Scientists have documented one of the most extensive melting events ever recorded in West Antarctica, an area that holds enough land ice to raise sea level by about 11 feet if it fully melted.

In a paper published Thursday, they described how the effects of a strong El Niño in the tropical Pacific combined with local weather patterns last year to melt surface ice across an area of Antarctica that is twice the size of California.

If climate models are correct, such melting is likely to happen more frequently in the years ahead, the authors said.

"If climate change proceeds as expected, the tropics are likely to see many more of these strong El Niño events," said David H. Bromwich, a research professor at Ohio State and a co-author of the study, published in the scientific journal Nature Communications.

One of the biggest surprises, Bromwich said, was that the melting occurred despite strong winds from the west, a common weather pattern that usually helps keep temperatures cold enough to prevent extensive melting. "What this particular event demonstrates is, almost regardless of what the westerly winds will be doing, we'll have large melting events in this particular area," he said.

Scientists have grown increasingly concerned that the West Antarctic Ice Sheet could disintegrate, eventually contributing about 11 feet to global sea level rise. The sheet is ringed by floating shelves of ice that hold back the glaciers behind them, and much of the focus of scientists so far has been on the erosion of these ice shelves by seawater from below as the oceans warm.

The new study, which looked at the massive Ross Ice Shelf, showed that surface melt could be another major problem.

Last year, scientists with Scripps Institution of Oceanography traveled to the area to measure how cloud cover affects warming at the surface of the ice, both by trapping heat below the clouds and by blocking sunlight. They began running their experiments just as a flow of warm air swept in from the Pacific Ocean, driven by the strong El Niño.

The coincidence allowed the scientists to combine satellite data with measurements from weather stations on the surface, providing key information about the various climatic factors at play. As the warm air currents hit the continent, temperatures on the ice hovered at or above freezing for several days in a row. Meanwhile, a layer of cloud cover settled in that was thin enough to let sunlight through but thick enough to trap much of its heat. The result was up to 15 days of melting in some parts of the ice sheet. In some spots, rain may even have fallen, the paper says.

The scientists estimate that it was one of the three largest surface melting events to occur since satellites began collecting data in 1978. They said it's likely that the meltwater re-froze, either at the surface or after filtering through cracks.

Meltwater can wreak havoc on an ice sheet by running through crevasses and eating away at the ice from within, said Robert M. DeConto, a climatologist at the University of Massachusetts. He pointed out that before the Larsen B ice shelf collapsed in 2002, scientists saw pools of meltwater accumulating at its surface.

Last year, DeConto and David Pollard published a controversial study arguing that the West Antarctic Ice Sheet could collapse over a matter of decades and dump enough water into the ocean this century to push sea level up by 3 feet. When taken along with melting elsewhere on the planet, that could mean a rise of 5 to 6 feet by 2100. Other research has suggested that the ice sheet is more stable, but part of what pushed DeConto and Pollard's models in a different direction was that they modeled exactly the type of melting that the new paper documented, and in the same place.

"It was eye-opening for me to see that much melt over that big of an area lasting that long," DeConto said. "The models start to show lots of melt events like this in the second half of the 21st century, but this is pointing out to me that this can happen already."



While the paper details how such a melting event can occur, it also highlights the many uncertainties for modeling future conditions. Accurate models will need to predict not only large-scale climatic events like the frequency and intensity of El Niños, but also local factors such as the density and patterns of cloud cover, a weakness in most models, Bromwich said.

DeConto said he has held out hope that his model was off, and that widespread surface melting might not happen for much longer. Seeing that it can happen already, though, doesn't bode well, he said.

"What this stuff makes you think is maybe we're not that far off," he said, "and who knows, maybe we're even underestimating."