Getting warmer

Posted: April 3, 2008

Courtesy: Antarctic Sun

By Peter Rejcek

Soon we may have to call it the Subantarctic Peninsula.

Scientists who monitor the ecosystem at the northern tip of the Antarctic Peninsula say a warmer, moist climate has migrated into their research area, virtually eliminating perennial sea ice there and driving the local Adélie penguin population to the brink of extinction.

“Our prediction now is that within the next five to 10 years there will not be any Adélies left at Palmer of any consequence,” said Bill Fraser, one of the principal investigators studying the ecosystem near Palmer Station, a U.S. research station located on Anvers Island off the coast of the peninsula.

The formal name of the ecosystem study is the Palmer Long Term Ecological Research (PAL LTER) program, a multi-disciplinary approach to understand the processes that affect climate and environmental change over a broad time scale. Established in 1990, the PAL LTER is part of a larger system of 26 LTER sites, mostly in and around the United States.

Researchers say the peninsula, particularly a 100,000-square-kilometer swath in the ocean that makes up the PAL LTER study site, is undergoing some of the most rapid climate change on the planet.

“It’s amazing what’s happening,” said Doug Martinson, another PAL LTER principal investigator. “It’s showing the most rapid rise in air temperature during winter time.”

The increase is about 6.5 degrees Celsius in the winter since the 1950s, rising more than five times faster than the global average. The life cycle of winter sea ice, on average, has dropped by three months per year, meaning it forms later and melts earlier. Year-round sea ice has virtually disappeared.

“We pretty much don’t have any perennial sea ice in our grid anymore, which has dramatic implications for the ecology and Bill Fraser’s penguins,” Martinson said. “That is just a staggering change in the sea ice distribution.”

It’s not only the atmosphere that’s heating up. Martinson, a senior research scientist with the Lamont-Doherty Earth Observatory at Columbia University, explained that air temperature alone doesn’t have enough heat capacity to cause the wholesale melting of glaciers in West Antarctica. “The real source of heat has to be the ocean,” he said.

Heating up
Between their own observations and data from other researchers, the PAL LTER scientists believe an upwelling of warmer, deep ocean water is coming on to the continental shelf along the peninsula. The shelf is the extended perimeter of the continent below sea level, ending at a point of increasing slope called the shelf break.

In most of the world, deep ocean water is colder than surface water. But in Antarctica, where the surface water temperature of the Southern Ocean is slightly below freezing (salinity prevents it from turning to ice), this current of deeper seawater is about 3.5 to 4 degrees Celsius above zero.

Volume for volume, water has a tremendous heat capacity compared to air — more than 4,000 times greater, according to Martinson. “The bottom line is that it’s a humongous amount of heat,” he said.

Intensified westerly winds are causing the upwelling, but it’s the Antarctic Circumpolar Current (ACC) that pushes the warmer water onto the shallow shelf. The ACC is the dominant ocean current of the Southern Ocean. In a sense, it isolates Antarctica, helping preserve its ice sheets by serving as a kind of buffer against warmer surface water. The closest place where it knocks against the continent? You guessed it — the Antarctic Peninsula.

“The ACC … just sort of slams into the continental shelf right there off the peninsula, so it’s a place where there’s an enormous heat transfer,” said Hugh Ducklow, the lead investigator for the PAL LTER. “It’s pumping all of this ocean heat into our region.”

Or as physical oceanographer Martinson explained it: “It’s like having this freight train of hot coals skirt right along the Antarctic Peninsula, the whole length, right along the continental shelf.”

That train track, the ACC, runs west to east, charging by numerous glaciers that pour out of West Antarctica. A recent NASA-led study said that the rate of Antarctic ice loss increased by 75 percent in the last 10 years as glacier flow increased.

That study’s lead investigator, Eric Rignot of NASA’s Jet Propulsion Laboratory, speculated that the losses, concentrated in West Antarctica’s Pine Island Bay sector and the northern tip of the Antarctic Peninsula, are the result of warmer ocean waters, which “bathe the buttressing floating sections of glaciers, causing them to thin or collapse.” Martinson's LTER ocean data support that speculation.

Rignot also noted that last year’s report by the Intergovernmental Panel on Climate Change (IPCC) did not properly account for Antarctica’s role in sea level rise estimates. Rignot’s team estimated Antarctic ice loss is now nearly as great as that observed in Greenland. The 2007 IPCC report estimated that the global average sea level could rise between 18 and 59 centimeters in the next century.

