Antarctica Gives Mixed Signals on Warming
Bijal P. Trivedi
|January 25, 2002|
Every climate model predicts that if global warming is indeed underway, the greatest warming will occur at the poles. For this reason, many scientists are closely watching these two regions for signs of ecological change.
But the situation isn't always clear. Based on several recent studies, Antarctica is either heating up or heading for the big chill.
Research published January 25 in the journal Science concluded that rising global temperatures have heated up the lakes on Signey Island, northwest of the Antarctic Peninsulaa narrow sliver of land that juts out toward South Americatriggering dramatic ecological changes.
Just last week, however, other researchers reported that, rather than melting and thinning, ice sheets in Antarctica have been thickening, and the continent has been cooling for the last couple of decades.
What does this say about global warming trends?
"In Antarctica, there is not a strong heating or cooling trend either way," said John Walsh, a polar climate specialist at the University of Illinois in Chicago.
"Antarctica is a tricky region and is more likely to do its own thing as the rest of the planet warms," he added. One reason for this, Walsh explained, is that compared with the Arctic, Antarctica is more susceptible to heat loss and gain from the ocean.
Walsh and his colleagues reported last week, also in Science, that contrary to previous reports, Antarctica is cooling.
If the whole of Antarctica were divided into a grid, said Walsh, about 60 to 70 percent of the squares would reveal a cooling trend, while warming would be seen in the other 30 to 40 percent of the overall area. "So there's a slight net cooling for the entire continent," Walsh said.
The total surface area of Antarctica is 5.1 million square miles (13.2 million square kilometers), but the area doubles in winter with the addition of sea ice, said Lloyd Peck, of the Natural Environment Research Council in Cambridge, United Kingdom.
"You can't expect an area three times the size of Australia to behave the same way throughout," said Peck, a co-author of the new study on the Signey Island lakes.
Walsh and his colleague Peter Doran, who collaborated on the recent study that concluded Antarctica is cooling, say past reports of global warming in the region were skewed by the nature of the measurements.
Much of the data, they said, came from research stations along the Antarctic Peninsula, which divides the Weddell and Bellingshausen Sea. But, Walsh pointed out, temperatures on the Peninsula are not reliable indicators of what is happening in the whole of Antarctica. "The Antarctic Peninsula is an unusual hotspot on the continent," said Dolan. This region has experienced some of the most rapid temperature changes on Earth in the last century2 degrees Celsius (3.6 degrees Fahrenheit) in the last 50 years.
According to Walsh and Doran, previous temperature trends for Antarctica were calculated by averaging 35 years of daily temperature readings from nearly three dozen stations. At least 15 of those stations are concentrated along the Antarctic Peninsula.
Doran and Walsh's team reanalyzed measurements from the past four decades and, in the words of Doran, "teased the cooling phenomena out of the data set."
Rising Lake Temperatures
In the January 25 report in Science, Peck and his collaborators said rising global temperatures have caused a warming of Antarctic lakes on Signey Island, resulting in less ice and a greater abundance of aquatic plant life than 15 years ago.
Signey Island, at the junction of the frozen Weddell Sea and the warmer Scotia Sea, has 17 lakes that vary widely in conditionsfrom predominantly ice-covered to nearly ice-free throughout the year.
In the last 40 years, the average air temperature at Signey Island has risen by one degree (nearly two degrees Fahrenheit). But photographic records suggest that ice cover on Signey has decreased by 45 percent since the 1950s, which supports the new report indicating that the lakes have warmed up.
"The average lake temperature has risen three times as fast as the air temperaturethat's the real surprise," said Peck. The finding is particularly jarring, he noted, because Signey's small size, its distance from human habitation, and the insulating ocean effect had led scientists to believe that such a place would change more slowly during climate fluctuations.
Peck's study reported that, compared with 15 years ago, there are now about 31 more days during which the lakes are ice free. "These lakes are an indication that the climate is changing fast," said Peck.
Warmer lake water, less ice, and more sunlight and nutrients have caused algae levels to quadruple their levels of 15 years ago.
Peck said he and his colleagues see the Antarctic lakes like the canaries of underground mines in relation to global warmingthey are highly sensitive to environmental conditions and can endure only minute climatic changes without "falling off their perch."
"We expect that lakes further north and south will go through similar changes," he added.
Thicker Ice Sheets
Another recent article in Science reported that some ice sheets in Antarctica have grown thicker, which seems antithetical to global warming. But Ian Joughin of the Jet Propulsion Laboratory in Pasadena, California, who led the study, said the finding has little relation to global warming.
He explained the phenomenon. Glaciers known as Ross ice streams empty into the Ross Sea. How quickly these glaciers move depends on the quality of the mud on which the glaciers are sliding. When the mud is warmer and wetter, a glacier is well lubricated and moves faster. But when the ice in the mud freezes, the mud is less slick and a glacier moves more slowly.
Using satellite measurements, Joughin found that the glaciers were thickening because they had slowed. "One ice stream stopped flowing 150 years ago and a second stream has slowed down considerably since the 1980s," he said.
Some glaciers undergo cycles of "bingeing and purging"they move slowly and increase their size, then speed up and move into the ocean. "Many models show that these binge-purge cycles can occur in the absence of climate-related changes," said Joughin.
There is currently no consensus on whether global warming will cause the ice sheets to grow or shrink, Walsh noted. Warming could lead to greater precipitation, resulting in a greater thickening of ice sheets.
Some scientists have predicted that if the climate continues to warm, the Ross ice shelfa floating shelf of ice produced by the glacier's dischargewill break away from the continent and melt, raising sea levels.
Joughin's research suggests this scenario is less likely. But he also cautions that "these are processes that occur over thousands of years, and our measurements represent only a few years."
"Each new piece of data helps us refine our models of how climate will affect the planet and its various ecosystems," Joughin said, adding: "It's an evolving process."
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