Image: Adelie penguins stand atop ice near the French station at Dumont d’Urville in East Antarctica Pauline Askin / Reuters file 14 hours
Less Ice or More? What You Need to Know About Antarctica’s Meltdown
By John Roach
In Antarctica, glaciers are sloshing seaward at an ever faster clip, ocean waters are warming, and, perhaps counterintuitively, sea ice is expanding, according to a batch of recent studies that paint a stark picture of climate change unfolding at the far southern reaches of the globe. For people in North America, the distant events raise the specter of higher seas sooner than climate models suggest.
Here are answers to key questions about what’s happening on that cold continent.
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How can Antarctica be losing ice overall if sea ice is growing?
About that sea ice, it is indeed expanding at the same time Antarctica is losing nearly the water-weight equivalent of Mount Everest every two years. The expansion fits with the expected natural variability in a warming world, according to a study accepted for publication Dec. 5 in the journal Geophysical Research Letters. In fact, some scientists say, warming may enhance sea ice growth.
Antarctic sea ice reached its maximum extent of 7.76 million square miles on Sept. 22, another record, according to the National Snow and Ice Data Center in Boulder, Colorado. The sea ice is typically three to six feet thick and most of it melts each summer as it drifts north toward warmer waters.
While that’s a lot of ice, the continent itself holds even more — about 2,500 times more, according to the Intergovernmental Panel on Climate Change, the world’s climate science body. On average, the Antarctic icecap is 1.3 miles thick and contains a whopping 70 percent of the world’s freshwater. If it all melted, it could raise global sea levels by nearly 200 feet, scientists report.
The panel’s most recent analysis put the current high end estimate of global sea level rise at about three feet by the end of this century; though many scientists — and scientific studies — indicate the number could actually be a “bit” higher, Richard Alley, a glaciologist at Pennsylvania State University, told NBC News.
“It is very, very clear that in the ‘most likely’ estimate there is still a huge amount of ice sitting in places where it could contribute to sea level rise,” he explained.
Alley was not involved in the current batch of papers, which, he said, bring “cleaner data, more data, [and] more agreement” to the basic narrative that West Antarctica’s glaciers are shrinking faster today because warmer waters are getting under the ice shelves. If this trend continues, it could commit the world to about 11 feet of sea level rise from West Antarctica alone.
So how fast is that continental ice melting?
In West Antarctica, according to one of the studies published Dec. 5 in the journal Geophysical Research Letters, the melt rate of glaciers has nearly tripled during the last decade, “which means that things are happening faster and the contribution to sea level is increasing more and more,” Isabella Velicogna, a coauthor of the paper from the University of California at Irvine, told NBC News.
The result stems from a 21-year analysis of four different techniques used to measure how much ice mass the glaciers discharge into the sea. “It is a very big signal and not one of those techniques misses it and all of (them) agree on the amount that is lost,” added Velicogna, who is also affiliated with NASA’s Jet Propulsion Laboratory.
Over the 21 years, the average amount lost was 83 gigatons per year, which translates to a Mount Everest’s-worth of water weight every two years. The rate of loss has accelerated an average of 6.1 gigatons per year since 1992. And from 2003 to 2009, the melt rate has increased an average of 16.3 gigatons per year.
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What’s contributing to that increase?
The melt rate is accelerating because warmer water is getting into the shallow West Antarctic shelf areas, according to a separate study published Dec. 5 in the journal Science.
Those warmer waters “melt the glaciers on the bottom and reduce the buttressing,” study lead author Sunke Schmidtko from the Helmholtz Centre for Ocean Research Kiel in Germany told NBC News in an email. “This makes the glaciers flow faster into the ocean, the ice loss is accelerated.”
Elsewhere in Antarctica, a strong coastal current prevents the warmer water from getting onto the shelf, though Schmidtko has his eyes on the southern Weddell Sea, on the east side of the Antarctic Peninsula. There the water is currently below freezing — 29 degrees Fahrenheit — but deep, warmer offshore waters are getting closer, he noted.
And what is causing the waters to warm? The answer appears to be tied to a shift in winds along with higher sea surface temperatures farther north. “That temperature rise and wind intensification is usually attributed to climate change,” Schmidtko said.
Are there other factors for sea ice growth?
Antarctic sea ice has increased over the past three decades; however climate models tend to show a decrease over the same period. The mismatch has caused head scratching throughout the climate science community and fueled skepticism about human-caused global warming.
Velicogna said the sea ice and the ice sheets are both sensitive to a changing climate, but “they evolve and respond in different ways; you cannot infer one from the other.”
The formation of Antarctic sea ice, according to Alley, is largely a wind-driven process beginning with winds that blow cold air from the icy landmass over the seas.
Recent studies have linked changes in regional wind patterns to the ozone hole, greenhouse warming, and natural variability. “Because Antarctic sea ice depends so much on wind, that right there complicates things because it is not primarily a temperature story,” Alley explained.
A further complication arises as glaciers dump more freshwater into the sea, which stratifies the ocean and keeps warmer water down low where it can’t melt sea ice.
“A big amount of melting in Antarctica actually favors growth of sea ice around it by putting extra freshwater near the surface,” Alley explained.