A new study warning that rapidly melting ice sheets in Greenland and Antarctica may push global sea level rise beyond 7.8 feet by 2100 has elicited alarm about the "profound consequences for humanity" and cast doubt on past projections.

"If we see something like that in the next 80 years we are looking at social breakdown on scales that are pretty unimaginable."

—Jonathan Bamber, lead author

"If we see something like that in the next 80 years we are looking at social breakdown on scales that are pretty unimaginable," lead author Jonathan Bamber, a professor of physical geography at the U.K.'s University of Bristol, told New Scientist.

The structured expert judgement study, published Monday by the peer-reviewed journal Proceedings of the National Academy of Sciences (PNAS), brought together 22 experts.

Under the experts' worst case scenario projection—based on global temperatures increasing to 5°C (9°F) above pre-industrial levels by 2100—rising seas would leave about 1.79 million square kilometers (691,123 square miles) underwater and displace up to 187 million people.

Detailing what this would look like on a global scale, BBC News reported:

Much of the land losses would be in important food growing areas such as the delta of the Nile. Large swathes of Bangladesh would be very difficult for people to continue to live in. Major global cities, including London, New York and Shanghai would be under threat. "To put this into perspective, the Syrian refugee crisis resulted in about a million refugees coming into Europe," said [Bamber]. "That is about 200 times smaller than the number of people who would be displaced in a two meter sea level rise."

The PNAS study's upper level projection is more than double the highest estimate from the Fifth Assessment Report of the Intergovernmental Panel on Climate Change's (IPCC). That 2013 U.N. analysis, which experts have long criticized as too conservative, concluded that under business-as-usual greenhouse gas emissions, global sea level could rise up to 3.22 feet by the end of this century.

"The IPCC report in 2013 only considered what is 'likely' to happen, which in scientific terms means they looked at 17-83 percent of the range of possibilities," explained BBC News. "This new study looks at a broader range of results, covering 5-95 percent of the estimates."

and the scientists think there's a 1/6 chance sea level rise by 2100 under business as usual will be considerably worse -- instead of ~1 meter sea level rise (3+ feet), potentially 2-3 meters (6-10 feet). pic.twitter.com/eZOvl39YSL SCROLL TO CONTINUE WITH CONTENT Never Miss a Beat. Get our best delivered to your inbox.





— Brad Johnson (@climatebrad) May 20, 2019

The experts who conducted the PNAS study found that sea level will most likely rise 3.6 feet by 2100, but in the final report the authors emphasized that considering the highest possible projection "is crucial for robust decision-making."

"The experts we assessed found a pretty significant chance of 'black swan' outcomes leading to extremely high levels of rise."

—Robert E. Kopp, co-author

"The central numbers in this study are about a foot higher than the IPCC's estimates," study co-author Robert E. Kopp said in a statement. "But the experts we assessed found a pretty significant chance of 'black swan' outcomes leading to extremely high levels of rise."

Kopp is a professor at Rutgers University–New Brunswick and a lead author of the IPCC's Sixth Assessment Report, set for publication in 2021. The IPCC released a special report last October that warned the international community had only about 12 years left to prevent climate catastrophe, but that report did not include updated projects for sea level rise.

Climate campaigners responded to the new study with both concern and calls to action:

Today it has been reported that global sea levels are due to rise by double what was originally predicted by 2100. This is why we have no other choice but to #YouthStrike4Climate. We are fighting for our lives.#FridaysForFuture @UKSCN1 https://t.co/eMJBQypxdu — YouthStrike4Climate (@Strike4Youth) May 21, 2019 The planetary slogan for the 21st century should be "Faster Than Expected."https://t.co/MKeTlQnrw3 — Bill McKibben (@billmckibben) May 21, 2019

"What we decide to do collectively as a species politically, globally, over the next decade," lead author Bamber told CNN, "is going to determine the future of the next generations in terms of the habitability of the planet and what sort of environment they live in."