The warming climate will push Antarctica’s ice sheets to the brink of irreversible melting, even if nations make good on the new commitments to cut carbon dioxide emissions they announced during the U.S.-led climate summit last month, according to two new studies published in Nature on Wednesday.

The studies show that the best chance of stopping the meltdown is to reach the Paris agreement’s most ambitious target to cap warming at 2.7 degrees Fahrenheit (1.5 degrees Celsius) above the pre-industrial level. The alternative is millennia of sea level rise that will swamp Pacific islands, flood shoreline cities and displace millions more people from coastal regions.

“Even the low-end scenarios for sea level rise are pretty bad,” said Andrea Dutton, a paleoclimate scientist at the University of Wisconsin-Madison and co-author of one of the studies. Major climate reports project that, even with rapid cuts in carbon dioxide emissions, global average sea level will rise at least 12 to 16 inches by 2100. 

In some regions, including the Western Gulf of Mexico and the Northeast United States, sea level will rise much higher than that, swamping thousands of square miles of coastal land, backing up storm drains and polluting water used for drinking and irrigation with salt. Sea level rise in parts of the tropical Pacific Ocean is projected to be as much as double the global average, which will submerge some of the far-flung islands that are home to 2.3 million people. 

Right now, the world is on a path to about 5.5 degrees Fahrenheit of warming—twice the amount of the Paris goal—which, “by the second half of this century means you clearly reach a threshold that changes ice sheet stability,” Dutton said. That would cause so much melting that sea level rise would increase from “millimeters to centimeters per year.” she said. “Coastal communities are totally unprepared to deal with that.”

The study Dutton worked on concluded that, with “global warming limited to 2 degrees Celsius (3.6 degrees Fahrenheit) or less, Antarctic ice loss will continue at a pace similar to today’s throughout the twenty-first century.” But at the current rate of warming, there would be an “abrupt jump in the pace of Antarctic ice loss after around 2060.”

That’s because Antarctica’s giant floating ice shelves are tenuously anchored to outcrops and ridges on the seafloor, and if the ocean warms enough to loosen those bonds, the flow of land-based ice held back by the floating shelves would accelerate toward the sea. 

The edges of the floating shelves, including ice cliffs towering hundreds of feet above the ocean, are also very susceptible to the warming atmosphere, said University of Exeter climate modeler Ed Gasson, a co-author of the Nature study.

“It’s a phase change,” he said, describing the climate threshold that would be breached somewhere between 2.7 and 3.6 degrees Fahrenheit of warming. “When you get rainfall instead of snowfall, and melting on the surface in the summer, that’s what starts the breakdown of the ice shelves.” The rain and meltwater seeps down into cracks in the ice and when it refreezes and expands, it acts like a wedge that can split off huge sections of the shelves, he said.

Related: Global Warming Is Changing the Winds Off Antarctica, Driving Ice Melt

A big question mark for many researchers is whether warming will boost snowfall over Antarctica, which could change the melt equation significantly if enough new snow and ice piles up to compensate for some of the losses. 

That’s why the total amount of sea level rise from land-ice melt is still a big unknown, said GEOMAR University of Kiel climate researcher Mojib Latif, who was not involved in the new studies. Projections will be more accurate when new climate models can accurately simulate the complex interplay between oceans, ice and the atmosphere, he said.

Such interactions include the possibility of unexpected climate feedback loops like the potential for ice shelf melting to fuel sea ice expansion with more freshwater, which freezes easier, explained Gasson.

Some of the current research with the latest models will shape revised sea level rise projections in the upcoming climate assessments from the Intergovernmental Panel on Climate Change.

Cracking Up and Melting Down

The Paris agreement’s goal of limiting warming to 2.7 degrees Fahrenheit “wasn’t just pulled out of the air,” climate researcher Bill Hare, chief scientist with Climate Analytics, posted recently on Twitter. 

The potential crackup and meltdown of Antarctica’s ice above that temperature is one of the things proving his point. Warming much more than that will also bring a steep increase in the number of deadly heat waves and droughts, the decline of coral reefs by as much as 99 percent, increased loss of biodiversity and other dangerous climate extremes, as detailed by a 2018 report from the Intergovernmental Panel on Climate Change.  

