Dynamic thinning of glaciers now reaches all latitudes in Greenland, has intensified on key Antarctic grounding lines, has endured for decades after ice-shelf collapse, penetrates far into the interior of each ice sheet and is spreading as ice shelves thin by ocean-driven melt.
So find Hamish D. Pritchard, Robert J. Arthern, David G. Vaughan & Laura A. Edwards in their advance online publication in Nature: Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets (or here).
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That Greenland has been melting rapidly is not something that will surprise many readers. The Arctic has seen more warming relative to the rest of the planet, due to among other factors, polar amplification. Although with a few exceptions in the denial fever swamps dramatic Arctic warming and Greenland melting are uncontroversial, the rate of melting has been a subject of interest. Melting of some of Greenland’s southeastern glaciers underwent a period of acceleration, but the acceleration seemed to have come to an end around 2005, according to presenters at 2008′s fall AGU meeting. Pritchard et al.’s findings are in broad agreement with this regional trend, but find the extent of dynamic melt was more widespread than has been previously assessed- especially in the northwest- and has penetrated in some areas more than 100km inland and to altitudes as high as 2000m. 81 out of 111 Greenland glaciers surveyed showed melt rates more than twice as fast as nearby flowing ice.
The findings in Antarctica were no less concerning. In discussions about Eastern Antarctic ice mass especially, it has often been pointed out that while ozone depletion, the Antarctic Circumpolar Current and other factors (see here and here) have buffered the interior of the continent from the kind of dramatic warming seen in the Arctic, the Southern Ocean itself is warming more rapidly than the global average. This is troubling because Pritchard et al. found that much of the Antarctic melt was due not to land surface warming, but ocean-driven warming [emphases mine]:
We infer that grounded glaciers and ice streams are responding sensitively not only to ice-shelf collapse but to shelf thinning owing to ocean-driven melting. This is an apparently widespread phenomenon that does not require climate warming sufficient to initiate ice-shelf surface melt. Dynamic thinning of Greenland and Antarctic ice-sheet ocean margins is more sensitive, pervasive, enduring and important than previously realized.
In other words, those who proclaim that the lack of the kind of warming seen in the Arctic means fears of Antarctic melting are overblown couldn’t be more mistaken. [UPDATE: Further discussion at Greenland and Antarctic ice sheet decay, continued]
And of course, glacial melt is not isolated to Greenland and Antarctica. Across the planet, glaciers are disappearing:
Dramatic visual evidence of the rapid decay of glaciers was recently on display at a TED presentation by photographer James Balog: