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Still Stuck in a Climate Argument

The Akademik Shokalskiy was trapped in the ice off East Antarctica after shifting winds caused loose pack ice to jam against the vessel.Credit...Andrew Peacock/Agence France-Presse — Getty Images

When a ship carrying scientists and adventure tourists became stuck in ice in the Antarctic late last month, climate change skeptics had a field day. On Twitter and other social media sites, they pointed out that a group whose journey was meant to highlight the effects of global warming was trapped by a substance that was supposed to be melting.

“Global warming idiots out of danger,” one noted when the ship’s 52 passengers were finally helicoptered to safety Thursday after more than a week on the ice.

The episode had little connection to climate change — shifting winds had caused loose pack ice to jam against the ship — and this was far from the first time that a ship had been trapped, even in the Antarctic summer. But sea ice cover in the Antarctic is changing, and scientists see the influence of climate change, although they say natural climate variability may be at work, too.

“The truth is, we don’t fully understand what’s going on,” said Ted Maksym, a researcher at the Woods Hole Oceanographic Institution.

Unlike the Arctic, where sharp declines in recent decades in the ice that floats on sea surfaces have been linked to warming, sea ice in the Antarctic has actually increased, scientists who study the region say. Averaged over the entire Antarctic coast, the increase is slight — about 1 percent a decade. At the same time, larger increases and decreases are being seen on certain parts of the continent.

“We’re constantly struggling against that statement, that Antarctic ice is increasing,” said Sharon E. Stammerjohn, a scientist at the Institute of Arctic and Alpine Research at the University of Colorado. “It misses key changes that are happening. And there are really strong climate signals in those changes.”

 Most of the sea ice changes are occurring in an area covering about a third of the Antarctic coast, from the Ross Sea to the Bellingshausen Sea and the Antarctic Peninsula, said Paul Holland, a researcher with the British Antarctic Survey. Areas around the Ross Sea, for example, have seen large increases in ice, while in the Bellingshausen and along the peninsula, ice cover has declined sharply. (The area where the research ship became stuck, west of the Ross Sea, has had a slight increase in ice cover over the past 35 years.)

Researchers agree that the changes in those seas are related to north-south winds that circulate clockwise around a stationary zone of increasingly lower-pressure air. That brings warmer air from the north into the Bellingshausen Sea and peninsula, pushing ice against the coast and melting some of it, and colder air from the south into the Ross Sea, which spreads the ice away from the coast and creates more of it.

But why that low-pressure air is getting lower is still a subject of debate.

Scientists say that increases in greenhouse gas concentration in the atmosphere, as well as depletion of atmospheric ozone, have changed temperature gradients from the tropics to the poles, which affects atmospheric circulation.

“There are clear signals of winds increasing due to climate change” in the Southern Ocean around the Antarctic, Dr. Stammerjohn said. Those intensifying winds might be affecting the low-pressure zone, she said, but there are also other factors that do not rule out natural variability.

“The jury is definitely still out on that,” Dr. Stammerjohn said.

Whatever the explanation, much of the Bellingshausen is now ice free for long periods each summer. That allows the relatively warmer waters of the Southern Ocean to flow more freely to the more permanent ice that extends from the land in glaciers and sheets.

“The combination of the warm ocean and the effects of waves on these glaciers may increase the rate of loss of glacial ice,” Dr. Maksym said.

The consensus now is that there is a net loss of ice from Antarctica’s ice sheets and glaciers, Dr. Maksym added, and it is the melting of this ice, rather than any loss of sea ice, that concerns scientists who study sea-level increases.

He cautioned that there was still a lot unknown about Antarctic sea ice, which has been studied far less than Arctic ice. In many ways the regions are opposites — the Arctic is an ocean largely hemmed in by land, while Antarctica is a land mass surrounded by a vast open ocean — so lessons learned from studying one do not necessarily apply to the other.

“The skeptics do have a good point,” Dr. Maksym said. “Why are we not paying as much attention to what’s going on in the Antarctic? There are good reasons to figure out why these changes are happening.”

A version of this article appears in print on  , Section D, Page 6 of the New York edition with the headline: Still Stuck in a Climate Argument. Order Reprints | Today’s Paper | Subscribe

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