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.


The Glacier / Sea Dance

October 2, 2009

Glacier – Fjord Dynamics in Greenland

Score one for the seals. In addition to using high-tech equipment and sophisticated research techniques, researcher Gordon Hamilton (University of Maine, Orono) will also get a hand (fin?) from the ocean-dwelling creatures for his new NSF-funded collaborative study. Hamilton and a team of glaciologists and oceanographers will spend the next three years studying glacier-fjord interactions at Helheim Glacier in east Greenland to better understand how warm ocean waters are affecting the dynamics of outlet glaciers draining the ice sheet. The seals, tagged with small sensors that record position, depth and temperature, will collect data about what goes on beneath the ocean’s surface.

Gordon Hamilton (PI from University of Maine) recovers a GPS instrument from Helheim Glacier during summer 2009 field work. Photo: Leigh Stearns

Gordon Hamilton (PI from University of Maine) recovers a GPS instrument from Helheim Glacier during summer 2009 field work. Photo: Leigh Stearns

Above water, the team will investigate the mutual relationship between the glacier and the ocean through field season surveys and year-round data collection in order to better understand how ocean currents and heat affect the ice sheet. The research team will use these and other data to map the circulation and properties of the fjord and adjacent offshore waters, and how these characteristics change with time.

The research comes as scientists modeling the impact of global warming and melting sea ice are challenged to estimate the rate of ice sheet mass loss caused by dynamic thinning. (For more information, see “Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets,” Pritchard et al.)

We talked to Professor Hamilton about his upcoming research.

Sermilik, Greenland, where Hamilton has also done research. Photo: Gordon Hamilton

Sermilik, Greenland, where Hamilton has also done research. Photo: Gordon Hamilton

PFS: Given your new NSF grant to explore ice sheet / ocean interactions and their influence on glacier motion, what did you think of the Pritchard article in Nature this week, suggesting “the most profound changes in the ice sheets currently result from glacier dynamics at ocean margins?”

Gordon Hamilton: It’s a nice paper because, for the first time, it takes a quantitative look at the entire coastlines of Greenland and Antarctica and shows that near-coastal changes are happening everywhere, not just in a few places. It really points to the importance of ice–ocean interactions in playing a major role in the health of the ice sheets.

PFS: The Pritchard article also mentions that “accelerated melt” is another relatively unknown area of glaciology. Can you explain what that is, and how your study may shed light on it?

Gordon Hamilton: In this case, they are referring to “submarine melting” which occurs wherever glacier ice comes into contact with the ocean (outlet glaciers and ice shelves, for example). Basically, any time you have ocean temperatures above the local freezing point (i.e., the freezing point adjusted for salinity and density), you have excess heat available to melt glacier ice. Because the ocean can carry so much more heat than the atmosphere, submarine melting can be much more effective than surface melting but it is very difficult to observe because of the obvious challenges in accessing the underwater portions of glaciers. Our work will help quantify the submarine melting component because we will be making direct measurements of ocean heat content, and the rate at which it is replenished, in the waters adjacent to a large outlet glacier.

PFS: The lack of knowledge about glacier/ocean interactions represents a large gap when it comes to modeling the impacts of melting ice sheets. How will your research improve modeling, and what specifically will you be looking for to develop a better understanding of these mechanisms?

Gordon Hamilton: Very true. There are just a couple of measurements of heat content and circulation in Greenlandic fjords, and most of these measurements have been isolated samples. We deployed some moored instruments as part of our pilot project, so we are just beginning to get a handle on how one-off measurements compare with a full year’s worth of data on fjord behavior. Seasonal and interannual variability is likely to be quite large, so we really need to quantify it so we can understand how much heat gets transported to these glacier fronts, when it gets transported, and if the rate of heat transport is changing with time. We will link these changes to observations of how the glacier behaves. For example, does the glacier calve more icebergs when the ocean is warmer, and does that cause the glacier to flow faster? These are basic things to know if we want to model how future ocean changes might affect the ice sheet.

PFS: Why did you choose Helheim Glacier?

Gordon Hamilton: Helheim is important because it drains about 5% of the entire ice sheet, it recently doubled its rate of mass loss, and because it discharges freshwater (in liquid and solid iceberg form) to a sensitive part of the North Atlantic Ocean where global ocean currents are formed.

PFS: Do the Pritchard et al findings potentially alter your research plan?

No, not really, but they do provide extra motivation in case anyone doubted the role of the ocean in potentially controlling the fate of the ice sheet.

