|The height of the Greenland ice sheet at present (left) and during the last interglacial (about 130,000 years ago), as simulated by the NCAR-based Community Climate System Model coupled with an ice-sheet model. Source: Bette Otto-Bliesner, NCAR|
A set of papers published in the 24 March issue of Science suggests that the ice sheets covering both the Arctic and Antarctic could melt more quickly than expected this century.
Two studies led by scientists at the National Center for Atmospheric Research (NCAR) and the University of Arizona blend computer modeling with paleoclimate records to show that by 2100 Arctic summers may be as warm as they were nearly 130,000 years ago when sea levels rose to 20 feet (6 meters) higher than they are today.
Bette Otto-Bliesner (NCAR) and Jonathan Overpeck (University of Arizona) based their findings on data from ancient coral reefs, ice cores, and other natural climate records, as well as output from the NCAR-based Community Climate System Model (CCSM), a powerful tool for simulating past, present, and future climates.
The National Science Foundation (NSF), which provides primary support to NCAR, funded the research. The study also involved researchers from the universities of Calgary and Colorado, the US Geological Survey, and Pennsylvania State University.
The historical data describe a period in Earth’s history characterized by a high level of Arctic warming. Based on those data, the modeling experiments Otto-Bliesner and Overpeck conducted provide important insights about possible future environmental changes in a warmer world that have the potential to significantly alter our natural and man-made environments.
Although the focus of our work is polar, the implications are global. These ice sheets have melted before and sea levels rose. The warmth needed isn’t that much more than present conditions.—Otto-Bliesner
The two studies show greenhouse-gas increases over the next century could warm the Arctic by 5-8 degrees Fahrenheit (3-5 degrees Celsius) in summertime-—about as warm as it was 130,000 years ago, between the most recent ice age and the one before it.
The difference 130,000 years ago is that there was an increase in solar radiation over the Arctic, caused by slight changes in the Earth-Sun orbit, which is a normal cycle that occur over tens of thousands of years. This time around the warming is man-made, caused by carbon dioxide emissions, but the effects on Arctic sea ice, permafrost, and icefields are forecast to be similar.—Dr. Shawn Marshall, University of Calgary
Although simulation results depend on the assumptions and conditions of different models, estimates of warming from the CCSM are within the range projected by other climate models, according to the authors.
Getting the past climate change correct in these models gives us more confidence in their ability to predict future climate change.—Otto-Bliesner
The CCSM suggests that during the interglacial period, melt water from Greenland and other Arctic sources raised sea level by as much as 11 feet (3.5 meters), says Otto-Bliesner. However, coral records indicate the sea level actually rose 13-20 feet (4-6 meters) or more. Overpeck concludes that Antarctic melting must have produced the remainder of the sea-level rise.
These studies are the first to link Arctic and Antarctic melting in the last interglacial period. Marine diatoms and beryllium isotopes found beneath the West Antarctic Ice Sheet indicate parts of the ice disappeared at some point over the past several hundred thousand years.
Overpeck theorizes that the rise in sea levels produced by Arctic warming and melting could have helped destabilize ice shelves at the edge of the Antarctic ice sheet and led to their collapse. If such a process occurred today, it would be accelerated by global-scale greenhouse-induced warming year round, Overpeck says. In the Arctic, melting would likely be hastened by pollution that darkens snow and enables it to absorb more sunlight.
A study by Robert Bindschadler, a glaciologist at NASA’s Goddard Space Flight Center confirms that warmer water temperatures are creeping into the Earth’s colder areas, increasing melting and accelerating ice flow in polar areas.
Temperatures collected from ships and buoys showed a warming of all oceans. That increase began before satellite sensors detected temperature increases of sea surfaces. Most of the warming was limited to the oceans’ upper 1,000 meters (.62 mile), except in the North Atlantic. In the cold North Atlantic waters, heat penetrated even deeper. This warming has increased the melting of sea ice in the North Atlantic.
These warm waters are beginning to melt the underside of the floating fringes of the Greenland ice sheet, even at great depths, It is these fringes that have been holding back vast stores of ice locked up in the Greenland ice sheet, and as this ice has been melting, the glaciers have hastened their flow to the sea.
