Greenland Sheds a ‘Mega-Berg’
Whenever I hear someone talk about mainstream scientists “hyping” the dangers of climate change, I think of this phrase in the sea level rise section of the most recent major report from the U.N. Intergovernmental Panel on Climate Change (IPCC): “Model-based range excluding future rapid dynamical changes in ice flow.” It shows up in a table (figure 3.1, page 45) of the report that projects how high global sea levels could rise by the last decade of this century due to climate change.
The table shows several different possible ranges based on different computer models, with the smallest projected rise at about .18 meters, or seven inches, and the greatest at .59 meters, or just under two feet. That’s the “model-based range” part.
The “excluding future changes” part refers to the fact that, at the time the report came out in 2007, scientists had a fair idea about how fast the ice sheets covering Greenland and Antarctica might melt in a warmer world, but not much understanding of how their motions might change. In both places, ice drains from the main sheets in the form of glaciers that flow slowly toward the sea, where they calve, dropping icebergs into the ocean. If that flow were to increase, the scientists knew, the extra land-based ice flowing into the ocean could speed up the rate of sea level rise, much as adding an ice cube abruptly raises the level of water in a glass.
Since the scientists couldn’t say anything with confidence about likely changes in glacier flow in a warming world, they declined to speculate at all. Some hype, huh?
Subsequent research, however, has cut down on the uncertainty: the best guess now is that sea level could rise more like three feet, and maybe as much as six, thanks to those and other changes in ice dynamics — and the research isn’t just theoretical. Scientists have been aware for nearly two decades that the glaciers in both Greenland and, to a lesser extent, parts of Antarctica, have begun moving faster. The changes in the past few years have been especially pronounced , dumping ice and melt water into the oceans more quickly than they had before.
Many of the icebergs that break off of Greenland are merely house-sized, or maybe apartment-building-sized. But just a few days ago, a megacity-size monster broke off the Petermann Glacier, in northern Greenland — 97 square miles’ worth of ice, or, as a blog post in The Washington Post put it (making sure to get in the local angle!) “about 40-percent larger than the District of Columbia.”
NASA satellite image from Aug. 5, 2010, shows the ice island calved from Greenland's Petermann Glacier. Credit: Andreas Muenchow, University of Delaware.
If you prefer a New York-centric view, University of Delaware oceanographer Andreas Muenchow told the campus newspaper UDaily that it’s four times the size of Manhattan, with a thickness of about half the height of the Empire State Building. “The freshwater stored in this ice island could keep the Delaware or Hudson rivers flowing for more than two years,” he said.
With many Greenland glaciers, faster flow comes in part from surface meltwater percolating down to lubricate the glacier’s underside, where it scrapes against bedrock, but also from the warming of the oceans. Some glaciers are “grounded” at their outlets; their leading edges sit on the shallow seafloor just offshore. But as the ocean warms, this grounded ice thins, and can even begin to float. All that against-the-seafloor friction is reduced, in effect removing the brakes holding back the whole river of ice behind it.
That’s probably what’s happening with Greenland’s Jakobshavn Glacier, which also calved a major iceberg earlier this summer. But the Petermann’s leading edge is already floating, so that explanation doesn’t work here. Instead, the floating tongue may simply have thinned enough in the warming water to have broken off at a weak point. Even so, wrote Mauri Pelto in a 2008 post at Realclimate.org, “The loss of this ice should then lead to acceleration, developing more crevassing and glacier retreat.”
In short, there’s more than one mechanism for dumping ice into the sea. It’ll be interesting to see what the IPCC has to say about sea level in its next report, due out in 2013.