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Monday, October 25, 2010

Konrad Steffen: Greenland’s contribution to sea level rise is increasing

Konrad Steffen: Greenland’s contribution to sea level rise is increasing

In this astonishing video, Jason Amundson captures the calving of a gargantuan iceberg from the face of the Jakobshavn Glacier in Greenland in the summer of 2007. (The video starts just with the sound of the event.) As the berg — which is hundreds of feet thick — calves from the glacier, it turns over and floats off, producing waves in the fjord more than 10 feet high. (Amundson and his colleagues describe a series of events like this in a Geophysical Research Letters paper.)

Today, a new report warns that Asian megacities like Bangkok are at increasing risk from flooding due to rising sea level in an increasingly warmer world.
And on Tuesday, NOAA issued its “Arctic Report Card” for 2010, which found, among other things, that ice in Greenland is melting faster now than ever before, helping to increase the rate of sea level rise.
With these reports as context (and as a ‘news peg’), I’d like to share what we learned during our weekly CEJ seminar yesterday from Konrad Steffen, a climate scientist who has made huge contributions to our understanding of the behavior of Greenland’s ice sheets. Steffen is director of the Cooperative Institute for Research in Environmental Sciences here in Boulder. His comments bear directly on the new sea level report and what the NOAA Arctic report card said about Greenland.
“We already know that 2010 is the second warmest year on record,” at least so far, Steffen notes. But in Greenland, it has been the very warmest. (With Greenland instrumental records going back to 1750.)
At what he calls “Swiss Camp,” his research station on the Greenland ice sheet (at about 1,000 meters elevation and 70° N), Steffen and his colleagues have observed a seesawing pattern of temperature change, with the variability explained by such factors as volcanic eruptions and the North Atlantic Oscillation.
But over the long term, there has been a clear warming trend of about 2 °C per decade, with a total warming at Swiss Camp of 4 °C since 1991.
Warming temperatures in Greenland have been expanding the area of the ice sheet that is subject to melting during the summer. Up until this year, 2007 saw the widest area of melt on record. Steffen is still analyzing the data for this year, but it looks like 2010 did indeed see a record amount of melt. (The authors of the Greenland report card have found the same thing.)
Overall, the total area of Greenland subjected to melting has increased 65% since 1979.
This map shows the difference between the duration of summer melting in Greenland in 2010 and the 1979-2007 mean. Pink through purple colors indicate an increase in the number of days in which these regions were subject to melting. (Source: NOAA’s Arctic Report Card for 2010)

Data from NASA’s orbiting ICESat show that inland, the Greenland ice sheet is actually gaining in elevation by about 2 cm per year. It sounds counterintuitive, but it actually makes sense.
As the atmosphere above Greenland warms, it can hold more moisture. And that means there has been more snowfall. Snow that falls above the elevation where melting occurs simply accumulates year by year and compresses into ice. So in these regions, the ice sheet is thickening — as a result of global warming.
But along the coasts, the elevation of the ice has decreased by tens of meters — and in some places, by up to 50 meters, according to Steffen. Thus, ice loss along the coasts is outpacing any growth inland.
One of the best place to see this ice loss is the Jakobshavn Glacier on the western coast of Greenland. “The locals used to call it the ‘Dead Glacier,’ because it never moved,” Steffen says. “But in 2001, that glacier suddenly became very active.”
It is now flowing from into the sea at the extraordinary speed of 14 kilometers per year. “If you stand there, you can see the ice moving,” Steffen says. And you also can hear it. “It sounds like a train going by.”
The glacier is helping to move massive amounts of ice out to sea, where it contributes to sea level rise.
Globally, the oceans are now rising at about 3.3 millimeters per year. Last decade, the rate was 1.7 mm per year. “So it has roughly doubled in the last decade,” Steffen says. And Greenland’s contribution to this sea level rise has risen from about 20% to 30% between 2004 and the present.
“It’s a huge change,” Steffen says.
Between 2003 and 2007, NASA’s Grace satellite showed that it was mostly eastern Greenland that was losing ice. But now, ice loss is steadily moving up the west coast, and considerably farther north than the Jakobshavn Glacier.
“All these glaciers are moving faster into the ocean,” Steffen says.
As glaciers farther north lose more and more ice, the implications are significant. The water locked up in the ice lost to the sea every day in summer from the Jakobshavn Glacier alone is equal to the amount of water used by New York City in an entire year. But the glaciers to the north potentially could discharge at a rate 10 times that amount, according to Steffen.
If those glaciers speed up enough, they have the potential to “drain most of northern Greenland in decades or a century,” Steffen says. That’s a wide range of uncertainty, to be sure. And it reflects gaps in scientists’ understanding of the ice dynamics. More about that in a minute. But first, based on what scientists know now, what can we expect in the way of sea level rise in coming decades?
The last IPCC report, published in 2007, said we could get up to 1 meter of rise by 2100. That would be the result of ice loss not just in Greenland, of course, but also Antarctica, as well as melting of mountain glaciers, and thermal expansion of sea water. But this prediction did not include growing understanding of ice dynamics in Greenland. As a result, sea level rise by 2100 could be higher than the IPCC’s most recent prediction.
But even 1 meter of sea level rise would pose “a major challenge,” Steffen says. To offer one example, with a storm surge from a hurricane, New York City could actually be looking at 1.5 meters of water sloshing ashore. And that could flood a good portion of the city.
With ice dynamics factored in, including accelerating coastal glaciers, what might we expect? Scientists cannot say with any confidence. But statistical modeling, based on what we know has happened in the past in Greenland, as well as on current observations of ice mass loss,  suggest that 2 meters of sea level rise, and even up to three, is possible, according to Steffen.
But he also cautions that physical models of ice behavior cannot yet accurately reproduce the kind of rapid flow that Jakobshavn Glacier has been undergoing, and the more northwesterly glaciers could undergo with additional warming. So 2-3 meters may be plausible, but the likelihood of that degree of sea level rise is not yet known. And without better physical models of ice flow through outlet glaciers like Jacobshavn, scientists also cannot say for sure how rapidly Greenland’s interior could deglaciate through this process.
These caveats are important to keep in mind. So is something else: “Our coastal societies developed in a time of stable sea level,” he says. But now, sea level is rising — and at an increasing rate. “That’s why this is scary.”

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