The former climate chief for NASA warns that future sea level rises could leave many if not all of the world's coastal cities "dysfunctional."

James Hansen, in a paper set for publication in the journal Atmospheric Chemistry and Physics Discussion, says that even if the world manages to attain the goal of limiting global warming to 2 degrees Celsius, sea level rises could still be dangerous for humanity.

Evidence shows average temperatures just a single degree warmer than today caused extreme storms thousands of years ago and caused sea level rises as high as 16 to 30 feet, Hansen and his study co-authors say.

"This is the biggest threat the planet faces," Hansen says. "If we get sea level rise of several meters, all coastal cities become dysfunctional. The implications of this are just incalculable."

During the last major interglacial period 120,000 years ago, global temperatures were also 2 degrees warmer than our preindustrial climate levels and sea levels reached levels that could wreak havoc today, he says.

Among cities threatened by sea level rises would be major U.S. cities including Boston, New York, Seattle and Miami, the researchers say. They also note it would be difficult to protect low-lying areas such as Bangladesh, European lowlands, and large portions of the United States' eastern coast and northeast China plains against such large increases in the sea level.

They say they base their projection of sea level rise on an expected acceleration in the melting of ice sheet coverings in Antarctica and Greenland, driven by warming resulting in increased concentration of atmospheric greenhouse gases from ongoing burning of fossil fuels.

Water from the melting ice sheets flowing into the world's oceans could change their circulation patterns, which in turn could result in even faster melting and a rise in sea levels greater than has been put forward in previous forecasts, the study authors say.

"We conclude that continued high emissions will make multimeter sea level rise practically unavoidable and likely to occur this century," they wrote. "Social disruption and economic consequences of such large sea level rise could be devastating."

James Hansen headed NASA's Goddard Institute for Space Studies until 2013 and is now with Columbia University's Department of Earth and Environmental Sciences.

The social and economic cost of the world's coastal cities becoming dysfunctional as a result of sea level rise would be devastating, he says. It is a possible future if we don't manage to mitigate the effects of climate change, he warns.

"I think that the major implication of that will be that we hand young people a climate system where it's not possible to avoid a large sea level rise," he says.

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