Photo: David Taylor/Science Photo Library (AP)
Antarctica is home to 90% of the world’s freshwater, trapped in the continent’s massive ice sheets—and the stability of much of that ice is seriously at risk from global warming. Two studies published in the journal Nature this week take a look at how climate change is shaking up conditions in ice sheets in Antarctica, spelling out the grim potential future of sea level rise.
The first study looks at how Antarctica’s two ice sheets are affected by what’s going on with their ice shelves, which serve as protective buttresses. Ice shelves extend out over the ocean, while the sheets cover land.
“Ice shelves are huge, hundreds- or even thousands-of-meters thick pieces of ice, and a few of them are as big as France,” said lead author Chad Greene, a postdoctoral research fellow at NASA’s Jet Propulsion Laboratory, in an email. “Ice shelves float on top of the ocean in hydrostatic equilibrium, so when an iceberg breaks off of an ice shelf, there’s no direct impact on sea level. But whenever an ice shelf calves, it gets a little bit smaller and a little bit weaker.”
Ice shelves normally have a healthy cycle of calving and are able to regenerate the ice they’ve lost. But climate change has helped to accelerate the calving process, weakening the ice shelves from below in warming water and making it harder for the shelves to replenish. To understand what this might mean for sea-level rise, Greene and his fellow researchers used satellite data to generate a series of high-resolution maps of Antarctica’s coastline over the past 25 years.
“What we found is that Antarctica’s ice shelves have been crumbling away at the edges,” said Greene. Overall, they determined that Antarctica has lost more than 14,280 square miles (37,000 square kilometers) of ice shelf area since 1997 (“That’s about the size of Switzerland,” Greene added). That means that the continent’s ice shelves have lost about 12 million metric tons over the past 25 years, about double previous estimates of loss. All this crumbling could spell bad news for the long-term stability of the continent’s ice sheets.
“Over the past quarter century, the shrinking and weakening of ice shelves has allowed Antarctica’s massive glaciers to speed up and increase their contribution to sea level rise,” said Greene. “The most significant impacts have been seen at West Antarctica’s Thwaites and Pine Island glaciers, and there’s no sign that either will be slowing down anytime soon.” (The Thwaites glacier is commonly referred to as the “doomsday glacier,” and it’s in pretty big trouble.)
Even ice sheets that were once thought to be stable are showing signs of stress. A second study out this week looks at the potential fate of an incredibly important ice sheet—the East Antarctic ice sheet, the biggest of the continent’s two ice sheets and the largest reservoir of freshwater on Earth. This ice sheet has traditionally been considered more protected than the western ice sheet—which includes the Thwaites and Pine Island glaciers—due to less exposure to warming ocean waters. But if the East Antarctic ice sheet ever does get threatened, that’s potentially disastrous news for the planet: the ice sheet holds enough water to raise sea levels by more than 170 feet (52 meters).
“We know that small mountain glaciers around the world are shrinking rapidly and contributing to sea-level rise,” Chris Stokes, the study’s lead author and a professor of geography at Durham University, said in an email. “We also know that the much larger Greenland Ice Sheet is also losing mass and contributing to sea level rise, as well as the Western part of the Antarctic Ice Sheet. However, we know much less about what might happen to the East Antarctic Ice Sheet.”
To get a better sense of what the future of the East Antarctic ice sheet could look like, Stokes and his co-authors did a review of previous work on how the ice sheet responded to past warm periods and current levels of change, added in with “a bit of new number-crunching based on computer simulations that predict how much this giant ice sheet might contribute to future sea-level rise,” he said.
There’s a bit of good news here: the authors say that the ice sheet will probably remain stable in the short term, and keeping warming under 2 degrees Celsius by 2100 will keep the ice sheet from collapsing over the long term. But the study also notes that the East Antarctic ice sheet is already showing signs of stress from climate change, and that the time to act is running out. Letting the world warm up beyond the bounds of the Paris Agreement, the study finds, could mean that the East Antarctica ice sheet could make sea levels rise by as much as 3 to 10 feet (1 to 2 meters) by 2300.
“The key conclusion from our work is that if we can satisfy the Paris Climate Agreement, we can almost certainly avoid a major sea-level contribution from East Antarctica,” said Stokes. “Hence, I think that amongst all of the doomsday stories that we hear about, our study at least offers some hope that we have a small window of opportunity over the next few decades to protect this ice sheet. As we conclude in the paper: the fate of the world’s largest ice sheet is very much in our hands.”
While these two papers deal with different scenarios, the message is clear: seriously curbing warming is crucial to helping us keep all above water.
“Antarctica is changing. Its ice shelves are falling apart, and sea levels are rising in response,” said Greene. “But as the Stokes et al. paper put it so well, there’s still time to act.”