Global warming is driving a shift in regional winds around the edges of Antarctica, and that's speeding up the meltdown of the West Antarctic Ice Sheet, new research shows.
Westerly winds that enable warmer ocean water to creep beneath the floating edge of the ice sheet have become more prevalent over the past 100 years, scientists found in a new study, published Monday in the scientific journal Nature Geoscience.
Paul Holland, a researcher with the British Antarctic Survey and lead author of the study, explained that the winds are changing because global warming is heating the tropics faster than Antarctica and the surrounding Southern Ocean. Contrasts between temperatures over different parts of the Earth are the main driver of wind. In this case, the increasing temperature differential from the equator to Antarctica is strengthening the westerly winds that circle around the frozen continent.
The way the westerly winds push the surface water allows warmer water from below to reach under the ice shelves, accelerating ice loss from the West Antarctic Ice Sheet.
"West Antarctica has seen a major and consequential acceleration of land ice flow into the ocean, and we know the acceleration is driven by oceanic melting," said Columbia University climate scientist Pierre Dutrieux, a co-author of the research.
The air above the continent is still very cold, but shifts in the regional winds can explain the changes in the flow of warm water that's melting the ice shelves from beneath, he said.
The new research combines data from satellites and climate modeling to quantify how the winds have shifted over the past century. It's the first study "that connects all the dots linking anthropogenic forcing and ice sheet acceleration," Dutrieux said.
When the westerly winds are stronger, they push the surface water offshore. "Something has to replace that surface water, and deep water from below upwells to replace it," explained Eric Steig, a University of Washington climate researcher and co-author of the study.
It's that deep water, which is warmer than the Southern Ocean's surface water, that's melting the floating edges of the ice sheets from below. Observations show that, historically, those melt-inducing conditions were associated with El Niño conditions, when sea surface temperatures in the Pacific are warmer than normal near the equator.
"Our results show that the winds are on average becoming more westerly in this region [West Antarctica], whether there is an El Niño or not. So when an El Niño does occur, its influence is on top of the already stronger westerlies. A moderate El Niño now may have just as strong an influence as a strong El Niño did 100 years ago. In the future, even a weak El Niño may be strong enough to cause melt-inducing conditions," he said.
More Emissions, Faster Melting
West Antarctica is about as big as the contiguous western United States and covered with ice that's over a mile thick in places. Its melting is projected to contribute at least 2 to 8 inches of global sea level rise by 2100, Dutrieux said.
If greenhouse gas emissions continue rising, the wind shift will continue, and there could be an additional increase in melting, Holland said. But curtailing greenhouse gas pollution now could reduce future sea-level contribution from this region.
Other recent studies have found that Antarctic glaciers and ice sheets are also at risk from atmospheric changes. Increased downslope winds were probably a big factor in the collapse of the Larsen B Ice Shelf along the Antarctic Peninsula. Another team of researchers in 2016 warned that human-caused global warming could lead to a rapid catastrophic fracturing of ice sheet edges, speeding up ice loss and leading to 3 feet of sea level rise by 2100.
Accurate information about sea level rise is critical for millions of people living along coastlines who must plan whether to build seawalls and pumping systems or to retreat from the flooding shoreline.
The potential for a surge of sea level rise from West Antarctica is so great that scientists have even studied whether the melting could be slowed by spraying the surface with man-made snow and ice. That research has helped to show how failure to curb greenhouse gas emissions now could mean a reliance on expensive and unproven technological measures to try to avert worst-case sea level rise in the future.
Ice Loss Has Been Accelerating
The new research adds more evidence that human-caused global warming, rather than just natural variability, is driving the accelerating ice loss from West Antarctica, said Eric Rignot, a glaciologist with the University of California Irvine and NASA's Jet Propulsion Laboratory.
Rignot led a 2014 study showing that ice discharge from West Antarctica into the Amundsen Sea increased 77 percent from 1973 to 2013. The scientists concluded that a large part of the West Antarctic Ice Sheet is irreversibly retreating, past the point of no return. Another study he led earlier this year found total ice loss from Antarctica had increased sixfold from 1979 to 2017.
"The issue is not whether the ice in the West Antarctic Ice Sheet will melt away, but how long will it take," he said.
Rignot said the new research reinforces the findings that global warming caused a step-change in the climate system in the 1980s. Earlier studies had suggested a fundamental change in West Antarctica winds around that time. "This is also when we see the mass loss from Antarctica starting to slowly ramp up," he said.
The Intergovernmental Panel on Climate Change's 2014 assessment acknowledged that human-caused global warming started pushing the climate system outside the range of natural changes in the 1980s. Many of the climate extremes seen since then can't be explained without the effect of greenhouse gas pollution, Rignot said.
Published Aug. 12, 2019