The viability of coastal megacities from Miami to Manila to Dhaka are intertwined with the fate of the vast West Antarctic Ice Sheet, and new, unnerving evidence shows that the ice loss from that ice sheet may raise global sea levels by twice the rate previously estimated for this century.

For example, Boston could see more than 1.5 meters, or about 5 feet, of sea-level rise in the next 100 years.



The study, published in the journal Nature on Wednesday, indicates that this ice sheet is far more sensitive to changes in the amount of greenhouse gases in the atmosphere as well as air and ocean temperatures than previously thought.

The bottom line finding is that sea level rise could go into overdrive during the next several decades, and become an existential problem for many coastal cities within the lifetime of current generations of people.

The study projects more than a meter, or 3.4 feet, of sea level rise from West Antarctica alone by the year 2100, along with an astonishing 15 meters, or 50 feet of sea level rise by the year 2500. By the middle of the 22nd century, the rate of sea level rise could exceed a foot per decade.

For perspective, that is about the amount of sea level rise that New York City has seen in the past century.

When combined with ice melt from Greenland and the expansion of seawater as the oceans warm, the study implies that previous sea level rise projections are off by about a factor of two.

The study offers a stark warning that the so-called "consensus" view of sea level rise provided in a U.N. panel's report three years ago may already be both outdated and far too optimistic. It builds on the work of other recent studies that have found that human-caused global warming has set in motion an essentially irreversible, long-term melting of the West Antarctic Ice Sheet.

"I am unnerved and rattled by this research"

The study, by two veteran glacier researchers, Robert DeConto of the University of Massachusetts at Amherst and David Pollard at Penn State University, warns that seas could rise by more than a meter, or 3.4 feet, by the end of this century, unless emissions of global warming pollutants are dramatically curtailed in the next few decades.

In fact, the simulations produced in the study show the complete collapse of the Larsen C ice shelf by 2055, followed by the rapid retreat of the West Antarctic Ice Sheet from all sides at once.

An iceberg embedded in sea ice as seen from the IceBridge DC-8 over the Bellingshausen Sea on Oct. 19, 2012. Image: NASA/James Yungel

The study is the first to use computer modeling that accurately accounts for a disconnect that had puzzled many glaciologists for years.

Scientists have long had evidence showing that during past periods with high greenhouse gas concentrations and relatively mild temperatures — including the Pliocene period about three million years ago, as well as the last interglacial between 130,000 and 115,000 years ago — global sea levels rose extremely rapidly, with global mean sea level of between 6 and 9.3 meters, or 19.6 to 30.5 feet, higher than today during the Pliocene.

Such high sea levels could only come from significant melting of both Greenland and Antarctica.

Yet ice sheet models that DeConto and Pollard had developed, along with other scientific groups' models, could not reproduce such melt rates or severity of the total melt during those periods.

Until now, that is.

The refined model used for this study was, in fact, able to simulate what scientists think happened to West Antarctica during those two spans of higher sea levels.

Giant, unstable ice cliffs

The new study takes into account the possibility that as floating ice shelves retreat inland, tall ice cliffs will form as a barrier between the ocean and the huge ice sheets that form a bowl behind them. Much of the West Antarctic Ice Sheet is well below sea level, so once those cliffs collapse, as the new model simulations project they will, the ice sheet will rapidly slip into the sea.

Giant crack seen in Pine Island Glacier in 2011. Image: NASA.

The study refers to such an occurrence as a "runaway Marine Ice Sheet Instability." Such instability is already being seen at at least three glaciers: the Helheim and Jakobshavn glaciers on Greenland and the Crane Glacier on the Antarctic Peninsula.

Other recent studies have focused on the role that relatively mild ocean waters are playing in eating away at marine-terminating glaciers and floating ice shelves from below.

This new study, though, finds that is important for destabilizing some glaciers and ice floes, but that over the longer-term, a bigger role will be played by warming air temperatures and meltwater, not to mention rainfall, on the top of the West Antarctic Ice Sheet. This will accelerate melting dramatically through 2100 and beyond, the study found.

The calving front edge of the Pine Island Glacier seen during an IceBridge flight on Oct. 29, 2014. Image: NASA/James Yungel

"Geological history and modern observations show that the grounding line, where the ice starts to float, may sit in one place for millennia despite changes in the environment, then pass some threshold and shift its position large distances rapidly, leading to thinning of non-floating ice and thus raising sea level," said Richard Alley, an ice sheet expert at Penn State who has published studies with Pollard, but was not directly involved in the new research.

The study does have an important silver lining, however, in that it shows that significant and rapid cuts in greenhouse gas emissions could prevent swift and severe sea level rise. However, the emissions cuts currently on the table as part of the Paris climate agreement, which is to be signed in a ceremony in New York in April, would not be sufficient.

Not a worst-case scenario

Pollard told Mashable in an interview that the study does not present a worst-case scenario, since the researchers chose to simulate a relatively slow rate of ice cliff collapse in West Antarctica compared to what they think is possible.

He also cautioned against feeling despair from seeing such dire scientific projections.

“This is so much higher than previous estimates are… that it’s really sobering and surprising jump of a previous estimates. It’s not the worst, but it’s a worser case than before in comparison to previous studies,” he said.

“It’s really just pointing out the possibility of these things, it’s not the last word, if anything it’s the first word.”

Pollard said such high amounts of sea level rise are not yet "locked in," and can be averted if countries were to significantly reduce greenhouse gas emissions.

