Climate Model Predicts West Antarctic Ice Sheet Could Melt Rapidly
For half a century, climate scientists have seen the West Antarctic ice sheet, a remnant of the last ice age, as a sword of Damocles hanging over human civilization.
The great ice sheet, larger than Mexico, is thought to be potentially vulnerable to disintegration from a relatively small amount of global warming, and capable of raising the sea level by 12 feet or more should it break up. But researchers long assumed the worst effects would take hundreds — if not thousands — of years to occur.
Now, new research suggests the disaster scenario could play out much sooner.
Continued high emissions of heat-trapping gases could launch a disintegration of the ice sheet within decades, according to a study published Wednesday, heaving enough water into the ocean to raise the sea level as much as three feet by the end of this century.
With ice melting in other regions, too, the total rise of the sea could reach five or six feet by 2100, the researchers found. That is roughly twice the increase reported as a plausible worst-case scenario by a United Nations panel just three years ago, and so high it would likely provoke a profound crisis within the lifetimes of children being born today.
Under the Ice Sheet
The vast West Antarctic ice sheet sits on bedrock that dips thousands of feet below sea level. New computer simulations suggest that the warming atmosphere and ocean could attack the ice sheet from above and below, causing sea levels to rise much faster than previously thought.
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The situation would grow far worse beyond 2100, the researchers found, with the rise of the sea exceeding a pace of a foot per decade by the middle of the 22nd century. Scientists had documented such rates of increase in the geologic past, when far larger ice sheets were collapsing, but most of them had long assumed it would be impossible to reach rates so extreme with the smaller ice sheets of today.
The long-term effect would likely be to drown the world’s coastlines, including many of its great cities.
New York City is nearly 400 years old; in the worst-case scenario conjured by the research, its chances of surviving another 400 years in anything like its present form would appear to be remote. Miami, New Orleans, London, Venice, Shanghai, Hong Kong and Sydney, Australia, are all just as vulnerable as New York, or more so.
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In principle, coastal defenses could be built to protect the densest cities, but experts believe it will be impossible to do that along all 95,000 miles of the American coastline, meaning that immense areas will most likely have to be abandoned to the rising sea.
The new research, published by the journal Nature, is based on improvements in a computerized model of Antarctica and its complex landscape of rocks and glaciers, meant to capture factors newly recognized as imperiling the stability of the ice.
The new version of the model allowed the scientists, for the first time, to reproduce high sea levels of the past, such as a climatic period about 125,000 years ago when the seas rose to levels 20 to 30 feet higher than today.
That gave them greater confidence in the model’s ability to project the future sea level, though they acknowledged that they do not yet have an answer that could be called definitive.
“You could think of all sorts of ways that we might duck this one,” said Richard B. Alley, a leading expert on glacial ice at Pennsylvania State University. “I’m hopeful that will happen. But given what we know, I don’t think we can tell people that we’re confident of that.”
Dr. Alley was not an author of the new paper, though it is based in part on his ideas about the stability of glacial ice. Several other scientists not involved in the paper described it as significant, with some of them characterizing it as a milestone.
But those same scientists emphasized that it was a single paper, and unlikely to be the last word on the fate of West Antarctica. The effort to include the newly recognized factors imperiling the ice is still crude, with years of work likely needed to improve the models.
Peter U. Clark of Oregon State University helped lead the last effort by a United Nations panel to assess the risks of sea level rise; he was not involved in the new paper. He emphasized that the research, like much previous work, highlighted the urgency of bringing emissions of carbon dioxide and other greenhouse gases under control.
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http://www.nytimes.com/2016/03/31/science/global-warming-antarctica-ice-sheet-sea-level-rise.html?_r=0
