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Friday, March 4, 2011

Jeff Masters: Greenland update for 2010: record melting and a massive calving event

Greenland update for 2010: record melting and a massive calving event

by Jeff Masters, wunderblog, March 4, 2011

No humans were present on the morning of August 4, 2010, in a remote fjord in Northwest Greenland, when the air vibrated with a thunderous crack as one of the largest icebergs in world history calved from the Petermann Glacier, the island's second largest ocean-terminating glacier. Where the glacier meets the sea, a 43-mile-long tongue of floating ice existed at the beginning of 2010. On August 4, 2010, a quarter of this 43-mile-long tongue of floating ice fractured off, spawning a 100-square-mile ice island four times the size of Manhattan, with a thickness half that of the Empire State building. According to Andreas Muenchow, associate professor of physical ocean science and engineering at the University of Delaware's College of Earth, Ocean, and Environment, the freshwater stored in this ice island could have kept the Delaware or Hudson rivers flowing for more than two years, or kept all U.S. public tap water flowing for 120 days. There was speculation that the ice island could find its way into the open Atlantic Ocean in two years, and potentially pose a threat to oil platforms and ships. However, as the ice island made its turn to get from the narrow Petermann Fjord to enter Nares Strait between Greenland and Canada, the mighty iceberg split into thousands of small icebergs that will not pose an unusual threat to shipping when they emerge into the Atlantic.

Figure 1. The 100-square-mile ice island that broke off the Petermann Glacier heads out of the Petermann Fjord in this image taken by NASA's Aqua satellite on August 21, 2010. Image credit: NASA. I've constructed a 7-frame satellite animation available here that shows the calving and break-up of the Petermann Glacier ice island. The animation begins on August 5, 2010, and ends on September 21, with images spaced about 8 days apart. The images were taken by NASA's Aqua and Terra satellites.

Petermann Glacier spawned smaller ice islands in 2001 (34 square miles) and in 2008 (10 square miles). In 2005, the Ayles Ice Shelf, about 60 miles to the west of Petermann Glacier, disintegrated and became a 34-square-mile ice island. The August 2010 Petermann Glacier calving event created the largest iceberg observed in the Arctic since 1962, when the Ward Hunt Ice Shelf on the north coast of Canada's Ellesmere Island calved off a massive 230-square-mile chunk. The Ward Hunt Ice Shelf also calved off a huge 21 square mile ice island a few days after the August 2010 Petermann Glacier calving event. According to an article in"Driftwood and narwhal remains found along the Ellesmere coast have radiocarbon dates from roughly 3,000 to 6,800 years ago, implying that the ice has been intact since those remains were deposited." All of the these calving events are evidence that the ice sheets in the Arctic are responding as one would expect to significantly warmer temperatures.

Warmer ocean temperatures cause significant melting of Greenland's glaciers 
At a talk last December at the world's largest conference on climate change, the American Geophysical Union (AGU) meeting in San Francisco, glacier expert Eric Rignot of UC-Irvine implicated ocean warming as a key reason for the calving of the Petermann Glacier's ice island. The ocean waters near the glacier have warmed by 1-2 °C over the past three years, he said, and all of the periphery of Greenland has seen ocean heat increases in recent years, with the result that 20-80% of all the mass lost by Greenland's glaciers in recent years could be attributed to melting of the glaciers by warmer waters attacking them from beneath. Ocean temperatures along the southwest coast of Greenland (60° N to 70° N, 60° W to 50° W) computed from the UK Hadley Center data set during 2010 were 2.9 °C (5.2 °F) above average -- a truly remarkable anomaly, and far warmer than the previous record of 1.5 °C above average set in 2003. Sea surface temperature records for Greenland began in the 1920s. 

A study earlier this year published in the journal Science (Spielhagen et al., 2011) found that ocean temperatures on the east side of Greenland are now at their warmest levels in at least 2,000 years. The researchers studied a sediment core containing fossil remains of planktic foraminifers, which vary as a function of water temperature. The study noted that not only have the waters flowing northward on the east side of Greenland warmed significantly, the volume of water flowing north has also increased, resulting in a large transport of heat into the Arctic. "Such an increased heat input has far-reaching consequences," they wrote.

Figure 2. Departure of sea surface temperature from average for 2010 from the NOAA Daily Optimum Interpolation SST Anomaly data set for October 2010. Areas colored red are warmer than the 1971-2000 average, areas colored blue are cooler than that average. A large region of record warm water temperatures extended along the west coast of Greenland, leading to record warm air temperatures and record melting along the western portion of Greenland in 2010. Ocean temperatures along the southwest coast of Greenland (60° N to 70° N, 60° W to 50° W) computed from the UK Hadley Center data set during 2010 were 2.9 °C (5.2 °F) above average -- a truly remarkable anomaly, surpassing the previous record of 1.5 °C set in 2003. Sea surface temperature records for Greenland began in the 1920s. Image credit:NOAA Visualization Lab.