“If we somehow jump to very strong mitigation and reductions in greenhouse gases…. Then our results are much more optimistic,” he said.

Scientists who were not involved in the new study said it is important because it accurately captures the events of the past, therefore leading to greater confidence that it is accurately projecting future ice loss and resulting sea level rise.

"I believe DeConto and Pollard have broken new ground with the full experiment of retreat through the deep basins including the cliff mechanisms," Alley said.



"Beyond some threshold, warming doesn’t just reduce the ice shelves but eliminates them to make a calving cliff," he said.

Ian Joughin, a researcher at the University of Washington's Polar Science Center who co-authored a study in 2015 that found that the "collapse" of parts of the West Antarctic Ice Sheet is now likely inevitable during the next 200 to 1,000 years, said the new study uses a model with a coarser resolution.

"If you think of the ice sheet as a ball sitting at the top of a hill, our model is more like starting from a point where some force of nature, whatever that may have been, has started the ball rolling down the hill. In their case, the ball is at the top of the hill and needs a push," he said of the new study, in an email.

A key component of the new study is how it handles the breaking up, or hydrofracture, of steep cliffs that could form around the edges of the West Antarctic Ice Sheet.

"Now, calving cliffs break, wait, wait, wait, break, but a too-high cliff will require less waiting, or perhaps almost none," Alley wrote in an email. "We haven’t “seen” a too-high cliff; the tallest ones observed today aren’t higher than the estimated threshold for more-rapid breakage, but retreat in Thwaites Glacier could give a much higher one. Prior modeling generally has not included all of those physical processes."

Frozen icebergs near Thwaites Glacier in West Antarctica seen during the Nov. 7, 2014, IceBridge mission. Image: NASA /James Yungel

"Beach-goers know that lifeguards won’t let kids dig really deep holes in the sand, because a too-high “cliff” can collapse, burying someone in the hole," he said. "Highway engineers work hard to transform cliffs into slopes to avoid rockfalls or landslides. Miners and quarry-workers have long known that too-high cliffs can crumble. I have flown along the 100-m/30-story-high cliff extending above sea level at Jakobshavn and marveled at the 10-story-high cracks in it. It waits between calving events, but it surely looks close to breaking."

Joughin, however, thinks the new study shows too rapid a breakup of these ice cliffs, and said the 200 to 1,000-year timeframe in his 2015 study may well hold up compared to the more rapid melt indicated in the new study.

In his view, the melt induced by the breaking up of cliffs at the edges of the West Antarctic Ice Sheet may take several decades longer than the new study estimates, for example.

Eric Rignot, a professor of earth system science at the University of California at Irvine, said the many uncertainties involved with projecting how the world's vast ice sheets will respond to global warming argues in favor of acting now to cut emissions of global warming pollutants.

"... We have never, us, humans, seen an ice sheet collapse. We have seen earthquakes, volcanoes, tsunamis, now stars exploding, or even black holes colliding, but we have never seen an ice sheet fall apart. So there are a lot things that we do not know about it, and frankly I am not sure it is worth running the experiment because we can only do that once," he said via email.

"We have still time to avoid the worst of it, but we have already opened a number of flood gates, one in West Antarctica, and several in Greenland," he said.

Alley compared the state of the science on projecting the future of the Antarctic ice sheet as akin to extended-range weather forecasts. His comments here, which were sent to several climate reporters, are worth excerpting in full (with a few minor edits):

You may recall many cases this winter, or last winter, when a US-east-coast weather person gave the extended outlook as something like “The models don’t agree well. Any single forecast is notably uncertain. But, ten days out, the most-likely of those unlikely forecasts is a major event.” You know that the weather forecasters generally have proven to have skill; exactly where, when and how big the storm would be usually shifted in the forecasts over the following week, but the major event usually happened... I think that the models don’t agree well, any single forecast is notably uncertain, but if we continue warming the world rapidly, the most likely outcome is a major event of large and rapid sea-level rise.

Personally, I really would be happier if we had the luxury of doing the research on this, without bothering the public until we have 95% confidence in an answer. All of us are fully aware how wrong it is to falsely yell “Fire” in a crowded theater. But, we also are fully aware how wrong it is to sit silently while a fire begins to spread in that theater. Right now, I do not believe humanity can continue with unchecked warming while confidently assuming that sea-level rise will be limited to roughly 3 feet (1 meter) in a century.



Instead, the recent modeling now favors the view that continuing rapid warming will cause sea-level rise to be larger, and perhaps much larger, especially if we look beyond the end of this century.

Benjamin Strauss, who directs the climate impacts program at the nonprofit research and journalism organization Climate Central, told Mashable that the study is hugely important. Strauss has led the development of sea level rise maps for cities in the U.S. and worldwide down to the ZIP code level.

"I am unnerved and rattled by this research," Strauss said.

Close-ups of the Amundsen Sea sector of the West Antarctic Ice Sheet in a high emissions scenario, showing the gradual disappearance of ice in this sector. Image: De conto and pollard, nature 2016.

"If this research stands, it marks a paradigm shift in sea level projections, and much for the worse. Yesterday's worst-case scenario becomes tomorrow's mid-range forecast."

Climate Central's analysis of the new research showed that it would raise tide levels by between 5 to 6 feet during this century in many U.S. East Coast cities, including New York and Boston.

Strauss is optimistic that the pace of technological and economic change can bring emissions under control faster than expected though, noting the debut on Thursday of Tesla's first mass-market electric vehicle as an example of this.

"I believe technological and economic change can take place much faster than we tend to expect — just like sea level change," he said.