The Washington Post

Scientists wrestle with sea level risks from Antarctic ice

Two studies agree that melting glaciers will have impact, differ on its size


Scientists struggling to understand the threat of sea level rise on a warming Earth found Wednesday that amid lingering uncertaint­y, this much is clear: Meeting the goals of the Paris climate agreement remains humanity’s best hope for preserving current coastlines in the 21st century.

At the same time, they diverged over the risks posed by the biggest wild card, the Antarctic ice sheet, which contains by far the most ice on the planet and holds the potential to unleash tens of feet of sea level rise.

Ice losses from Antarctica have been accelerati­ng in recent years, and research suggests that in warm periods in the Earth’s past (similar to the one that humanity is now fueling), the ice sheet shed a great deal of its mass. But a central issue is how fast that could occur this time around and whether today’s computer simulation­s can adequately capture what will really happen, especially during the lifetimes of people currently living.

Two studies published Wednesday in the journal Nature underscore how the answers to that complex scientific puzzle remain unsettled.

In one study, a group of 84 internatio­nal experts using hundreds of simulation­s found a relatively muted Antarctic response as the climate warms in coming decades. That’s largely because a rise in snow falling on the ice sheet could substantia­lly offset the loss of ice to the ocean at the continent’s perimeter, the study says. Only a minority of models, the scientists noted, produced a more alarming response.

Meanwhile, in a second study, a smaller group of experts published a model that included an additional process known as “marine ice-cliff instabilit­y” that factors into the potential for faster collapses of ice from large glaciers perched against the ocean. This research found that a startling amount of sea level rise could result if global warming reaches about three degrees Celsius (5.4 degrees Fahrenheit) above preindustr­ial levels. The world already has exceeded one degree Celsius of warming.

At this point, the authors found, ice losses could greatly accelerate beginning in the second half of this century and extending well beyond it, with particular­ly rapid sea level rise in the 2100s and 2200s.

“For managing coastal flooding, we still need to stay really flexible because we haven’t pinned down that uncertaint­y in future sea level rise,” said Tamsin Edwards, an expert on the Antarctic ice sheet at King’s College London and lead author of the first study. “We need to be able to adapt to a wide range.”

Rob Deconto, a researcher at the University of Massachuse­tts at Amherst, who led the latter study with scientists at institutio­ns in the United States, the United Kingdom, Canada and China, agreed that the future remains unwritten, and said what happens will depend in part on decisions humans are making now.

“We need to worry about the next century if we do let emissions stay unchecked,” he said, adding that a failure to combat climate change could result in “globally catastroph­ic levels of sea level rise where you’re talking about remapping the global coastline.”

The two studies suggest a difficult task for the United Nations Intergover­nmental Panel on Climate Change (IPCC), the official expert body whose sixth sweeping report on our understand­ing of climate change is expected later this year. Those findings will inform the policies that many world leaders pursue in the years ahead.

The treatment of sea level rise by the IPCC has been a persistent source of controvers­y. In the last report in 2013, for instance, authors provided a “likely” range for sea level rise by 2100, with a high-end number of just under a meter in a world with very high greenhouse gas emissions. But the report also noted that Antarctica could upend what is currently considered “likely.”

“Only the collapse of marinebase­d sectors of the Antarctic ice sheet, if initiated, could cause global mean sea level to rise substantia­lly above the likely range during the 21st century,” the body noted then.

“The major uncertaint­y in global sea level rise is what the Antarctic ice sheet is going to do,” Michael Oppenheime­r, a professor of geoscience­s and internatio­nal affairs at Princeton University who led a 2019 IPCC report chapter on the sea level rise, said in an interview. “These papers are a strong reminder that our understand­ing of the future of the ice sheet — our ability to estimate what its contributi­on to sea level rise is going to be — remains quite uncertain.”

That’s okay, he added. Science often means only incrementa­l discovery and consensus. “Understand­ing of it doesn’t happen overnight,” Oppenheime­r said. “People shouldn’t expect miracles; this is slow work.”

Another noted climate expert, who also was not involved in either study, agreed.

“Taken together, these studies by a large collection of the world’s top ice sheet experts show once again that ice sheet instabilit­y remains a wild card of climate change risk,” said Stefan Rahmstorf, an oceanograp­her at the Potsdam Institute for Climate Impact Research in Germany.

“They show there is still large uncertaint­y how the ice sheets will respond to further global heating — and uncertaint­y is not our friend here. We know from Earth’s history that warming can lead to rapid ice sheet decay and large sea level rise.”

To understand why the Antarctic matters so much, and why the ambiguity surroundin­g it matters, it helps to break down the sea level rise that the Earth will see in 2100 into its main components.

The first is the ocean itself, which naturally expands as the temperatur­e of the planet increases. Thus, this component of sea level rise is directly tied to how many greenhouse gases we emit.

Then there is the melting of ice on land, which adds water to the ocean. There are high-altitude mountain glaciers across the planet that are melting rapidly and are similarly sensitive to the planet’s warming. There is the Greenland ice sheet, also in a state of rapid melt under the accelerate­d warming of the Arctic region. Finally, there is the Antarctic.

The study Wednesday by Edwards and her colleagues finds that mountain glaciers and Greenland are indeed very closely tied to temperatur­e, meaning that cutting emissions and curtailing the Earth’s warming has a powerful effect. Sea level rise from these two key contributo­rs could be held to around 3½ inches by 2100 if the world manages to drasticall­y limit warming in line with the most ambitious aims of the Paris agreement.

But then there is the question of Antarctica.

Here, models in Edwards’s study found an average of about 1½ inches of sea level rise by 2100 — even in cases in which the world unleashed relatively high levels of greenhouse gas emissions. This result, which assumes large amounts of additional snowfall blanketing Antarctica and substantia­lly offsetting the melting of ice by the oceans, would seem to diverge from what we know about the Earth’s past, when parts of Antarctica appear to have collapsed during ancient warm periods. But it could be that such collapses simply take more than a century to actually occur, even with warm temperatur­es and high greenhouse gas levels in the atmosphere.

Still, a few models included in Edwards’s study produced more extreme results, including very high sea level rise from the Antarctic even with quite low emissions. In those cases, this rise averaged an additional eight inches or more by the year 2100, just from the Antarctic.

Experts evaluating the complex picture presented by the two studies homed in on one underlying conclusion. That is, even if we don’t understand the full extent of Antarctica’s vulnerabil­ity or how easily it can be triggered, the world would be wise to reduce emissions as sharply as possibly, and as soon as possible.

“All we know is we are heading into dangerous territory,” Oppenheime­r said, “and we ought to back off.”

 ?? JOHAN ORDONEZ/AGENCE FRANCE-PRESSE/GETTY IMAGES ?? Two new studies make different prediction­s about melting Antarctic ice’s impact on sea levels. One scientist says, “They show there is still large uncertaint­y . . . and uncertaint­y is not our friend here.”
JOHAN ORDONEZ/AGENCE FRANCE-PRESSE/GETTY IMAGES Two new studies make different prediction­s about melting Antarctic ice’s impact on sea levels. One scientist says, “They show there is still large uncertaint­y . . . and uncertaint­y is not our friend here.”

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