New Antarctica ice loss model doubles sea-level rise estimates

Source: Camille von Kaenel, E&E reporter • Posted: Friday, April 1, 2016

A melting Antarctica alone could raise oceans by more than 3 feet by the end of the century if greenhouse gas emissions continued unabated, roughly doubling previous total sea-level rise estimates, according to new research.

Scientists previously thought glacial melt in Antarctica would raise sea levels only by a little or only in the far future. But that’s changing as researchers learn more about the different ways the continent’s ice sheets could dump water into the oceans.

The new paper, published in the journal Nature yesterday, comes at a time of increasing concern among sea-level rise researchers about Antarctica.

“The Antarctic retreat could happen much faster and much more than we previously thought,” said David Pollard, one of the lead authors and a researcher at Pennsylvania State University. “This is just the Antarctic contribution to future sea-level rise, but if there’s anything to this, and if we really follow the business-as-usual [emissions] scenarios, then this contribution would exceed all other contributions over next years.”

Pollard and his colleague Robert DeConto from the University of Massachusetts, Amherst, updated models with new discoveries and compared them to past sea-level rise events.

Rising seas could put much of the world’s coastlines, including major cities from New York City to Miami, underwater or at risk of flooding and storm surges.

‘We’ve been waiting for this’

“We’ve been waiting for this paper. It has societal implications right off the bat,” said Ben Horton, a sea-level rise researcher at Rutgers University. He advises Delaware on sea-level rise, he said, and suggested to colleagues that they wait on a policy proposal to include projections from the new models.

The researchers stressed that a range of scenarios exist, based on how quickly the world cuts back on greenhouse gas emissions from burning fossil fuels. If emissions were rapidly decreased, Antarctica would cause sea levels to rise less than a foot by 2500.

In the worst-case scenario, sea levels could rise around 50 feet by 2500. Even in a moderate scenario in which emissions stop after 2500, the long “thermal memory” of the ocean would prevent the ice sheet from growing back for centuries.

The Intergovernmental Panel on Climate Change found that sea levels could rise between 11 and 38 inches by 2100 due to thermal expansion and melting glaciers. Many sea-level scientists have deemed those estimates conservative, with several studies suggesting higher numbers (ClimateWire, Feb. 23).

In recent years, sea-level researchers have started focusing on ice sheets at the poles because they remain little-understood yet could quickly disintegrate (ClimateWire, Jan. 4). In particular, they have sounded alarms over the potential runaway collapse of the Thwaites and other glaciers in West Antarctica.

One process increasingly in the spotlight has water creeping up below the ice shelves, melting them from below and further lubricating their slide into the ocean.

Solving a historical riddle

In their paper, DeConto and Pollard also highlighted two other “underappreciated” mechanisms chipping away at the ice sheets. Surface meltwater can slip through cracks in the ice shelves, accelerating their breakup in a process called hydrofacturing. For example, the Antarctic Peninsula’s Larsen B Ice Shelf suddenly and dramatically broke apart in 2002.

Ice cliffs towering more than 300 feet can also form where the ice shelves meet the ocean. They can then collapse under their own weight, as observed at the Jakobshavn and Helheim glaciers in Greenland, DeConto said.

“There have been a lot of mechanisms that cause ice to disappear, which we are just now beginning to understand and account for, and all of them tend to make the ice disappear faster and raise sea levels more quickly, so as we learn about these mechanisms and better understand how they may affect future sea-level rise, we’re going to continue to see upward revisions of the amount of possible sea-level rise,” said Josh Willis, a scientist at NASA’s Jet Propulsion Laboratory leading a project called Oceans Melting Greenland.

The idea that a small variation in temperature could cause massive ice loss at the poles has precedent: Around 120,000 years ago, during the last interglacial period, sea levels rose 6 to 9 meters (20 to 30 feet) higher than today as the planet warmed by zero to 2 degrees Celsius (3.6 Fahrenheit). During the Pliocene Epoch 3 million years ago, which saw a carbon dioxide atmospheric concentration similar to today’s, sea levels rose by up to 10 to 30 meters (32 to 98 feet).

Yet models that may parallel modern observations of ice loss have not matched those historic records. The mismatch has puzzled DeConto and Pollard for years.

Now they appear to have solved it. After adding the new mechanisms to their model, they were able to calibrate it to past records. They found that Antarctica contributed 3 to 7 meters to the last interglacial sea-level rise event, for example. The rates of ice loss — and regrowth — also matched.

“It would be nice to say this is a culmination paper,” DeConto said. “But we’re already thinking about the next steps and how this needs to be improved.”