PFS: On a lighter note, you have Greenlandic colleagues who will outfit seals with sensors to collect information on ocean temperature and depth. Can you tell us more about the logistics of this? What do you expect to learn from the seals? Any chance you can outfit them with cameras, as well, so we can get seal-cam pix of the fjord or ocean?

Gordon Hamilton: This is something new for us, but it’s a fairly standard technique for marine biologists who want to understand how and where seals dive for food. The tags will measure depth and water temperature. Seals are much more effective at making these kinds of measurements than scientists because they live there year-round and are continually diving for food, whereas we only visit the area for short field seasons. We use them as oceanographic platforms to build up a really long and detailed record of oceanic conditions. Each tag should last for a few months, so it will allow us to look at seasonal patterns in heat content of the offshore waters adjacent to the Greenland Ice Sheet. Plus it makes a great collaboration for glaciologists and oceanographers and marine biologists to be working together. I’m not sure about the practicalities of installing seal-cams, but we do plan to install cameras at a few key points overlooking the glacier and fjord. The cameras will have telemetry capability, so we should be able to monitor fjord conditions in near real-time.

OFS: As part of your outreach program, you’re planning to present a non-technical report to the Greenlandic community. How might the report benefit them?

Gordon Hamilton: In southeast Greenland, where we are working, the way of life for a huge percentage of the population is dependent on hunting and fishing in the coastal and fjord waters. These waters are starting to change character, so different species of fish (warmer-water species) are moving in and other types are disappearing. Local fisherman have to adapt to these conditions. Also, the warming waters are probably playing a role in the recent speed-up of the many of the ice sheet’s outlet glaciers, meaning there are more icebergs in the fjord year-round. Icebergs are a real hazard to safe travel for these hunters, so they will be very interested in our predictions for future conditions.

 

The front of the Helheim Glacier. Photo:Gordon Hamilton

The front of Helheim Glacier. Photo:Gordon Hamilton

PFS: You’ve done extensive glaciology in Antarctica. How does working in Greenland compare to your antarctic research? Are you doing glacier/fjord studies on the southern continent as well?

Gordon Hamilton: This project just deals with Greenland, but some colleagues have recently started looking at similar processes in the Amundsen Sea of West Antarctica. And we are thinking about starting related work beneath the Ross Ice Shelf, close to where several large outlet glaciers enter from East Antarctica.

Working in Greenland is quite different from Antarctica. For one, real populations of people live in villages around Greenland’s coast; Antarctica only has scientific research stations. So the local population provides an interesting cultural perspective. Plus it is just a few hours from the east coast of the US, meaning it is a lot easier and quicker to conduct fieldwork there than in Antarctica.


Glacier Quakes

August 18, 2009

Meredith Nettles of Lamont-Doherty Earth Observatory, a scientist studying glacier dynamics in Greenland, sent a link to her project Web site the other day. There, in addition to basic information on her NSF-funded study, you can find a few pictures from the first of two field trips this year and a sheaf of photos from previous years as well.

The team accesses its monitoring sites via helicopter, landing on the scarred surface of the glacier. Source: Nettles Web site

The team accesses its monitoring sites via helicopter, landing on the scarred surface of a southern Greenland glacier. Source: Nettles Web site

Nettles and colleagues Gordon Hamilton (University of Maine) and Jim Davis (Smithsonian), along with Danish and Spanish colleagues and technical experts from UNAVCO, have placed Global Positioning Systems (GPS) networks on two of Greenland’s most active outlet glaciers, Helheim and Kangerdlugssuaq, both on the island’s southeastern coast.  These glaciers dump massive quantities of fresh water into the Arctic Ocean, and back around 2005 scientists noticed that they (and other glaciers in Greenland’s south) seemed to be flowing a lot faster all at once:  a statue placed on Helheim Glacier in January 2002 would have advanced an impressive six kilometers toward the ocean by year’s end; but during 2005 the same statue would have raced seaward some 11 kilometers—about half a football field a day, Gordon Hamilton estimated.  In addition to accelerated advance, the science team observed that the glacier—about 700 meters (nearly half a mile) thick from muddy bottom to tumbled top, seemed to be thinning rapidly as well, which suggested further destabilization.

The calving front of Helheim Glacier, 2006. Photo: Meredith Nettles

The calving front of Helheim Glacier, 2006. Photo: Meredith Nettles

The seasonal processes driving these changes are what the Nettles collaboration is attempting to discover. For the past few years, researchers have gone to one or both of the glaciers and placed GPS instruments on the ice to create the monitoring networks. (On the Helheim, the team has also placed time-lapse cameras and instruments to monitor climate, seismic and tidal activity.)  These have then collected precise information about the glaciers’ movements, sending data via radio signal to a collecting station on a rock outcrop which in turn sends data back to the Nettles lab via Iridium phone.

