January 23, 2009

Glacier Slowdown in Greenland: How Inconvenient

In this week’s Science magazine, science writer Richard Kerr reports on some of the goings-on at this past December’s annual meeting of the American Geophysical Union.

While he didn’t cover our presentation at the meeting in which we described our efforts at creating a reconstruction of ice melt across Greenland dating back into the late 1700s (we found that the greatest period of ice melt occurred in the decades around the 1930s), Kerr did cover some other recent findings concerning the workings of Greenland’s cryosphere in his article titled “Galloping Glaciers of Greenland Have Reined Themselves In.”

Here is how Kerr starts things off:

Things were looking bad around southeast Greenland a few years ago. There, the streams of ice flowing from the great ice sheet into the sea had begun speeding up in the late 1990s. Then, two of the biggest Greenland outlet glaciers really took off, and losses from the ice to the sea eventually doubled. Some climatologists speculated that global warming might have pushed Greenland past a tipping point into a scary new regime of wildly heightened ice loss and an ever-faster rise in sea level.

And some non-climatologists speculated disaster from rapidly rising seas as well.

During his An Inconvenient Truth tour, Gore was fond of spinning the following tale:

[E]arlier this year [2006], yet another team of scientists reported that the previous twelve months saw 32 glacial earthquakes on Greenland between 4.6 and 5.1 on the Richter scale - a disturbing sign that a massive destabilization may now be underway deep within the second largest accumulation of ice on the planet, enough ice to raise sea level 20 feet worldwide if it broke up and slipped into the sea. Each passing day brings yet more evidence that we are now facing a planetary emergency - a climate crisis that demands immediate action to sharply reduce carbon dioxide emissions worldwide in order to turn down the earth’s thermostat and avert catastrophe.

Oh how things have changed in the past 2 years.

For one, the “team of scientists” that reported on the Greenland earthquakes now think that the earthquakes were the result of processes involved with glacial calving, rather than something “underway deep within the second largest accumulation of ice on the planet” (Nettles et al., 2008).

For another, Gore’s “massive destabilization” mechanism for which the earthquakes were a supposed bellwether (meltwater lubrication of the flow channel) has been shown to be ineffective at producing long-term changes in glacier flow rate (e.g. (Joughin et al., 2008; van de Wal et al., 2008).

And for still another, the recent speed-up of Greenland’s glaciers has even more recently slowed down.

Here is how Kerr describes the situation:

So much for Greenland ice’s Armageddon. “It has come to an end,” glaciologist Tavi Murray of Swansea University in the United Kingdom said during a session at the meeting. “There seems to have been a synchronous switch-off” of the speed-up, she said. Nearly everywhere around southeast Greenland, outlet glacier flows have returned to the levels of 2000. An increasingly warmer climate will no doubt eat away at the Greenland ice sheet for centuries, glaciologists say, but no one should be extrapolating the ice’s recent wild behavior into the future.

The last point is driven home by new results published last week (and described in our last WCR and in our piece over at MasterResource) by researchers Faezeh Nick and colleagues. They modeled the flow of one of Greenland’s largest glaciers and determined that while glaciers were quite sensitive to changing conditions at their calving terminus, that they responded rather quickly to them and the increase in flow rate was rather short lived. Nick et al. included these words of warning: “Our results imply that the recent rates of mass loss in Greenland’s outlet glaciers are transient and should not be extrapolated into the future.”

All told, it is looking more like the IPCC’s estimates of a few inches of sea level rise from Greenland during the 21st century aren’t going to be that far off—despite loud protestations to the contrary from high profile alarm pullers.

Maybe Gore will go back and remove the 12 pages worth of picture and maps from his book showing what high profile places of the world will look like with a 20-foot sea level rise (“The site of the World Trade Center Memorial would be underwater”). But then again, probably not—after all the point is not to be truthful in the sense of reflecting a likely possibility, but to scare you into a particular course of action.

References:

Intergovernmental Panel on Climate Change, 2007. Climate Change 2007: The Physical Science Basis. Solomon, S., et al. (eds.), Cambridge University Press, Cambridge, U.K., 996pp.

Joughin, I., et al., 2008. Seasonal speedup along the western flank of the Greenland Ice Sheet. Science, 320, 781-783.

Kerr, R. A., 2009. Galloping glaciers of Greenland have reined themselves in. Science, 323, 458.

Nettles, M., et al., 2008. Step-wise changes in glacier flow speed coincide with calving and glacial earthquakes at Helheim Glacier, Greenland. Geophysical Research Letters, 35, doi:10.1029/2008GL036127.

Nick, F. M., et al., 2009. Large-scale changes in Greenland outlet glacier dynamics triggered at the terminus. Nature Geoscience, DOI:10.1038, published on-line January 11, 2009.

van de Wal, R. S. W., et al., 2008. Large and rapid melt-induced velocity changes in the ablation zone of the Greenland ice sheet. Science, 321, 111-113.




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