Mapping of Greenland may aid understanding of sea-level mystery

Greenland appears green in this world map.
Greenland appears green in this world map.

A University of Alberta Arctic ice researcher is closing in on some real understanding about the process that might be feeding rising sea levels.

Using satellite microwave data, Martin Sharp, a professor in the Department of Earth and Atmospheric Sciences, has successfully mapped the summer melt extent and duration of the summer melt across Greenland for a five-year period stretching from 2000 to 2004.

“What we’re interested in is the problem of explaining why the rates of global sea level rising have more or less doubled since the early 1990s compared with the early part of the 20th century,” he said.

Sharp, whose findings were recently published in the Journal of Geophysical Research, says there is mounting evidence that indicates the increasing sea level is probably due to the substantial increase in the rate in which Greenland is generating water, both by melting and by carving ice bergs into the ocean.

While climate change is the leading hypothesis of the day, the processes that are accelerating the melting of Greenland’s ice cap are still largely misunderstood.

“Some of this water is getting to the bottom of the ice sheet, even in places where it is a kilometre-and-a-half thick, and at sub-freezing temperature pretty much through the whole thickness,” said Sharp. “It’s kind of counter-intuitive that water would find its way through, but it looks like that is what may be happening.”

He says that water at the base of the ice sheet acts as lubricant for the glaciers above to flow faster. An increased rate of glacial movement means a surge in the rate at which icebergs are being dropped into the water. This, in turn, may be contributing to rising sea levels, he suggests.

Sharp hypothesizes that this increase might very well be destabilizing the flow of glaciers, overwhelming the natural drainage system at the bottom of the ice sheet, increasing the water pressure, and, in effect, ‘floating’ the ice off the bedrock, allowing it to speed up.

While it’s true the speed of glacial flow has seen an increase from eight kilometres per year to 15 over the last two decades, the onset of another interesting phenomenon may be helping to shape the tale. Sharp says seismic monitoring stations have measured a substantial increase in the number of large earthquakes being reported in areas where Greenland seems to be losing mass more rapidly.

“The suggestion is those earthquakes are generated by increased slip between rock and the ice underneath,” he said. “The glaciers are moving like a large landslide and causing tremors in the crust beneath it when it does.”

“It’s a hypothesis, but there is not a whole lot of direct evidence that says this is the case,” said Sharp, who is now in search of a grad student to help compile all the data.

“We know where those earthquakes took place, so now we can ask the question: Does the occurrence of those earthquakes actually coincide with periods of unusually high melt?”

Previous attempts at mapping Greenland’s ice sheet were unreliable, resulting in data points whose imagery had a resolution that covered more than 4,000 square kilometres of ice shelf.

So three years ago, Sharp began tapping into another satellite source that has been available since 1999, which produces a resolution of a more manageable 16 square kilometres.

Over that area, a satellite sends down a signal, some of which is absorbed by the surface while some is scattered back to the satellite. If there is any water at the surface at all, a very large portion of the energy hitting the surface is absorbed and doesn’t come back. The result is daily resolutions of the distribution of ice melt across the whole ice sheet.

Sharp broke Greenland down into nine different regions to better determine exactly when it was melting where in each year. For each region, Sharp says he will be able to calculate the average length of melt season and then back-calculate the air temperature. Once the inputs are created, Sharp looks forward to determining exactly what is driving these melt events.

“We are asking if it is actually true that the amount and extent of melting in Greenland has increased over the period of interest and, in particular, is it increasing in the places where it looks as if the flow of the glacier has been sped up,” said Sharp.

Ancient Flood Disrupted Ocean Circulation And Triggered Climate Cooling

As the giant North American ice sheets melted an enormous pool of freshwater, many times larger than all of the Great Lakes, formed behind them. About 8400 years ago this pool of freshwater burst free and flooded the North Atlantic. About the same time, a sharp century long cold spell is observed around the North Atlantic and other areas. Researchers have often speculated that the cooling was the result of changes in ocean circulation triggered by this freshwater flood. The sudden addition of so much freshwater would have curtailed (suppressed) the sinking of deep water in the North Atlantic and as a consequence less warm water would be pulled north in the Gulf stream.

In a new study in Science (published online in Science Express 6 December) Kleiven and co workers confirm that the deep ocean was disturbed in just the way previous workers had speculated. Using a marine core from south of Greenland, which monitors the southward flowing deep waters formed in the North Atlantic they show that there is a sudden disruption in the deep circulation pattern at the time of the flood outburst.

