Greenland ice melt turns out to be more complicated, and better studied, so this is a revision of what I posted on September 18, 2012.
The top surface melt may either move slowly downwards, re-freezing and consolidating in firn, the form of old snow that is compressing into ice, or it may move as meltwater, or perhaps some other fate, for which I have yet to find a study. Sublimation?
The massive surface melt in the summer of 2012 was remarkable because nearly every surface experienced some melting, at least in place. The scientists monitoring Greenland melt had earlier classified Greenland ice as dry-snow facies, the combined percolation and wet-snow facies, ice facies, transient melt areas and moraine. Dry snow facies are surfaces where if snow falls, it accumulates, never melting. Percolation facies are surfaces that melt and refreeze, forming pipes and lenses. The
As a recent abstract by McGrath et al (2013) clarifies, "Extrapolation of this observed trend now suggests, with 95% confidence
intervals, that the dry snow facies of the Greenland Ice Sheet will
inevitably transition to percolation facies. There is a 50% probability
of this transition occurring by 2025."
In the percolation facies is were the lakes and moulins form.
As the Greenland ice sheet melts around the edges and off its top surface, transient meltwater lakes form on the top of the ice at some locations. Sometimes the transient lakes discharge in transient mighty rivers that cut across the ice sheet surface and may join with more permanent rivers or perhaps better-termed seasonally recurrent rivers. In the summer of 2012 an engorged ice melt river destroyed a bridge in Kangerlussuaq. How many months of the year do the ice melt rivers flow? I don't know. How much melt water stays on the ice sheet surface all the way to ground surface and from there to the ocean, and how much melt water diverts through the moulins to under-ice channels before it reaches the ocean? I don't know those answers, either, and it would be great if someone was gathering real data on those questions.
In other instances, the meltwater lakes accumulate for only a few days or weeks in summer, then flush themselves down the inverse-chimneys, the moulins, that develop under the lakes and deep into the ice, as the melt water opens up cracks in the ice. In a moulin, the melt water creates a deep vertical shaft that extends to the base of the ice sheet. The moulin water is thought to travel through under-ice channels to the ocean. How well established is that fate? Does all the water go to the ocean?
Tracking ice melt comes with some tools, as ice melt is fresh water, low in salts and conductivity, and as it is from snow fall, it has a lighter oxygen isotope signature than does seawater.
Is it part of the freshwater runoff accumulation found in the seas near shore? Apparently yes.
The East Greenland Current and in consequence, the West Greenland Current, are affected by plumes of glacial melt water that can be detected far into the current by oxygen isotope studies.
Is the meltwater also pooling in the great central lowland of Greenland that is hidden under the ice? The data from the 1990s did not suggest that it was. The Greenland ice core samples from GISP, GSIP2, GRIP, NGRIP, and NEEM did not hit a lake of any sort before approaching bedrock.
However the times are a-changing. In the summer of 2012, the entire top ice surface of Greenland had a melt layer, unlike previous years' observations in which center of the continental sheet did not lose a layer. This years' loss of annual ice record is as startling as if a geologist could watch the loss of a geologic stratum.
Also in this past year, researchers at the other pole were able to drill down in Antarctica, to the ancient Lake Vostock, deep under the ice sheet. Is there another lake like Vostock forming under Greenland's melting ice sheet?
Like Antarctica, Greenland has a low bedrock area in mid-sub-continent as much as 500 meters below current sea level.
Researchers suggest that the Greenland meltwater from surface lakes may be lubricating the icesheet's bottom surface. With the slopes of that mid-continent depression, not all the ice sheet would slide towards the sea.
Two to five areas of Greenland bedrock have channel patterns linking the mid-continent depression to the sea.
These channels could unplug at some point in the mix of sea level rise and ice melt.
For now, the Greenland ice cores suggest, but don't prove, an absence of a preexisting mid-continental lake.
Perhaps it's one of those answers that is all of the above or some of the above.
In breaking news on November 30, 2012, a report published in Science, A reconciled estimate of Ice-Sheet mass balance, shows that Greenland's ice sheet is melting five times faster than the models had predicted.
Most recent update May 22, 2013. Joan Savage