As coastal communities prepare for the impacts of climate change, a new report warns that ice loss from Antarctica and Greenland could cause far more sea level rise than previously thought, and it says planners should not ignore that peril.

If planners are working with a mid-range projection of sea level rise, their efforts might protect coastal regions from the most likely scenarios depicted in climate models, but that still leaves a lot of risk, say the authors of the study published Monday in the journal Proceedings of the National Academy of Sciences.

“Coastal decisions by and large require long lead times, and it would be nice if we could wait for the science to clear up, but we can’t,” said Michael Oppenheimer, an atmospheric scientist at Princeton University and one of the authors of the study.

“If you knew there was a one-third or even 10 percent chance a plane would crash, you wouldn’t get on it. It’s the same with sea level rise,” he said.

The authors write that, for planning purposes, it would be prudent to use scenarios that anticipate 6.5 feet of sea level rise by the end of the 21st Century—more than double the likely upper limit put forward by the Intergovernmental Panel on Climate Change.

That projection of 6.5 feet of sea level rise is based on a worst-case emissions scenario in which little is done to rein in greenhouse gases and the planet warms as much as 5 degrees Celsius (9°F) above pre-industrial times.

That amount of sea level rise could inundate nearly 700,000 square miles—almost equal to the entire land mass of Indonesia—“including critical regions of food production and displacement of up to 187 million people,” the authors write. “A [sea level rise] of this magnitude would clearly have profound consequences for humanity.”

Meeting the Paris climate agreement‘s ambitions—which would require governments to take steps that keep warming under 1.5°C—would result in as much as 2 feet of sea level rise. Another half degree, up to 2°C warming, would add 4 inches on top of that.

Better Understanding of the ‘Known Unknowns’

When the Intergovernmental Panel on Climate Change was publishing its fifth assessment report in 2014, there were still a lot of “unknown unknowns” about ice sheet behavior, to borrow a phrase from former Secretary of Defense Donald Rumsfeld.

In the intervening years, experts have learned about new dynamics in the ice sheet that could impact the loss of ice and resulting sea level rise. That includes both positive feedbacks that could rapidly accelerate the loss of ice, and negative feedbacks, which could slow it down.

In Antarctica, for example, recent detailed satellite data analyses suggest that warming ocean water is influencing East Antarctic glaciers more than in the past and that they have been losing ice faster over the past decade. In West Antarctica, research published last week found nearly a quarter of that region’s ice is now thinning and the largest glaciers, Pine Island and Thwaites, are losing ice five times faster than they were in 1992. The discovery of a giant cavity expanding under the Thwaites Glacier has also raised new questions about how warming water will affect the ice. On the Greenland Ice Sheet, scientists are developing new understanding of melting feedback loops involving microbes, among other changes.

But just how fast could the ice be lost, and how much sea level rise could result from it? That’s still hard to model. With the increase in “known unknowns,” the amount of uncertainty around sea level rise has grown.

It’s counterintuitive, says lead author Jonathan Bamber of the University of Bristol, but not actually that rare. “When you start to dig into these questions, you realize, ‘Ah, there are things we didn’t even know were important and now we know they are.’”

Asking Experts in the Field

The authors behind Monday’s report took a different approach to understanding the risks of sea level rise by conducting what’s called a structured expert judgement study. This approach allowed the authors to look at a range of possible outcomes based on the knowledge of those who know the topic best, Bamber said.

Here’s how it worked: Groups of experts gathered at meetings in the United States and in the United Kingdom where they were tested on their ability to make estimates pertaining to sea level rise. “We calibrated our experts on their ability to assess their own certainty or uncertainty in processes and behaviors that they know something about but not everything,” said Bamber.

Then they were presented with questions around sea level rise from ice loss in Antarctica and Greenland, resulting in the findings about potential sea level rise.

The structured expert judgement approach is a well-known method—it resembles a method used previously by the IPCC in assessing the urgency of climate crisis and led to the recognition that ratcheting up global ambition to keep global warming to 1.5°C could significantly decrease the impacts of climate change, as opposed to 2°C. 

“We’re still not at the level where we can make really confident predictions of what’s going to be happening over the next 50 or 100 years,” said Mark Serreze, the director of the National Snow and Ice Data Center, who was not a part of the PNAS study.

“The devil is in the details, and they’re very complex things in terms of the dynamics of the ice sheets. It’s a very tough nut to crack,” he said. “So—uncertainty. What do you do? Throw up your hands and say I don’t know? So you can bring in these other approaches like expert judgement.”

The findings of a structured expert judgment aren’t meant to replace research into sea level rise, the authors wrote in the paper, but should be considered “complementary insight.”