The amount of sea level rise that many of us will experience in our lifetimes may be more than double what was previously anticipated, unless we sharply curtail greenhouse gas emissions, according to a new study that factors in emerging, unsettling research on the tenuous stability of the Antarctic Ice Sheet.

Importantly, the study highlights that cuts we could still make to greenhouse gas emissions during the next several years would significantly reduce the possibility of a sea level rise calamity after 2050.

Published Wednesday in the open access journal Earth's Future, the study is the first to pair recently discovered mechanisms that would lead to the sudden collapse of parts of the Antarctic Ice Sheet, such as the disintegration of floating ice shelves and mechanical failure of tall ice cliffs facing the sea. The study also goes a step further by showing how the new projections could play out city by city around the world.

Researchers from several institutions, including Rutgers University, Princeton, Harvard, and the nonprofit research group Climate Central found that sea level rise predictions that incorporate a faster -- even sudden -- disintegration of huge parts of the Antarctic Ice Sheet would yield far more dire projections.

This is especially the case when compared to the consensus put forward by the U.N. Intergovernmental Panel on Climate Change (IPCC) in 2014.

The IPCC did not incorporate the possibility that the Antarctic Ice Sheet could become unstable as air and sea temperatures warm, and essentially crumble into the sea, in rapid succession.

More than 4 feet of sea level rise

Specifically, the new study finds a median sea level rise projection of 4 feet and 9 inches during the 21st Century if greenhouse gas emissions remain on their current high trajectory.

Or, when expressed as a range, the study shows that a high emissions scenario that takes new Antarctic melt mechanisms into account would yield between 3 and 8 feet of global average sea level rise by the year 2100. This contrasts with the projection that does not include the rapid Antarctic melt mechanisms, which shows just 1.6 to 3.9 feet of sea level rise through 2100.

The new study paints a far more alarming picture compared to what the IPCC found, which was a median projection of two feet and five inches of sea level rise by 2100 under a high emissions scenario. Most sea level rise predictions since that report was published have indicated that figure was too low, however.

The study shows that global average sea level is projected to increase by one foot by the year 2050, and several more feet by the year 2100, depending on the significance of any cuts to greenhouse gas emissions. If the temperature targets in the Paris Climate Agreement are met -- which is a big if at the moment -- then we're unlikely to trigger a rapid Antarctic meltdown, the study found.

Prior sea level rise projections have not included the recently discovered mechanism of marine ice-cliff instability in Antarctica. Instead, those projections relied on other assumptions of how significantly the Greenland and Antarctic Ice Sheets might melt during the course of the 21st Century. Nor have they included all of the ways that floating ice shelves might disintegrate rapidly, either, weakening inland ice.

Two of the authors of the new paper, Rob DeConto of the University of Massachusetts and David Pollard of Pennsylvania State University, have published studies raising unsettling questions about the stability of Antarctic ice.

In an interview at the American Geophysical Union's annual meeting in New Orleans, coauthors Robert Kopp and DeConto emphasized that our understanding of the physics of Antarctic ice melt mean that there is a large amount of uncertainty regarding sea level rise projections for post-2050, so large and complex, in fact, that it's referred to as "deep uncertainty."

Ice cliffs and shelves

One of these mechanisms involves ice shelves that hold back large quantities of land-based ice, but that are vulnerable to melting from relatively warm water below and fracturing from melt ponds that can form during the summer on the surface of the ice.

Such melt ponds appeared en masse shortly before the collapse of the Larsen B Ice Shelf in 2002.

“The meltwater can move around, which can put different stresses onto the ice shelf, and the meltwater itself can get into crevasses in the ice shelves, and that meltwater getting into crevasses can make the crevasses go deeper and deeper and deeper," DeConto said. "Eventually they’ll penetrate so much of the ice shelf that it won’t be able to hold itself together and it’ll break up.”

“In a future, warmer world we’ll start to see summers where there’s days, weeks, months where there’s persistent meltwater and even rain falling on the ice shelf surfaces and we’ll start to see ponding,” DeConto said. “You could have these ice shelves break up really suddenly… We’ve seen these happen in the past.”

When floating ice shelves shrink or even disappear, water can penetrate further inland, especially in areas like the West Antarctic Ice Sheet, which is shaped like a bowl and situated below sea level, Kopp said.

The other mechanism concerns the collapse of massive ice cliffs that exist around Antarctica, where huge glaciers end abruptly in the sea. These cliffs can only grow to certain heights before they're inherently unstable, DeConto said. This mechanism is known as marine ice-cliff instability.

“Ice is only so strong, we know something about the strength of ice, if a cliff is tall enough, just it’s own weight will create stresses that overcome the strength of the ice and it will fail at the edge, it will break,” DeConto said.  

“We realized there are lots of places around the Antarctic… where if you lose ice shelves you’re going to have ice cliffs tall enough that they would start to break up mechanically, mechanical failure of these ice cliffs," he said.

While there is considerable debate about the new Antarctic findings, researchers have found that it more closely matches historical data of past melt events, DeConto said, which lends it some street cred. The deep uncertainty regarding sea level rise projections post-2050 means that the specific figures in this study should not be taken as precise forecasts.

The new study combines DeConto and Pollard's findings with sea level rise projections other researchers published in 2014, which allows for detailed sea level rise mapping scenarios of what the projections would look like if they become reality.

153 million people

The study calculates that, absent protections like sea walls and other flood mitigation measures, water could permanently inundate land that is currently home to 153 million people. In other words, the number of people that lives on land that could be underwater by the year 2100 equals nearly half the U.S. population, according to Climate Central.

It's likely, however, that we may not know how much sea level rise we are in for until around midcentury, according to the new research. And by that point, greenhouse gas emissions from the burning of fossil fuels will already have locked us into such amounts, making emissions reductions in the face of uncertainty even more vital today.

Because of this, Climate Central created a series of graphics that illustrate the new projections under low and high greenhouse gas emissions scenarios. A high emissions future looks, well, rather wet and potentially even unsurvivable for many coastal cities.

"The takeaway is that our current understanding does not constrain sea level rise rates after 2050 or so," said coauthor Michael Oppenheimer, a climate scientist at Princeton University, in an email.

"A significant chunk of the 3 to 4 meters of sea level equivalent in the West Antarctic Ice Sheet could be in play during that period and beyond (and some in the East Antarctic Ice Sheet) and we simply don’t know how much and how fast, but very high rates are plausible, potentially doubling or even tripling global and local rates formerly estimated by IPCC, and radically shortening flood return intervals."

"The good news is that at this point, we have a pretty good fix on what will happen through 2050," Oppenheimer said. "That means the time to implement the first phase of adaptation is NOW - we have a credible sea level range for that period. But we need to implement resilience flexibly while we start planning for the period beyond 2050 because projections are unlikely to narrow for the later period for a long time."

In other words, as with so many climate change impacts, when it comes to sea level rise, it's up to us to determine how severe we want this problem to get before we do something about it.

Editor's note: This story was updated to correct an error in the headline regarding the current population that would be affected by 8 feet of sea level rise. The original figure of 153 million was too low.