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While Arctic ice is melting at a record pace, a team of NASA-led researchers say they can explain why Antarctic sea ice has been edging in the opposite direction. That paradox has puzzled scientists for years and given climate-change deniers fodder to dispute global warming.
The group found that the icy winds blowing off Antarctica, as well as a powerful ocean current that circles the frozen continent, are much larger factors in the formation and persistence of Antarctic sea ice than changes in temperature.
The mighty Southern Ocean Circumpolar Current prevents warmer ocean water from reaching the Antarctic sea ice zone, helping to isolate the continent. The winds within that ice zone keep the water extremely cold, enabling the sea ice cover to grow in recent years even as global temperatures have risen markedly.
The findings are based on satellite readings of Antarctic sea ice movement and thickness, as well as new, detailed interpretations of charts showing the shape of the sea bottom around Antarctica. They were published online this month in the journal Remote Sensing of Environment.
Arctic sea ice and glaciers around the world have been dwindling quickly. And scientists have published dire warnings that several ice shelves in West Antarctica are being undermined by warm currents where they connect to the ocean floor. That melting phenomenon is expected to lead to significant, unavoidable sea rise over centuries.
Sea ice doesn't have a big effect on sea level—it grows and melts seasonally. The Antarctic ice shelves, by contrast, are the floating extensions of huge, land-based ice sheets and glaciers. And as they fall apart, the flow of land ice toward the sea accelerates, speeding up sea-level rise.
Antarctic sea ice has grown somewhat over the past 10 years. Between 2012 to 2014, it reached record-high extents each year during the winter. It topped 7.78 million square miles in September 2014, the largest extent since satellites started keeping accurate measurements in 1979.
Oklahoma Republican Sen. James Inhofe who calls global warming a hoax, along with other prominent deniers of global warming, has said scientists should focus on what is "not melting."
Since the late 1970s, the Arctic has lost an average of 20,800 square miles of sea ice per year, while the Antarctic has gained an annual average of 7,300 square miles.
The Antarctic freezing trend has not been captured well by climate models. So scientists have been trying to understand why planetary warming has not melted Antarctic sea ice like it has in the Arctic. In the new study, Son Nghiem, a researcher with NASA's Jet Propulsion Laboratory, evaluated satellite data to zero in on an answer.
"The polar sea ice paradox is really a challenge for the science community," Nghiem said. The big difference between his research and prior studies is that the findings are based on observations and measurements, rather than modeling, he said.
Nghiem's findings bolster earlier work that suggested similar explanations for the freezing phenomenon.
The satellite readings show that as sea ice forms early in the season, wind blowing off the cold Antarctic ice cap pushes it offshore and northward. As the ice then moves away from the shore, it breaks and is pushed around by wind, eventually forming thicker ridges of ice—a sort of reef—that protects the constantly forming younger ice from being eroded by wind and waves.
"The ice at the front of the ice pack is the older ice," Nghiem said. "It's thicker and rougher...it forms a great wall that protects the ice inside, so the internal ice opens up, stretches out. In the open water, ice can grow 10 times as fast as at the front," he said.
The study also found that the Southern Ocean Circumpolar Current, which helps determine sea-ice extent, is steered by submerged ridges and canyons along the edge of the Antarctic continental shelf, rather than by global warming or other climatic conditions.
Even as the global temperature average has risen significantly in recent decades, especially in the Arctic, the warming has not been as great in Antarctica. There, conditions are still conducive to sea ice formation.
"The central continent is still very cold. Even if the air masses flowing off Antarctica warm a little bit, the air is still more than cold enough to form ice," said Marcel Nicolaus, an ice physics scientist at the Alfred Wegener Polar Research Institute in Bremerhaven, Germany, who was not involved in the new research.
Additionally, the snowpack on top of the Antarctic ice is much thicker than in the Arctic, Nicolaus said. That means there is less formation of dark-colored surface melt ponds, which amplify the melting and warming in the Arctic, he explained.
As the Antarctic sea ice reached record levels, scientists floated several hypotheses, including possible changes in the ozone hole over Antarctica, or increased amounts of fresh water—which freezes more easily—on the surface of the ocean around Antarctica. At the same time, they said it's important to remember the big differences between the poles. Assessing sea ice dynamics at opposite ends of the Earth is not an apples-to-apples proposition.
The biggest difference is that the Arctic sea ice forms in a huge ocean surrounded by the northern hemisphere land masses, while the Antarctic sea ice forms as a fringe around a vast frozen continent.
"One has to say that Arctic sea ice is completely different from Antarctic sea ice, which almost melts completely back each summer," said Lars Kaleschke, an ice researcher with the Center for Earth System Research and Sustainability at the University of Hamburg. "The processes of ice formation are completely different.
"We have more snow in the Antarctic, which speeds ice formation (by pushing thin ice underwater) and protects the ice from melting," he said. "And the Antarctic is surrounded by the circumpolar current, which isolates the Antarctic from the rest of the world."
Kaleschke said he doesn't doubt that the factors Nghiem's team pinpointed are key to the dynamics in the Antarctic, but he thinks other forces are in play.
"One of the most convincing things I see is the freshening of the ocean from precipitation," he said. Increased snowfall over the region is consistent with global climate models because a warmer atmosphere holds more moisture.
"It comes down over the Antarctic Ocean over the sea ice zone. There, this leads to freshening of the surface layer. That leads to stratification, which leads to an increased formation of sea ice, because it's effectively shielded [by snow]," he said.
The fact that researchers are still debating the reasons for Antarctic sea ice expansion shows the need for more data and more studies.
"The climate models do not get it right at this point," Kaleschke said. "The models project a decrease of Antarctic sea ice, which is in contrast with observations."