If you ever wondered what the worst-case scenario for climate change would look like, a set of researchers shared your curiosity. Their answer, which they published Friday, is scary: sea levels nearly 200 feet higher than today—enough to submerge an 18-story building—putting some of the world's largest cities under water.
That's what's in store if all available fossil fuel reserves are burned, releasing about 10,000 gigatons of carbon and almost completely melting the Antarctic ice sheet, according to the research published in the peer-reviewed journal Science Advances.
"Global sea level rise of that magnitude is unprecedented in human civilization," said Ricarda Winkelmann, a climate scientist at the Potsdam Institute for Climate Impact Research in Germany and the lead author of the study. "Dozens of major coastal cities would be threatened, including New York, Tokyo, and Shanghai."
Winkelmann and colleagues found that seas could rise nearly 10 feet each century in the first 1,000 years under this scenario, and would continue to rise for thousands of years after that. Some scientists reviewing the study, though, said researchers may have even underestimated the speed of ice melt and sea level rise.
"We usually think about our generation, maybe the next generation," Winkelmann said. "It is hard for people to think of Earth a thousand years in the future. But what we are doing right now, the CO2 we are emitting into the atmosphere, could change the face of our planet a thousand years from now."
Antarctica has not been ice-free since approximately 100 million years ago, when dinosaurs roamed the earth, Winkelmann said. Until recently, scientists have focused mostly on how the Greenland ice sheet, which is one-eighth the size of its southern neighbor and therefore more susceptible to rapid thawing, will respond to global warming. But large sections of Antarctica once considered stable have begun melting at an alarming rate in the past few years.
Scientists estimate that the world has already emitted two-thirds of the greenhouse gases that can be released before global warming exceeds 2 degrees Celsius since the beginning of the Industrial Age. That's the threshold after which climate impacts are expected to be catastrophic, and is the target for international climate treaty negotiations in Paris this December.
But with much of the world declining to cut emissions as much or as quickly as scientists say is needed, Winkelmann and colleagues at the Carnegie Institution for Science and the University of California modeled what would happen to the Antarctic ice sheet if nations were to use up all the oil and gas available before completing a transition to renewable energy sources such as wind and solar.
They found that if the world succeeds in limiting emissions to an additional 600 gigatons of carbon, most likely holding warming to 2 degrees, much of Antarctica would remain intact and its contribution to global sea level rise would be just 6 and a half feet.
Burn all 10,000 gigatons of remaining carbon and the outcome is "terribly scary," said Hal Wanless, a climate scientist at the University of Miami. "The results of this study are sobering."
But Wanless said the model Winkelmann and her colleagues used probably underestimated how quickly Antarctica's ice will melt and seas will rise.
"Ice feels change early and it feels it quickly," Wanless said. "When we went from the last ice age to today, there were serious rapid pulses of rise. That's what we should expect now, several feet in a short period of time. Not the millennial time scale shown in the study. There is no way we aren't going to have a major rise this century."
Michael Mann, a climate scientist at Penn State, also said the modeling results seemed too conservative, particularly in the immediate decades and centuries to come.
"The authors show a maximum sea level rise from Antarctic melt of only a small fraction of a meter per century for a 2C warming target," Mann said. "But there is considerable empirical evidence that we are likely already now committed to at least three to four meters of sea level rise from West Antarctic ice loss, due to warming already in the pipeline and the destabilization of the ice shelves."
In other words, there might a worse worst-case scenario than even this study predicted.