James Hansen, the former NASA scientist whose congressional testimony put global warming on the world’s agenda a quarter century ago, is now warning that humanity could confront “sea level rise of several meters” before the end of the century unless greenhouse gas emissions are slashed much faster than currently contemplated. 

This roughly ten feet of sea level rise—well beyond previous estimates—would render coastal cities such as New York, London and Shanghai uninhabitable.  “Parts of [our coastal cities] would still be sticking above the water,” Hansen says, “but you couldn’t live there.”

This apocalyptic scenario illustrates why the goal of limiting temperature rise to 2 degrees Celsius is not the safe “guardrail” most politicians and media coverage imply it is, argue Hansen and 16 colleagues in a blockbuster study they are publishing this week in the peer-reviewed journal Atmospheric Physics and Chemistry. On the contrary, a 2 C future would be “highly dangerous.”

If Hansen is right—and he has been right, sooner, about the big issues in climate science longer than anyone—the implications are vast and profound. 

Physically, Hansen’s findings mean that the earth’s ice is melting and its seas are rising much faster than expected. Other scientists have offered less extreme findings; the United Nations Intergovernmental Panel on Climate Change (IPCC) has projected closer to three feet of sea level rise by the end of the century, an amount experts say will be difficult enough to cope with. (Three feet of sea level rise would put runways of all three New York City area airports underwater unless protective barriers were erected. The same holds for airports in the San Francisco Bay Area. 

Worldwide, approximately $3 trillion worth infrastructure vital to civilization such as water treatment plants, power stations, and highways are located at or below three feet of sea level, according to the Stern Review, a comprehensive analysis published by the British government. 

Hansen’s track record commands respect. From the time the soft-spoken Iowan told the U.S. Senate in 1988 that man-made global warming was no longer a theory but had in fact begun and threatened unparalleled disaster, he has consistently been ahead of the scientific curve. 

Hansen has long suspected that computer models underestimated how sensitive earth’s ice sheets were to rising temperatures. Indeed, the IPCC excluded ice sheet melt altogether from its calculations of sea level rise. For their study, Hansen and his colleagues combined ancient paleo-climate data with new satellite readings and an improved model of the climate system to demonstrate that ice sheets can melt at a “non-linear” rate: rather than an incremental melting as the earth’s poles inexorably warm, ice sheets might melt at exponential rates, shedding dangerous amounts of mass in a matter of decades, not millennia. In fact, current observations indicate that some ice sheets already are melting this rapidly.

“Prior to this paper I suspected that to be the case,” Hansen told The Daily Beast. “Now we have evidence to make that statement based on much more than suspicion.”