Just at the time of the flood, the chemical properties of the deep ocean shift suddenly to values not observed at any other time in the last 10,000 years. The chemical changes suggest that at the site south of Greenland, the new deep waters formed in the North were completely replaced by older deepwater coming from the south.

This suggests that deep waters from the North Atlantic were too shallow or weak to influence this site for about century following the flood outburst after which time the deep ocean snapped back to its near modern state. This is what researchers had predicted and what computers have simulated the ocean needed to have done in order to help bring about the cold spell.

Kleiven et al., strengthen the connection between the deep ocean changes and the climate anomaly by showing that the sharp cooling at their location falls within the century long disruption in deep circulation. If the cooling had fallen outside the period of disrupted circulation, the role of the ocean and related heat transport could have been ruled out as the major driver of the cooling.

Perhaps even more importantly, they show that deep circulation is altered over just a few decades or less demonstrating that the the deep ocean changes fast enough to drive the sudden jump in climate seen at this and other times in the past.

There is no modern or future equivalent source for freshwater to cause a mega flood like that which occurred 8400 years ago. Yet, the fact that these deep ocean changes clearly occur on timescales rapidly enough to impact human societies underscores the importance of determining just how much freshwater is needed to bring about such dramatic changes—given the concerns that melting of the Greenland Ice Sheet may accelerate as the globe warms.

Greenland Melt Accelerating

An iceberg calved from a glacier floats in the Jacobshavn fjord in southwest Greenland. A new CU-Boulder study indicates Greenland continues to lose ice mass, and the rate of loss is accelerating.
An iceberg calved from a glacier floats in the Jacobshavn fjord in southwest Greenland. A new CU-Boulder study indicates Greenland continues to lose ice mass, and the rate of loss is accelerating.

The 2007 melt extent on the Greenland ice sheet broke the 2005 summer melt record by 10 percent, making it the largest ever recorded there since satellite measurements began in 1979, according to a University of Colorado at Boulder climate scientist.

The melting increased by about 30 percent for the western part of Greenland from 1979 to 2006, with record melt years in 1987, 1991, 1998, 2002, 2005 and 2007, said CU-Boulder Professor Konrad Steffen, director of the Cooperative Institute for Research in Environmental Sciences. Air temperatures on the Greenland ice sheet have increased by about 7 degrees Fahrenheit since 1991, primarily a result of the build-up of greenhouse gases in Earth’s atmosphere, according to scientists.

Steffen gave a presentation on his research at the fall meeting of the American Geophysical Union held in San Francisco from Dec. 10 to Dec. 14. His team used data from the Defense Meteorology Satellite Program’s Special Sensor Microwave Imager aboard several military and weather satellites to chart the area of melt, including rapid thinning and acceleration of ice into the ocean at Greenland’s margins.

Steffen maintains an extensive climate-monitoring network of 22 stations on the Greenland ice sheet known as the Greenland Climate Network, transmitting hourly data via satellites to CU-Boulder to study ice-sheet processes.

Although Greenland has been thickening at higher elevations due to increases in snowfall, the gain is more than offset by an accelerating mass loss, primarily from rapidly thinning and accelerating outlet glaciers, Steffen said. “The amount of ice lost by Greenland over the last year is the equivalent of two times all the ice in the Alps, or a layer of water more than one-half mile deep covering Washington, D.C.”

The Jacobshavn Glacier on the west coast of the ice sheet, a major Greenland outlet glacier draining roughly 8 percent of the ice sheet, has sped up nearly twofold in the last decade, he said. Nearby glaciers showed an increase in flow velocities of up to 50 percent during the summer melt period as a result of melt water draining to the ice-sheet bed, he said.

“The more lubrication there is under the ice, the faster that ice moves to the coast,” said Steffen. “Those glaciers with floating ice ‘tongues’ also will increase in iceberg production.”

Greenland is about one-fourth the size of the United States, and about 80 percent of its surface area is covered by the massive ice sheet. Greenland hosts about one-twentieth of the world’s ice — the equivalent of about 21 feet of global sea rise. The current contribution of Greenland ice melt to global sea levels is about 0.5 millimeters annually.

The most sensitive regions for future, rapid change in Greenland’s ice volume are dynamic outlet glaciers like Jacobshavn, which has a deep channel reaching far inland, he said. “Inclusion of the dynamic processes of these glaciers in models will likely demonstrate that the 2007 Intergovernmental Panel on Climate Change assessment underestimated sea-level projections for the end of the 21st century,” Steffen said.

