Climate scientists have cracked the mystery of why Antarctic sea ice has managed to grow despite global warming—but the results suggest the trend may rapidly reverse, a new study says.

Satellite data show that, over the past 30 years, Arctic sea ice has declined while Antarctic sea ice has mysteriously expanded, according to study leader Jiping Liu, a research scientist at Georgia Tech in Atlanta.

"We've seen this paradox, but we don't know why—here we gave an explanation," Liu said.

The new analyses are based on climate models and sea-surface temperature and precipitation observations from 1950 to 2009. They show that, in the 20th century, ocean warming boosted precipitation in the upper atmosphere over the Antarctic region, which fell as snow.

More snow made the top layers of the ocean less salty and thus less dense. These layers became more stable, preventing warm, density-driven currents in the deep ocean from rising and melting sea ice. (Test your ocean IQ.)

Global Warming to Speed Up Antarctic Melting?

The data show that Antarctic sea ice growth in the 20th century might be mostly dictated by natural processes, Liu noted.

But that won't be the case for the 21st century, since human-caused global warming is predicted to dominate the Antarctic climate and trigger faster melting of sea ice, he said. (See a map of global warming impacts worldwide.)

As increasing greenhouse gases continue to warm the oceans off Antarctica, more Antarctic precipitation will turn to rain, which rapidly melts snow and ice, according to the study, published this week in the journal Proceedings of the National Academy of Sciences.

The more the ice melts, the more the sun's rays are absorbed into the dark ocean instead of being reflected back into the atmosphere, according to the study. This will further warm the ocean and melt even more sea ice.

The scientists predict the transition from natural variability to greenhouse-gas warming will begin soon: "I cannot give you a precise year—but definitely in this century," Liu said. (See a greenhouse effect interactive.)

The oceans off Antarctica are the world's most biologically productive, and declining sea ice could have a "substantial impact on the Antarctic marine ecosystem," Liu said. For instance, many Antarctic species rely on sea ice for hunting and overall survival. Antarctic penguins—many of which may disappear if warming continues—are among the animals at risk, conservationists say.

A loss of sea ice could also throw a wrench in how ocean water travels around the world, Liu noted: The oceans off Antarctica include Earth's coldest, densest water, which is one of the "dominant driving forces" for the ocean's global conveyor belt, a circulation pattern that provides nutrients for up to three-quarters of marine life.

Ozone Hole Missing in Sea Ice Study

The study results aren't surprising, since they're in line with previous predictions that Antarctic sea-ice loss would accelerate, said Walt Meier of the National Snow and Ice Data Center (NSIDC) in Boulder, Colorado.

"That's been the conventional wisdom—this is a modeling study [that] does actual physics that confirms" this idea, Meier said.

The paper also helps to dispel a common misconception—that the strong decline in Arctic ice and increase in Antarctic ice causes a net zero effect, Meier said.

That's not the case, because the two polar ecosystems are so different, he said. Arctic ice is multiyear, persisting through the seasons, while Antarctic ice forms and melts each year and has always been governed more by wind and ocean circulation than air temperatures, he said.

That's not to say the melting effects of temperature, as seen in the Arctic, won't also occur in the Antarctic—they'll just take longer to show up, he said.

But Kevin Trenberth, a senior scientist at the Boulder, Colorado-based National Center for Atmospheric Research, said that the paper has a major omission: the influence of the hole in the ozone layer.

Bright summertime clouds spurred by the hole have acted as shields against global warming, scientists say. Recent research suggests the hole may finally be closing, following the phase-out of ozone-depleting chemicals called chlorofluorocarbons. And as the sun-reflecting clouds dissipate, temperatures in the Southern Hemisphere may rise faster than currently predicted by models.

The ozone hole has been a reason "why Antarctica has not warmed the same way as other parts of the world," Trenberth said by email.

"How the ozone hole recovery comes about in the future is a major factor in expected developments, as shown in some model simulations—but these aspects are not dealt with in this paper."