Record warmth and melting in Greenland during 2010
Greenland's climate in 2010 was marked by record-setting high air temperatures, the greatest ice loss by melting since accurate records began in 1958, and the greatest mass loss of ocean-terminating glaciers on record. That was the conclusion of the 2010 Arctic Report Card, a collaborative effort between NOAA and European Arctic experts that comes out each year. 

Was 2010 the warmest year in Greenland's history? That is difficult to judge. We know it was also very warm in the late 1920s and 1930s in Greenland, but we only have two stations, Godtahab Nuuk and Angmagssalik, with weather records that go back that far (Figure 3.) Godtahab Nuuk set a record high in 2010, but temperatures at Angmagssalik in 2010 were similar to what was observed during several years in the 1920s and 1930s. 

Marco Tedesco of the City College of New York's Cryosphere Processes Laboratory remarked that last year's record warmth and melting in Greenland began when an unusually early spring warm spell reduced and "aged" the snow on the surface of the ice sheet, so that the snow became less reflective, allowing it to absorb more heat from the sun. This accelerated snow melt even further, exposing the bare ice, which is less reflective than snow and absorbs more heat. This feedback loop extended Greenland's record melting season well into the fall.

Figure 3. Historic temperatures in Greenland for the six stations with at least 50 years of data, as archived by NASA.Three of the six stations set record highs in 2010. However, only two of the six stations (Godtahab Nuuk and Angmagssalik) have data going back beyond the 1930s, which was a period of warmth in Greenland similar to the warmth of the current decade. Godtahab Nuuk set a record high in 2010, but 2003 still ranks as Angmagssalik's hottest year on record.

Figure 4. The 2010 summer melt season was lasted more than 40 days longer (purple colors) than the mean melt season from 1979 to 2007. Image credit: Arctic Report Card.

Why Greenland matters: sea level rise
The major concern with a warming climate in Greenland is melting of the Greenland ice sheet, which currently contributes about 25% of the observed 3 mm/year (1.5 inches per decade) global rise in sea level. Higher sea levels mean increased storm surge inundation, coastal erosion, loss of low-lying land areas, and salt water contamination of underground drinking water supplies. Greenland ice mass loss is accelerating over the long term, according to independent estimates using three different techniques (Figure 5), with more mass being lost each year than the previous year. 

According to Rignot et al. (2011), ice mass loss is also accelerating in Antarctica, and "the magnitude of the acceleration suggests that ice sheets will be the dominant contributors to sea level rise in forthcoming decades, and will likely exceed the IPCC projections for the contribution of ice sheets to sea level rise in the 21st century." As I discussed in a 2009 blog post, "How much will global sea level rise this century?", the IPCC in 2007 estimated that global sea level would rise 0.6-1.9 feet by 2100, but several studies since then predict a higher range of 1.6-6.6 feet.

During the warm period 125,000 years ago, before the most recent ice age, roughly half of the Greenland ice sheet melted. This melting plus the melting of other smaller Arctic ice fields is thought to have caused 7.2-11.2 feet (2.2-3.4 meters) of the 13-20 foot (4-6 meters) sea level rise observed during that period. Temperatures in Greenland are predicted to rise 3 °C by 2100, to levels similar to 125,000 years ago. If this level of warming occurs, we can expect sea levels to rise 13-20 feet several centuries from now. There's enough water locked away in the ice sheet to raise sea level to rise 23 feet (7 meters), should the entire Greenland ice sheet melt.

Figure 5. Loss of mass from Greenland's ice sheet in gigatons per year from 1992 through 2009, as computed from satellite gravity measurements from the GRACE satellites (red line) and from a mass balance method. The mass balance method computes the amount of snow on the surface, the amount of ice mass lost to wind and melt, and the amount of ice lost computed from glacier velocity and ice thickness. Adding together these terms gives the total amount of ice lost or gained over the ice sheet. The acceleration is given in gigatons per year squared. Another paper by Zwally et al. (2011) used a third method, laser satellite altimetry, to determine Greenland mass loss. Between 2003 to 2007, the ice sheet lost 171 gigatons of mass per year. Between 1992 to 2002, Greenland was only losing only 7 gigatons per year. Image credit: Rignot et al. (2011, Geophys. Res. Lett.).
Rignot, E., et al., 2011: Acceleration of the contribution of the Greenland and Antarctic Ice Sheets to sea level rise, Geophysical Research Letters, in press, doi:10.1029/2011GL046583.

Spielhagen et al., 2011: Enhanced Modern Heat Transfer to the Arctic by Warm Atlantic Water, Science (28 January 2011), Vol. 331, No. 6016, pp. 450-453; DOI: 10.1126/science.1197397

Zwally, J., et al., 2011: Greenland ice sheet mass balance: distribution of increased mass loss with climate warming; 2003-2007 versus 1922-2002, Journal of Glaciology, Vol. 57, No. 201.

Wunderground's climate change section has a Greenland web page with detailed information and references.

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