Hundreds of American cities face sea level threat

Ben Strauss, Climate Central - Measurements tell us that global average sea level is currently rising by about 1 inch per decade. But in an invisible shadow process, our long-term sea level rise commitment or "lock-in" — the sea level rise we don’t see now, but which carbon emissions and warming have locked in for later years — is growing 10 times faster, and this growth rate is accelerating.

An international team of scientists led by Anders Levermann recently published a study that found for every degree Fahrenheit of global warming due to carbon pollution, global average sea level will rise by about 4.2 feet in the long run. When multiplied by the current rate of carbon emissions, and the best estimate of global temperature sensitivity to pollution, this translates to a long-term sea level rise commitment that is now growing at about 1 foot per decade.

In a new paper published in the Proceedings of the National Academy of Sciences , I analyze the growth of the locked-in amount of sea level rise and other implications of Levermann and colleagues’ work. This article and its interactive map are based on this new PNAS paper, and they include extended results.

To begin with, it appears that the amount of carbon pollution to date has already locked in more than 4 feet of sea level rise past today’s levels. That is enough, at high tide, to submerge more than half of today’s population in 316 coastal cities and towns (home to 3.6 million) in the lower 48 states.

By the end of this century, if global climate emissions continue to increase, that may lock in 23 feet of sea level rise, and threaten 1,429 municipalities that would be mostly submerged at high tide. Those cities have a total population of 18 million. But under a very low emissions scenario, our sea level rise commitment might be limited to about 7.5 feet, which would threaten 555 coastal municipalities: some 900 fewer communities than in the higher-emissions scenario.

To develop such figures, I combined my sea level debt findings with analysis from Climate Central’s Surging Seas project, which is a national assessment and mapping of coastal vulnerability in the U.S. based primarily on elevation and census data.

Nationally, the largest threatened cities at this level are Miami, Virginia Beach, Va., Sacramento, Calif., and Jacksonville, Fla.

If we choose 25 percent instead of 50 percent as the threat threshold, the lists all increase, and would include major cities like Boston, Long Beach, Calif., and New York City. The lists shrink if we choose 100 percent as the threshold for calling a community “threatened.”

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