If the world’s largest carbon polluters don’t cut emissions more aggressively, Antarctic ice melt could speed up dramatically around the middle of this century, triggering “rapid and unstoppable” sea-level rise for hundreds of years to come, a new modeling study has found.
Nearly 200 nations have submitted emissions reductions targets, called Nationally Determined Contributions, under the Paris Agreement. But while the global climate accord calls for limiting warming to 3.6 degrees Fahrenheit (2 degrees Celsius), the initial Paris Agreement pledges place the world on track for at least 5.4 degrees Fahrenheit (3 degrees Celsius) of warming this century. Last month, President Joe Biden and several other world leaders increased their nations’ emissions reduction goals to be more in line with the Paris temperature targets.
Research published Wednesday in the journal Nature shows just how much of a difference hitting those targets could make for Earth’s largest ice sheet. If humanity succeeds in limiting global warming to 2°C, the study concludes that Antarctica will continue to lose ice at a steady pace throughout the 21st century.
But if the world stays on its current path to exceed 2°C, Antarctica might experience an abrupt jump in melting and ice loss around 2060, nearly doubling its contribution to sea level rise by 2100. This, the study finds, is due to runaway processes that could be triggered in the coming decades if emissions aren’t reined in, particularly at the imperiled West Antarctic ice sheet.
“This is another fairly quantitative study that says ‘here is the line,’” says Ted Scambos, a glaciologist at the University of Colorado, Boulder, who wasn’t involved with the new paper. “If we keep our foot on the gas pedal, figuratively, we're going to get into a situation where the West Antarctic ice sheet will be on the road to elimination.”
Ice sheet tipping points
Since the early 1990s, Antarctica has lost roughly three trillion tons of ice. Today, the rate of loss is accelerating as warm ocean water melts and destabilizes the floating ice shelves that hold back West Antarctica’s glaciers, causing those glaciers to flow more quickly into the sea. But while minimizing global warming is clearly the best way to limit future Antarctic ice losses and sea level rise, scientists still aren’t sure how much ice will disappear—and how quickly—at different levels of warming.
That uncertainty is partly due to processes that may cause large Antarctica glaciers to enter a state of runaway retreat as the floating ice shelves that buttress them thin or disappear.
The first process, called the marine ice sheet instability, occurs when a glacier that empties into the ocean melts and retreats over bedrock that becomes deeper further inland. Much of West Antarctica is like that: The ice sits in a basin that’s below sea level (hence “marine”), like a giant scoop of ice cream in a shallow bowl.
As the edges of that ice melt away toward the deeper and taller center, the front of the glacier becomes thicker and exerts more pressure on the ice shelf holding it back. This causes the entire ice stream to ooze out to sea faster. The reality of this instability is widely accepted by climate scientists; it’s the reason they’re nervous about West Antarctica. The process may be underway already at Thwaites Glacier, which sits at the heart of West Antarctica and holds back several feet of global sea level rise.
The other process, marine ice cliff instability, is considered more speculative. As the ice retreats toward the center of the basin and loses its protective, floating ice shelves entirely, very tall cliffs of it can end up towering out of the sea. Beyond a certain height, perhaps 100 meters (328 feet) these cliffs could start collapsing under their own weight. If this process were to occur at Thwaites Glacier or elsewhere, it could lead to far more rapid ice losses and sea level rise.
To better understand Antarctica’s future and how these instabilities could shape it, the new study used an ice sheet physics model that incorporates both. It builds off of earlier modeling work published in 2016, but includes more sophisticated physical processes and interactions between Antarctic ice and other parts of the Earth system.
What a difference emissions make
The authors ran their models forward several centuries in time under a scenario where warming is limited to the Paris Agreement goal of 2°C, as well as the aspirational 1.5°C warming target. They also looked at a scenario in which temperatures climb by 3°C, roughly in line with the initial batch of global emissions reductions pledges under Paris.
The findings show that setting more ambitious climate targets could make a huge difference for Antarctica’s future. Antarctic ice melt is currently adding about half a millimeter a year (around .02 inch) to global sea level rise. If humanity limits warming to 1.5 or 2°C by stabilizing temperatures around the middle of the century, the study found, that contribution will rise to about 2 millimeters a year and between 80 and 90 millimeters total by 2100—a little over 3 inches.
But if we overshoot the Paris targets, our future looks considerably worse. In a 3°C warming scenario, crucial ice shelves holding back Thwaites and other West Antarctic glaciers could destabilize, triggering instabilities. Around 2060, models suggest, Antarctica could reach an inflection point where ice losses and sea level rise shoot up due to the onset of marine ice sheet and marine ice cliff instabilities.
Under this scenario, the ice sheet could be responsible for closer to 6 inches of global sea level rise by 2100. At that point, Antarctic melt causes the seas to rise by 5 millimeters a year—more than double what occurs at lower warming levels. By 2300, in this scenario, Antarctic melt alone will have added 5 feet to global sea levels, compared with about 3 feet if temperatures are stabilized at 1.5°C.
Even dialing atmospheric carbon levels back down is unlikely to stop runaway ice retreat if Antarctica crosses a tipping point. The authors found that if the initial Paris Agreement pledges were combined with atmospheric carbon removal technology starting later this century, sea levels would still rise for hundreds of years to come. Unless that technology is deployed at scale within the next 40 years, the total amount of sea level rise will still be higher than it would if global warming was limited to 2°C or less. Currently, technology for pulling carbon out of the air is in a very early stage of development.
“Once you hit this threshold, you can’t go back,” says study co-author Andrea Dutton, a sea level researcher at the University of Wisconsin-Madison. “You’re committed. That’s the big takeaway.”
No single study can perfectly predict Antarctica’s future, however. That’s one of the key takeaways of another paper published Wednesday in Nature, which analyzed hundreds of glacier and ice sheet modeling studies in order to predict the future contribution of all land-bound ice on Earth—including the Greenland and Antarctic ice sheets, mountain glaciers and ice caps from 19 regions around the world—to sea level rise under different carbon emissions scenarios.
This study also found a stark difference between what happens to Earth’s ice if the Paris Agreement targets are met versus if we overshoot them: In a 3°C warmer world, land ice adds twice as much to global sea levels by the end of the century compared with if temperatures only rise 1.5°C.
But when it comes to Antarctica, the second study found no clear relationship between emissions levels and ice losses across all of the models it looked at. This, the authors say, is due to uncertainties in how the competing processes of additional snowfall (which adds mass to the ice sheet) and additional melting will impact Antarctica on balance as Earth warms.
Tamsin Edwards, a climate scientist at King’s College London and the lead author of the second study, says this does not mean Antarctica isn’t being impacted by climate change. “What we‘re saying is when you look at lots of different climate models and ice sheet models, the amount that Antarctica responds to climate change varies very widely,” Edwards said in a press briefing.
The new paper by Dutton and her colleagues projects levels of Antarctic melt that are higher than the average projections in Edwards’ analysis, but “within the range” of a group of more pessimistic models, Edwards says. None of those other models included the marine ice cliff instability process—rather, they predicted greater levels of ice loss for other reasons, including more melting beneath Antarctica’s ice shelves or on top of them.
Whichever model winds up being most accurate, Dutton says it’s important to understand that the Antarctic ice sheet has an intrinsic tipping point.
“And there’s a real possibility we’re very close to it,” she says. “And we need to do everything in our power to prevent that from happening.”