News•July 29, 2013

Sea Level Rise 'Locking In' Quickly, Cities Threatened

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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.

We have two sea levels: the sea level of today, and the far higher sea level that is already being locked in for some distant tomorrow.

In a new paper published Monday in the Proceedings of the National Academy of Sciences (PNAS), 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.

A quick tour of the interactive map on this page shows that Florida is by far the most vulnerable state under any emissions scenario. Louisiana, New Jersey and North Carolina would also face enormous difficulties. If we call a place “threatened” when at least half of today’s population lives below the locked-in future high tide line, then by 2100, under the current emissions trend, more than 100 cities and towns would be threatened in each of these states.

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.”

But each fraction is arbitrary, and true critical levels will depend on geography and economics. Some places when partly or wholly below sea level may be defensible, at least to some degree — like New Orleans with its network of levees and flood barriers. Other places may be indefensible with well under 25 percent of exposure. For example, South Florida will be very difficult to protect, due in large part to the porous bedrock underlying it.

Overall, this analysis does not account for potential engineering solutions; it is based simply on elevations.

The low-emissions scenario could reduce impacts substantially — by almost threefold — but is profoundly ambitious compared to current trends and policy discussions. It includes a halt to global emissions growth by 2020, followed by rapid global emissions reductions, and a massive program to remove carbon from the atmosphere, resulting in net negative emissions — atmospheric clean-up — by late in the century.

The big question hanging over this analysis is how quickly sea levels will rise to the committed levels. Neither Levermann and colleagues' analysis, nor my new paper, address this question.

In a loose analogy, it is much easier to know that a pile of ice in a warm room will melt, than to know exactly how fast it will melt.

Levermann and company do put an upper limit of 2,000 years on how long it will take the sea level commitments described here to play out. Recent research indicates that warming from carbon emitted today is essentially irreversible on the relevant timescales (in the absence of its massive-scale engineered removal from the atmosphere), and will endure for hundreds or thousands of years, driving this long run unstoppable sea level rise.

On the other hand, our sea level rise commitment may be realized well before two millennia from now. The average rate of global sea level rise during the 20th century was about half a foot per century. The current rate is 1 foot, or twice that. And middle-of-the-road projections point to rates in the vicinity of 5 feet per century by 2100.

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