Based on current emissions and global warming trends, the US already has “locked in” sea level increases of at least four feet above current levels before the next century, according to a Climate Central analysis.

That’s enough to submerge more than half the population in 316 US coastal cities and towns when high tide rolls in. Those communities currently are home to about 3.6 million people.

“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,” writes Ben Strauss, vice president for Climate Impacts and Director of the Program on Sea Level Rise at Climate Central. “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 high-emissions scenario.”

Right now, the water is encroaching on coastal developments in the US by about one inch per decade, but Climate Central anticipates increases of closer to one foot per decade in the near-term future.In its analysis, Climate Central considers a place to be “threatened” if at least half of the current population lives below the sea levels expected at high tide by 2100.

Florida is particularly vulnerable, but Louisiana, New Jersey and North Carolina also are home to plenty of threatened communities under that definition.

If current carbon pollution trends continue, Climate Central suggests that more than 100 cities and towns will be threatened in each of these states.

Nationally, the biggest threatened cities include Miami, Virginia Beach, Va.; Sacramento, Calif.; and Jacksonville, Fla.

The graphic below shows the areas that would be flooded at high tide in 2100, under the scenario described above. The threatened areas are represented in blue:

 

If you use a lower threshold for giving communities a  threatened stats –- say, if 25% of the current population lives below the projected high-tide sea levels for 2100 — you can add major cities including Boston, Long Beach, Calif., and New York City to the list.

The analysis doesn’t account for any solutions that cities may be putting into place for protection, such as the network of levees and flood barriers in New Orleans or a series of measures that have been proposed for New York City. It does suggest that areas such as South Florida will be especially difficult to protect, because of its geological makeup.

The impact would be reduced significantly under a low-emissions scenario that calls for a halt to global emissions growth by 2020, followed by rapid reductions and clean-up actions. But that is extremely unlikely given the current political climate.

The average rate of global sea level increases was about one half-foot per century during the 20th century, just half the current rate.

Middle-of-the-road projections suggest average per-century increase of about 5 feet by 2100, notes Climate Central.

“Such rates, if sustained, would realize the highest levels of sea level rise contemplated here in hundreds, not thousands of year — fast enough to apply continual pressure, as well as threaten the heritage, and very existence, of coastal communities everywhere,” writes Strauss.

The Climate Central analysis was published in the Proceedings of the National Academy of Sciences. Read the complete report:

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