Helicopter surveys indicate there has been an increase in cylindrical, vertical shafts in Greenland’s ice known as moulins, which drain melt water from surface ponds down to bedrock, he said. Moulins, which resemble huge tunnels in the ice and may run vertically for several hundred feet, switch back and forth from vertical to horizontal as they descend toward the bottom of the ice sheet, he said.

“These melt-water drains seem to allow the ice sheet to respond more rapidly than expected to temperature spikes at the beginning of the annual warm season,” Steffen said. “In recent years the melting has begun earlier than normal.”

Steffen and his team have been using a rotating laser and a sophisticated digital camera and high-definition camera system provided by NASA’s Jet Propulsion Laboratory to map the volume and geometry of moulins on the Greenland ice sheet to a depth of more than 1,500 feet. “We know the number of moulins is increasing,” said Steffen. “The bigger question is how much water is reaching the bed of the ice sheet, and how quickly it gets there.”

Steffen said the ice loss trend in Greenland is somewhat similar to the trend of Arctic sea ice in recent decades. In October, CU-Boulder’s National Snow and Ice Data Center reported the 2007 Arctic sea-ice extent had plummeted to the lowest levels since satellite measurements began in 1979 and was 39 percent below the long-term average tracked from 1979 to 2007.

CIRES is a joint institute of CU-Boulder and the National Oceanic and Atmospheric Administration.

New satellite study shows dramatic melting of Greenland ice during summer of 2007

Newly published research that includes satellite data from three separate sources shows that the seasonal melt on Greenland’s ice sheet during the summer of 2007 was a stunning 60 percent more than the previous high, set in 1998.

The new information, which differs from other studies by including information beginning in the early 1970s, is consistent with other indicators of worldwide global climate change, according to the author of the study, Thomas L. Mote, a climatologist from the University of Georgia.

“What we found was really quite remarkable,” said Mote, a professor in UGA’s department of geography and its Climatology Research Lab. “This work includes the longest satellite record anyone has for Greenland. No one piece of evidence ever tells the whole story, but when you put them together, they point in the same direction.”

Mote’s research was just published in the journal Geophysical Research Letters.

Perhaps just as dramatic as the huge increase in snow melt is that Greenland had as many as 50 more days of melt than average, and the melting season began a full month earlier than normal.

Mote has studied snow melt in Greenland for more than a decade, but even he was unprepared for the dramatic melt that occurred last summer. He compared data from three satellite sources:

  • The Special Sensor Microwave/Imager (SSM/I), which provided information from 1987 to present;

  • The Scanning Multichannel Microwave Radiometer (SMMR), which supplied data from 1979 to 1987; and

  • The Electrically Scanning Microwave Radiometer (ESMR), for data recorded in 1973, 1974 and 1976 respectively.

While other researchers had used data from SSM/I and SMMR, none had reexamined the older information generated from the ESMR satellite, and that longer record allowed Mote to see farther into the recent past of Greenland’s ice-sheet history.

“To be honest, there’s just not much useful satellite data prior to the 1980s,” said Mote.

There have been other warm periods in Greenland’s recent past, Mote says, most notably in the 1930s, though it’s difficult to say how much melt may have occurred then. But a dramatic change is now underway in a land that covers more than 800,000 square miles but has a permanent population of less than 60,000.

Just why the huge increase in melt occurred in the summer of 2007 is not yet entirely clear, said Mote. Certainly, increasing surface temperatures are part of it. Coastal meteorological stations showed higher-than-average temperatures for most of the season. But another culprit may be changes in the surface of the ice sheet itself.

Data show that the average number of melt days has been steadily increasing since 1997, and this may have allowed the ice sheet to become more susceptible to further melting. Large streams of water actually flow through chasms and cracks down to the land’s surface and cause the ice sheet to become unstable.

Another possible mechanism may be an increase in temperature of the snow or less snow accumulation in recent years, said Mote.

He points out that the noticeable summer melt of ice in Greenland is consistent with satellite observations that have pointed to decreasing sea ice across the Northern Hemisphere since 2000.

“The most recent Intergovernmental Panel on Climate Change assessment report concludes that changes in surface melting have contributed to a loss of mass in Greenland,” said Mote, “which they report is ‘very likely’ a contributor to global sea rise level.”

Researchers will be watching Greenland’s ice sheet warily next summer to see if the trend continues.