snow or firn and refreeze with none of these effects. Here we quantify the existing water storage capacity of the percolation zone of the Greenland ice sheet and show the potential for hundreds of gigatonnes of meltwater storage. We collected in situ observations of firn structure and meltwater retention along a roughly 85-kilometre-long transect of the melting accumulation area. Our data show that repeated infiltration events in which melt water penetrates deeply (more than 10 metres) eventually fill all pore space with water. As future surface melt intensifies under Arctic warming, a fraction of melt water that would otherwise contribute to sea-level rise will fill existing pore space of the percolation zone. We estimate the lower and upper bounds of this storage sink to be 322±44 gigatonnes and 1,285(+388-262) gigatonnes, respectively. Furthermore, we find that decades are required to fill this pore space under a range of plausible future climate conditions. Hence, routing of surface melt water into filling the pore space of the firn column will delay expansion of the area contributing to sea-level rise, although once the pore space is filled it cannot quickly be regenerated.
Co-author Neil Humphrey of Wyoming University is quoted in the university news sheet
“We’re not saying Greenland is not melting,” Humphrey says. “What we’re saying is it will be one to two decades longer before we start seeing the melt.”
That’s because Humphrey and other researchers’ data -- collected on the western flank of the Greenland Ice Sheet from 2007-2009 -- shows that the water generated by repeated recent melt events penetrates deeply into the snow and firn (partially compact snow). This fills the pore space and diminishes the amount of meltwater that actually runs off into the ocean.
As future surface melt intensifies due to Arctic warming, a fraction of meltwater -- that would otherwise add to the rise in sea levels -- fills tens of meters of existing pore space of the percolation zone. The percolation zone is a region of the accumulation area that is perennially covered by snow and firn, Humphrey says.
What happens to the water that "penetrates deeply into the snow and firn"?
“The snow is so deep and so cold that, even though it’s melting, the melt infiltrates into the lower, colder snow and refreezes,” Humphrey says. “We calculate there is one to two decades of pore space within the snowpack. You get denser snowpack. After 10 or 20 years, it (the pore space) fills up.”
However, after this healthy dose of science, the prof. drops into the realm of fantasy (is he concerned about protecting his funding?)
“While other people (scientists) are predicting up to a one-half foot sea rise by 2050, we’re actually saying our data shows that any rise that will occur will be delayed by one or two decades,” says Humphrey of the paper he termed as “controversial.” “A half-foot rise is significant. Half of Florida would be under water. New Orleans would be gone.”
A "half-foot" is of course six inches, or 15 cm. Is he really suggesting that half of Florida is just 15 cm above current high tides? Just when I thought that scientists were abandoning advocacy in favour of traditional research and actual fact.....

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