Ice losses from Antarctica have been accelerating in recent years
Scientists struggling to understand the threat of sea level rise on a warming Earth found on Wednesday that amid lingering uncertainty, this much is clear: Meeting the goals of the Paris climate agreement remain 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 fuelling), 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 on 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 included an additional process in their model, known as "marine ice-cliff instability," which factors in 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 3°C above preindustrial levels. The world already has exceeded 1°C 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, 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 the year 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 marine-based 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, 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 OK, 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 the year 2100 into its main components.
The first is simply 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 additional 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 on 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 3 1/2 inches by the year 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 around 1 1/2 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 levels of sea level rise from the Antarctic even with quite low emissions. In those cases, this rise averaged an additional 8 inches or more by the year 2100, just from the Antarctic.
None of these models included a process known as the "marine ice-cliff instability," which some experts believe is already underway in parts of Greenland and that could someday affect the Antarctic.
It refers to the idea that larger and thicker glaciers can become chronically unstable once their ice shelves shatter and crumble into the ocean, leaving behind tall and fragile cliffs overlooking the ocean.
These are "potentially game changing processes in a warming world, with a warmer ocean surrounding Antarctica, and a warmer atmosphere," said DeConto.
DeConto's study, which does include this instability, suggests an Antarctic trigger may exist somewhere around 3°C of warming.
Holding warming to the Paris agreement targets of "well below" 2°C, compared to preindustrial levels, generally only produced a few inches of sea level rise from Antarctica by 2100 in his simulations.
But higher levels of warming began to generate much bigger losses in some simulations, leading to dramatic ice retreat in the region of West Antarctica.
"These results demonstrate the possibility that rapid and unstoppable sea-level rise from Antarctica will be triggered if Paris Agreement targets are exceeded," the study states.
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."