Here researchers install a GPS instrument in the middle of nowhere--actually a northern section of the Helheim). Photo: J. Vilendal Petersen

Here researchers install a GPS instrument in the middle of nowhere--a northern section of the Helheim). Photo: J. Vilendal Petersen

The networks have captured information about so-called glacier quakes, phenomena discovered less than a decade ago by Nettles and colleagues monitoring other seismic information. The team noticed that seismic signals were being recorded in clusters around the coast of southern Greenland, an area traditionally associated with little seismic activity. Further study revealed that the seismic activity was caused by sudden, fierce movement of glaciers lurching forward, but the physical processes where not known.

Since then, Nettles and others have learned a bit about these quakes. Nettles talked with Popular Mechanics earlier this year, explaining how glacier quakes work:

“We saw a couple last summer from our helicopter, near the calving front. We were at the outlet to the Helheim glacier, in a system of fjords with sheer rock walls that are 500 meters [more than 1600 ft] tall. Typically, you start to see a rift open up in the glacier and then this big block of ice starts to roll over. The block that breaks off might be a couple of kilometers long and it’s the full thickness of the glacier, which is about seven hundred meters—mainly underwater. . . . It takes a couple of minutes to fall, and as it’s rolling, it has to move this thick melange of ice and water that’s in front of it out of the way. You start to see the icebergs moving very, very fast down the fjord or, if they’re close to the calving front, you see them being popped up, straight towards the helicopter. Then you see just tons of water streaming off of the new iceberg as it is being formed. We have instruments to detect the resulting tsunami about 35 or 40 kilometers away.”

Not for the faint of heart.

Instruments located on Helheim Glacier, Greenland.

Instruments located on Helheim Glacier, Greenland.

After capturing a season’s worth of data on the GPS networks, the Nettles team is back in the field this week removing the Kangerdlugssuaq network and winterizing that on the Helheims. She indicated that she is pleased with the data capture. See for yourself by clicking “Telemetry Status” on the project Web page.


Iceland’s Vatnajökull: Europe’s Newest National Park

June 20, 2009

Field Notes contributor Rachel Walker recently spent a week in Iceland visiting some of the country’s natural wonders and will be writing several posts on her explorations.

On June 7, 2008, Iceland established Vatnajökull National Park. Europe’s newest and largest national park, it joins the existing Skaftafell National park, Jokulsargljufur National Park, and the Vatnajökull glacier.

A map of Iceland shows Vatnajokull in the southeastern part of the country.

A map of Iceland shows Vatnajokull in the southeastern part of the country and the volcanic trends that are so formative to the country's geography.

The new park covers a 12,000 square kilometer area—more than 12 percent of the Iceland’s surface, and Iceland’s tallest peak, Hvannadalshnúkur (2110 m), is located in the southern periphery.

The enormous area is a natural wonder smorgasborg: raging waterfalls, expansive peaks, glacial valleys, volcanoes, hot springs, and, of course, glaciers.

The Svartifoss waterfall spills over columnar basalt in Vatnajokull National Park.

The Svartifoss waterfall spills over columnar basalt in Vatnajokull National Park.

 

The Jokulsarlon lagoon, where ice falling off of the glacier drops into a giant lake at one of many tongues of the Vatnajokull glacier.
The Jokulsarlon lagoon, where ice falling off of the glacier drops into a giant lake at one of many tongues of the Vatnajokull glacier.
View from the Foss Hotel Skafatell, located in the national park, on a sunny day.

View from the Foss Hotel Skafatell, located in the national park, on a sunny day.

Iceland’s glaciers are retreating. From 1958 to 2000, the Vatnajölull glacier has retreated 328 square kilometres, shrinking from 8,538 square kilometres to 8,160 square kilometres. Still, this namesake glacier remains Europe’s largest. At its thickest, Vatnajökull is about 1,000 meters, and on average it measures between 400 and 500 meters. It covers seven active volcanoes, which cause enormous floods when they erupt (in 1996, the Grimsvotn volcano erupted so violently it lifted the glacier and caused torrents of floodwater to burst forth; the destruction caused significant death and obliterated a major section of the road).

In short, this is a fascinating area with myriad research subjects. It’s also a beautiful place to take a walk. But given the volatile weather, getting a good glimpse of the natural wonders is not guaranteed.