“Sea level is going to rise much faster than [IPCC estimates],” Martinson cautioned.

The disappearance of ozone, a greenhouse gas, has actually caused the atmosphere above Antarctica to cool, he explained. But the surrounding ocean hasn’t cooled off, and may be slightly warmer in recent years. The disparity has whipped up the westerly winds.

Back to the deep ocean water: That seawater is part of vast conveyor belt that begins in the Gulf of Mexico, chugs across the Atlantic Ocean and then begins to cool around Iceland, where it sinks down and separates from the surface water. It takes a spin around the Arctic before ending up at the bottom of the Southern Ocean, where it merges with the Antarctic Circumpolar Current.

The variable that Martinson wants to quantify revolves around heat flux between that deep ocean water and the cold surface where the glaciers float. How much heat vents to the atmosphere and how much heat contributes to glacial melt?

“It’s like ice cubes in a glass. They melt fast. Even cold water melts them fast,” Martinson said.

Each austral summer since 1993, the PAL LTER scientists have conducted a science cruise in a grid pattern along their study area, which extends about 200 kilometers off shore from the archipelago and about 500 kilometers south to Marguerite Bay. From those cruises, the team has data showing the deep-water warming trend — but they’re after a full-length film instead of a handful of important still shots.

“When we go down on the cruise every single year, it’s just a snapshot,” Martinson said. “If something exciting happens the day before the ship is there, or when the ship is not there, which is most of the year, we miss it.”

In conjunction with an International Polar Year project called SASSI (Synoptic Antarctic Shelf-Slope Interactions Study), Martinson deployed four moorings on the annual PAL LTER cruise this past January from the Antarctic Research Supply Vessel Laurence M. Gould. The moorings, which include another one that was set in 2007, will monitor water temperature over depths of 50 to 400 meters year-round to quantify the heat flux phenomenon.

SASSI itself is a program to monitor simultaneously how the deep ocean water makes its way onto the continental shelf around Antarctica and whether the mechanism is the same across the entire continental shelf, which should help polar scientists understand the physical processes involved, according to Martinson.

“We want to find out not only what the changes have been [on the peninsula] but exactly how and why they’re happening,” explained Ducklow, a marine ecologist and co-director of The Ecosystems Center at the Marine Biological Laboratory in Woods Hole, Mass.

Martinson’s moorings are one component. Another new addition is an autonomous underwater vehicle (AUV) called a Slocum glider. The submersible robot, tested on the 2007 LTER cruise, will be able to carry a number of sensors that measure things like seawater salinity and temperature. Next season, the researchers hope to launch a small fleet of the AUVs.

“We’re trying to get away from what you see during one month from the ship,” Ducklow explained. “We’re going to get a vastly expanded spatial and temporal view of properties in our region.”

However, the biggest shift in the long-term study may prove to be more strategic than philosophical. The PAL LTER scientists have proposed extending their study area farther south along the peninsula, where they believe the ecosystem has seen less effect from climate change.

“By going farther south, into an area where the warming hasn’t really migrated into yet, we hope to have a better insight into what things used to be like,” Ducklow said. “We’ll be in place down there with observations over the next decade when the climate change carpet continues to unfold farther to the south.”

Fraser, a seabird ecologist from the Polar Oceans Research Group in Montana, said the prospect of moving into this sort of virgin territory is exciting because by the time the PAL LTER began in 1990, a process the scientists refer to as climate migration had already begun. Climate migration assumes that whole ecosystems will shift to a new location that better matches the original climate and environment.

“We can test very specific hypotheses about how we think that system is going to change as it warms due to climate migration,” Fraser said. “We are really seeing the entire mega-fauna of the Subantarctic starting to move into our region and displacing … polar species like Weddell seals and Adélie penguins.”

In fact, except for a more permanent ice cover, the new site to the south possesses similar characteristics to the current study area, even down to the bathymetry, the underwater topography of the area. A grid of deep canyons off Charcot Island mirrors a similar feature farther north. That’s important because the researchers believe the canyons play a role in the upwelling event driving the warm water onto the continental shelf.

“We want to be on the ground floor of change in that area,” Fraser said of the southern expansion. “In other words, we want to document it from the beginning.”

The PAL LTER scientists might have missed that chance in 1990, but Martinson noted the current study area has been a unique experience. “The opportunity to actually have a full sampling program in place in an area undergoing change this dramatic is once in a hundred lifetimes. It’s just pure serendipity that we’re in the right place at the right time to monitor exactly how this physical change is impacting the ecosystem.”

- Antarctic Sun