Earth’s geologic record reinforces the current warnings. Global temperatures during the Pliocene Epoch, from about 2.6 to 5.5 million years ago, were similar to today’s, but sea level was about 20 feet higher. The only way that climate models can recreate those conditions is by including a rapid disintegration of Antarctica’s ice shelves, Dutton said.

Research published in 2020 hinted that some ice shelves retreated by as much as 6 miles per year during rapid climate shifts in the geologic past, and that sea level rose at a rate of about 1.5 to 2.3 inches per year for about 500 years at the end of the last ice age. That would be a quantum increase from today’s rates of glacial melting in Antarctica and global sea level rise. The fastest current ice shelf retreat has been measured at between about a half mile to 1.2 miles per year in West Antarctica.

Related: Antarctic Ocean Reveals New Signs of Rapid Melt of Ancient Ice, Clues About Future Sea Level Rise

The difference between 2.7 and 3.6 degrees Celsius warming may not seem like all that much, but it has a “huge planetary scale impact,” she said. During the peak of the last ice age, the global average temperature was only about 5 degrees Fahrenheit cooler than today, yet ice sheets a mile thick covered most of North America. 

Another way to think of it is in terms of your own body temperature, she added. “If you had a long-term 3.6 degree fever, you’d be pretty sick,” she said. In Antarctica, when air temperatures are near freezing over the surface of the ice sheet, those small increments make the difference between ice remaining frozen or thawing. Once you pass the threshold, “it increases melt exponentially.”

Steps to reduce emissions and avoid that threshold should be the big policy priority, Gasson said. If the signatories to the Paris agreement can hold warming to its least ambitious target of 2 degrees Celsius of warming, meltwater from Antarctica will raise seas between 2.4 and 4.3 inches. But at the current trajectory toward 3 degrees Celsius (5.4 degrees Fahrenheit) of warming, that range would be 6.6 to 8.2 inches.

Another Study Highlights Uncertainty

The other Nature study published Wednesday did a statistical analysis of climate simulations developed by 62 research institutions in 15 countries to show how all global land ice, including the polar ice sheets and mountain glaciers, will respond to different levels of greenhouse gas emissions and temperature increases.

Lead author Tamsin Edwards, a climate scientist at King’s College, London, said the research aimed to highlight some of the uncertainties associated with the future of Antarctica’s ice, because for policy makers and planners, the unknowns can be just as important as what is known. 

Despite the intense research into increasing sea levels, officials responsible for coastal flood preparations are still faced with a wide range of sea level rise scenarios. Narrowing projections of how much the water will rise would require detailed modeling of every individual glacier, ice sheet and ice shelf, down to the shape and gradient of the valleys they flow through—a near impossible task.

The new research is also specifically intended to refine sea level rise projections, including uncertainties, in the upcoming IPCC report, Edwards said. In the policy guidance language of the reports, the now-familiar Representative Concentration Pathways (RCPs), will be replaced by updated socio-economic scenarios. Those won’t highlight, for example, increasingly less likely futures with unbridled fossil fuel burning.

The study shows that, in fact, meeting the goal of the Paris agreement makes a huge difference, not just to polar ice masses, but to 200,000 or so mountain glaciers which together cover an area about the size of Texas.

Capping warming at close to the Paris agreement’s goal of 2.7 degrees Fahrenheit would cut the contribution of melting land ice to sea level rise in half, to five inches, as opposed to the 10 inches expected by 2100 under current climate projections. Meeting that target could reduce sea level rise from melting Greenland ice by 70 percent.

If we stick to the path of warming as projected based on current pledges to reduce emissions, Dutton said, “we will reach this trigger where we start those processes and lose the ice shelves. If we decide after that to reduce emissions, the ice sheet doesn’t care. It will keep melting. It’s not reversible, at least on a human time scale.”

“There are tipping points, and we don’t want to go there,” she added. “The problem with thresholds is, you don’t know it until you’ve passed it. The point is, we need to act now to avoid passing it.”

And while melting ice sheets at the ends of the Earth may seem remote, Dutton sees their impacts as far from abstract.

“To me it’s about people,” she said. “We have choices. It’s about how we are going to survive on this planet.”