Coastal communities around the world should gear their climate resilience planning for a “catastrophic” two metres (6.5 feet) of sea level rise by 2100, more than double the likely outcome most recently projected by the Intergovernmental Panel on Climate Change (IPCC), if nothing is done to reverse the greenhouse gas emissions driving the climate emergency, according to a survey of expert judgement published this week in the Proceedings of the National Academy of Sciences.

“The results were worse than we anticipated,” lead researcher Jonathan Bamber told The Independent. “Five degrees warming by 2100 is a pretty awful scenario,” he added, and “we’re closer to business as usual than what we would hope.”

Under that scenario, “parts of London, Los Angeles, New York, and Rio de Janeiro could be submerged, leaving 1.79 million square kilometres of land lost globally,” The Independent states.

The higher estimate is not guaranteed, co-author Michael Oppenheimer, an atmospheric scientist at Princeton University, told InsideClimate News. But “if you knew there was a one-third or even 10% chance a plane would crash, you wouldn’t get on it. It’s the same with sea level rise,” he said.

“Coastal decisions by and large require long lead times,” he added, “and it would be nice if we could wait for the science to clear up, but we can’t.”

While responding to the IPCC’s mid-range projections of ice loss from Greenland and the Antarctic will protect communities from the agency’s most likely future scenarios, the practice will leave them at serious risk if the eventual outcome is worse.

The study’s projection “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 5.0°C (9.0°F) above pre-industrial times,” InsideClimate explains. The resulting sea level rise “could inundate nearly 700,000 square miles—almost equal to the entire land mass of Indonesia,” taking out critical regions of food production and displacing as many as 187 million people.

By contrast, ICN notes that sea level rise would hit 0.6 metres (two feet) under a 1.5°C scenario, 0.7 metres (nearly 2.5 feet) at 2.0°C. Following the release of last year’s IPCC report on 1.5°C pathways, Oxfam Regional Director Raijeli Nicole noted that “every tenth of a degree of warming is a choice between life or death” when “climate change has set our planet on fire, millions are already feeling the impacts, and the IPCC just showed that things can get much worse.”

InsideClimate News summarizes the scientific advances that have occurred since 2014, when the IPCC reported there were still many “unknown unknowns” about the behaviour of ice sheets. “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,” writes reporter Sabrina Shankman. “That includes both positive feedbacks that could rapidly accelerate the loss of ice, and negative feedbacks, which could slow it down.”

The balance between those factors is still up for grabs, the latest study finds.

“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, director of the U.S. National Snow and Ice Data Center, who was not a part of the 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.”

The structured expert judgement study involved 22 ice sheet specialists estimating plausible ranges for future sea level rise. InsideClimate News says the approach is intended to complement, not replace research, but it’s a well-known technique that resembles other IPCC methods.