A recent assessment in the changes in speed and the amount of snow and ice around Greenland confirms a large melting of outflow glaciers and acceleration of ice flow. Three large glaciers, the Kangerdlugssuaq, Helheim and Jakobshavns Isbrae have been melting at a rapid rate over the past several years.
Jakobshavns, the largest outlet glacier on Greenland’s east coast, has been annually thinning at 15 meters (49.2 feet) since 1997. The other two glaciers have also been thinning. Kangerdlugssuaq at 40 meters (131.2 feet) per year and Helheim at 25 meters per year (82.0 feet), which can’t be explained by normal melting. All of these glaciers have also been accelerating. Scientists are seeing similar behavior in Antarctica as well.
In another paper, a team from Harvard University and Columbia University report an unexpected offshoot of global warming: “glacial earthquakes” in which Manhattan-sized glaciers lurch unexpectedly, yielding temblors up to magnitude 5.1 on the moment-magnitude scale, which is similar to the Richter scale.
Glacial earthquakes in Greenland, the researchers found, are most common in July and August, and have more than doubled in number since 2002.
People often think of glaciers as inert and slow-moving, but in fact they can also move rather quickly. Some of Greenland’s glaciers, as large as Manhattan and as tall as the Empire State Building, can move 10 meters in less than a minute, a jolt that is sufficient to generate moderate seismic waves.—Göran Ekström, Harvard
As glaciers and the snow atop them gradually melt, water seeps downward. When enough water accumulates at a glacier’s base, it can serve as a lubricant, causing blocks of ice some 10 cubic kilometers in size to lurch down valleys known as outlet glaciers, which funnel all of Greenland’s glacial runoff toward the surrounding sea.
Our results suggest that these major outlet glaciers can respond to changes in climate conditions much more quickly than we had thought. Greenland’s glaciers deliver large quantities of fresh water to the oceans, so the implications for climate change are serious. We believe that further warming of the climate is likely to accelerate the behavior we’ve documented.—Meredith Nettles, Columbia
Seismometers worldwide detected 182 earthquakes in Greenland between January 1993 and October 2005. The research team examined the 136 best-documented of these seismic events, ranging in magnitude from 4.6 to 5.1. All 136 temblors were found to have originated at major valleys draining the Greenland Ice Sheet, implicating glacial activity in the seismic disturbances.
Of the 136 earthquakes analyzed, more than a third occurred during the months of July (22 earthquakes) and August (24 earthquakes). By comparison, January and February each saw a total of only four earthquakes between 1993 and 2005. Non-glacial earthquakes in polar regions show no seasonal variability.
Greenland’s overall number of glacial earthquakes also increased markedly between 1993 and 2005. Annual totals hovered between 6 and 15 through 2002, followed by sharp increases to 20 earthquakes in 2003, 24 in 2004, and 32 in the first 10 months of 2005. A single area of northwestern Greenland, where only one seismic episode was observed between 1993 and 1999, experienced more than two dozen glacial quakes between 2000 and 2005. Polar regions have not experienced increases in non-glacial earthquakes in recent years.
While the glacial earthquakes appear most common in Greenland, the scientists have also found evidence of glacial earthquakes originating at mountain glaciers in Alaska and at glaciers located in ice streams among the edges of Antarctica.
“Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-Level Rise,”; Jonathan T. Overpeck, Bette L. Otto-Bliesner, Gifford H. Miller,Daniel R. Muhs, Richard B. Alley, Jeffrey T. Kiehl; Science 24 March 2006:Vol. 311. no. 5768, pp. 1747–1750; DOI: 10.1126/science.1115159
“Simulating Arctic Climate Warmth and Icefield Retreat in the Last Interglaciation,”; Bette L. Otto-Bliesner, Shawn J. Marshall, Jonathan T. Overpeck, Gifford H. Miller, Aixue Hu; Science 24 March 2006:Vol. 311. no. 5768, pp. 1751–1753; DOI: 10.1126/science.1120808
“Hitting the Ice Sheets Where It Hurts”; Robert Bindschadler; Science 24 March 2006:Vol. 311. no. 5768, pp. 1720–1721; DOI:10.1126/science.1125226
“Seasonality and Increasing Frequency of Greenland Glacial Earthquakes”; Göran Ekström, Meredith Nettles, Victor C. Tsai, Science 24 March 2006:Vol. 311. no. 5768, pp. 1756–1758; DOI: 10.1126/science.1122112