NEW YORK (NYTIMES) - The South Pole, the most isolated part of the planet, is also one of the most rapidly warming ones, scientists said Monday (June 29), with surface air temperatures rising since the 1990s at a rate that is three times faster than the global average.
While the warming could be the result of natural climate change alone, the researchers said, it is likely that the effects of human-caused warming contributed to it.
The pole, home to a US research base in the high, icy emptiness of the Antarctic interior, warmed by about 0.6 degrees Celsius, per decade over the past 30 years, the researchers reported in a paper published in Nature Climate Change. The global average over that time was about 0.2 degrees Celsius per decade.
Although parts of coastal Antarctica are losing ice, which contributes to sea level rise, the pole is in no danger of melting, as the year-round average temperature is still about minus-50 degrees Celsius. But the finding shows that no place is unaffected by change on a warming planet.
Analysing weather data and using climate models, the researchers found that the rising temperatures are a result of changes in atmospheric circulation that have their origins thousands of kilometres away in the western tropical Pacific Ocean.
"The South Pole is warming at an incredible rate, and it is chiefly driven by the tropics," said Kyle R. Clem, a postdoctoral researcher at Victoria University of Wellington in New Zealand and the lead author of the study.
While climate change resulting from emissions of carbon dioxide and other greenhouse gases has very likely played a role, the analysis showed that natural climate variability could account for all of the extreme swing in temperature, effectively masking any human-caused contribution.
"The Antarctic interior may be one of the few places remaining on Earth where the anthropogenic signal cannot be easily teased out due to such extreme variability," Clem said. "But you're very, very unlikely to get a warming trend that strong without increasing greenhouse gases."
Temperature records at the pole have been kept since 1957, when the first US base was completed there. For decades, average temperatures were steady or declining. Strong westerly winds that circled the continent served as a barrier, preventing warmer air from intruding into the interior.
But that changed near the end of the 20th century, Clem said, when sea-surface temperatures in the western tropical Pacific began to rise, part of a natural oscillation that occurs on a time scale of decades.
The warming ocean heated the air, which caused ripples of high and low pressure in the atmosphere that reached all the way to the Antarctic Peninsula, more than 8,000km away. Scientists call these kinds of long-distance links teleconnections.
Coupled with the stronger westerly winds, which are part of another long-term pattern, the ripples led to stronger storms in the Weddell Sea, east of the peninsula. These rotating, or cyclonic, storms, swept warmer air from the South Atlantic Ocean into the interior of the continent.
Stronger storms in the Weddell Sea have also led to a recent decline in sea ice in the region.
Clem said the warming was not uniform across the Antarctic Plateau, the enormous expanse that covers most of the interior, including the pole, with an average elevation of nearly two miles (3.2km). But the only other permanent base on the plateau, Russia's Vostok station about 1,280km from the pole, has also recorded rapidly rising temperatures, he said.
The ripples from the tropical Pacific also had an effect on the Antarctic Peninsula, which for most of the late 20th century had been one of the fastest-warming areas in the world. But in the past few decades the rate of warming there has declined significantly.
In an email message, two researchers at the University of Colorado, Sharon E. Stammerjohn and Ted A. Scambos, said that while the rest of the world has been warming steadily over the past five decades, Antarctica has seen major swings and probably always has. Neither scientist was involved in the research, but they wrote a commentary on the study published in the same issue of the journal.
As ocean temperatures in the tropical Pacific switch toward cooling, they said, the rate of warming at the South Pole will likely decline as well, but not by as much as it would have without human-caused climate change.
In an interview, Stammerjohn said "warming at the South Pole is significant because it's the most remote place on the planet." "But it's still never going to get above freezing," she said. "We don't have to worry too much about losing ice at the pole just yet. But definitely the coastlines are another matter."
Especially along the coast of West Antarctica, warm water brought up from depth by the action of wind is melting ice shelves from underneath, which ultimately leads to sea level rise.
Stammerjohn said there was more and more evidence that the way the planet is responding to warming was changing the atmosphere and ocean circulation on a large scale.
"And that's what's contributing to the warmer waters at depth," she said.
"There's going to be a lot of variability superimposed on that, but the direction, and the projection, would be toward more and more warm water and more ice sheet loss." "It's so easy to think that Antarctica is isolated and remote and is not going to respond to climate change," Stammerjohn said.
While the impact at the South Pole may not be all that significant, ice loss along the coast has huge implications.
"It's the one that's going to change our sea level dramatically," she said.
Warming at South Pole, she said, is "the ultimate canary in the coal mine, one that we can no longer ignore".