What Lies Beneath

December 18, 2009

For years, scientists thought that melted water beneath Greenland’s coastal glaciers such as the Jakobshavn and Helheim lubricated the giant sheets of ice above, accelerating their plunge into the ocean and contributing to loss of sea ice. Turns out, that was an over-simplified explanation, said Ian Howat, assistant professor of earth sciences at Ohio State University.

Speaking in a press conference Wednesday at the annual meeting of the American Geophysical Union (AGU), the NASA-funded, CPS-supported scientist explained that the subsurface dynamics beneath glaciers is significantly more complex than previously thought.

“In the science community it’s been accepted that basal lubrication due to increased melting and warming is responsible for accelerating glacial advance and breaking off,” said Howat. “We’re finding out that’s not true.”

A calving glacier drops huge ice chunks into the sea. Photo: Martyn Clark, National Snow and Ice Data Center

Specifically, a complex, subglacial “plumbing” system involving the ocean, meltwater, and ice evolves, which drives the glacial calving. In fact, early evidence from Howat’s research suggests that ocean changes have a greater impact on the rate at which outlet glaciers spill into the sea than does meltwater.

Much of the melt water comes from early summer hot temperatures, which melt the glacier’s surface. The water flows through cracks in the ice to the ground surface.

Ian Howet in the field. Photo: Ohio State University

In the early summer, the sudden influx of water overwhelms the subglacial drainage system, causing the water pressure to increase and the ice to lift off its bed and flow faster—up to 100 meters per year, he said. The water passageways quickly expand, however, and reduce the water pressure so that by mid-summer the glaciers flow slowly again.

Inland, this summertime boost in speed is very noticeable, since the glaciers are moving so slowly in general. But outlet glaciers along the coast, such as the Jakobshavn, are already flowing out to sea at rates as high as 10 kilometers per year — a rate too high to be caused by the meltwater.

“So you have this inland ice moving slowly, and you have these outlet glaciers moving 100 times faster. Those outlet glaciers are feeling a small acceleration from the meltwater, but overall the contribution is negligible,” Howat said.

His team looked for correlations between times of peak meltwater in the summer and times of sudden acceleration in outlet glaciers, and found none. So if meltwater is not responsible for rapidly moving outlet glaciers, what is? Howat suspects that the ocean is the cause.

Through computer modeling, he and his colleagues have determined that friction between the glacial walls and the fjords that surround them is probably what holds outlet glaciers in place, and sudden increases in ocean water temperature cause the outlet glaciers to speed up.

However, Howat said meltwater can have a dramatic effect on ice loss along the coast. It can expand within cracks to form stress fractures, or it can bubble out from under the base of the ice sheet and stir up the warmer ocean water. Both circumstances can cause large pieces of the glacier to break off, and the subsequent turbulence stirs up the warm ocean water, and can cause more ice to melt.


Rapid Coastal Erosion Correlated to Diminishing Sea Ice

December 16, 2009

Retreating sea ice leaves the Alaskan coast vulnerable to the full force of the ocean. Photo: Benjamin Jones, USGS

Rapid erosion of the northern coastline of Alaska midway between Point Barrow and Prudhoe Bay is accelerating at a steady rate of 30 to 45 feet a year, according to CPS-supported scientists presenting a study at the annual American Geophysical Union meeting this week in San Francisco. As the coast erodes, frozen blocks of silt and peat that contain 50 to 80 percent ice topple from bluffs into the Beaufort Sea during the summer.

The acceleration is caused by a combination of large waves pounding the shoreline and warm seawater melting the base of the bluffs, said CU-Boulder Associate Professor Robert Anderson, a co-author on the study. Once the blocks fall they melt within days and sweep silt material out to sea.

Anderson, along with collaborators Cameron Wobus of Stratus Consulting and Irina Overeem of CU’s Institute of Arctic and Alpine Research (INSTAAR) have studied the coastline for the past two summers with Office of Naval Research support. Equipped with two meteorology stations, a weather station, time-lapse cameras, detailed GPS and wave sensors outfitted with temperature loggers, they documented the summer ocean/shore dynamic.

Triple Whammy

Declining sea ice, warming sea water, and increased waves create a “triple whammy” that expedites erosion. For the majority of the year, the Beaufort Sea is covered with sea ice that disconnects from the coast during the summer. These ice-free summer conditions are lasting for longer periods of time, allowing warmer ocean water to lap the coast and weaken the frozen ground. And the longer that sea ice is not connected to the coastline, the further the distance grows between the ice and the shore.  This open-ocean distance between the sea ice and the shore, known as “fetch,” increases both the energy of waves crashing into the coast and the height to which warm seawater can come into contact with the frozen bluffs, said Anderson.

The shoreline bluffs are made up of contiguous, polygon-shaped blocks, primarily made of permafrost and measuring roughly 70 to 100 feet across. Ice “wedges” (created by seeping summer surface water that annually freezes and thaws) are driven deep into the cracks between individual blocks each year. The blocks closest to the sea are undermined as warm seawater melts their base, and eventually split apart from neighboring blocks and topple during stormy conditions, said Anderson.

Impacts of Erosion

As the coastline submits to the ocean, old whaling stations, military and oil related infrastructure and entire towns threaten to fall into the sea. In addition, the loss of sea ice alters ocean systems and diminishes habitat for creatures like the polar bear.

According to a 2009 CU-Boulder study, Arctic sea ice during the annual September minimum is now declining at a rate of 11.2 percent per decade. This year, only 19 percent of the ice cover was more than two years old — the least ever recorded in the satellite record and far below the 1981-2000 summer average of 48 percent.


Follow

Get every new post delivered to your Inbox.

Join 94 other followers