This is how rising seas will reshape the face of the United States

Buildings near the ocean in North Miami, Fla., a state with a high risk of flooding as sea levels rise, according to a recent report. (Joe Raedle/Getty Images) Enlarge

In a new study, a team of scientists who specialize in studying rising seas bring the implications of their research right to the U.S.’s doorstep — calculating just how many American cities and municipalities are at risk of being flooded in the future, as well as how many may already be committed to that fate.

The striking result is that millions of Americans may already live on land destined to be someday — albeit perhaps in a very distant future — reclaimed by the sea. But the number for whom this is true will rise dramatically if carbon dioxide emissions continue unchecked — or, if recent concerns about the destabilization of the ice sheet of West Antarctica turn out to be well founded.

“Future emissions will determine which areas we can continue to occupy or may have to abandon,” note the report’s researchers, led by Benjamin Strauss of Climate Central in Princeton, N.J. The work appeared Monday in the Proceedings of the National Academy of Sciences and was co-authored by Scott Kulp of Climate Central and Anders Levermann of the Potsdam Institute for Climate Impact Research in Germany.

The analysis, “Carbon choices determine US cities committed to futures below sea level,” turns on a critical number: For every one degree Celsius of warming, the scientists estimate that we should expect 2.3 meters of long-term, eventual sea-level rise, playing out over millennia. That calculation is based on much research and represents the “state of the art,” said Stefan Rahmstorf of the Potsdam Institute, who was not involved in the study but has published previously with Levermann. “It is the best estimates we can make with the understanding that we have today about the processes leading to sea-level rise.”

The authors do not say how fast the sea-level rise could occur — the basic assumption is that the estimate of 2.3 meters plays out over 2,000 years, as the planet’s huge masses of ice slowly adjust to a change in its temperature. But much of the sea-level rise could happen a lot faster than that. Its precise timing is a key question for scientific inquiry right now.

Using this understanding of the link between warming and eventual ice melt, the authors estimate that with current carbon emissions, the world has probably already committed to 1.6 meters of long-term sea-level rise — more than five feet. And if you take into account existing carbon-spewing infrastructure and the added emissions that the study implies will exist in the future, that rises to more than seven feet.

How much more sea-level rise takes place, in their analysis, depends on how much more carbon we emit and — critically — whether a loss of the vast West Antarctic ice sheet is already inevitable, as some recent research has suggested.

Of the West Antarctic ice sheet, Strauss said: “Its contribution is going to be measured in inches per century, until it’s measured in feet per decade. And the question is, how long is the fuse and has it been lit yet?”

Indeed, Antarctic scientists recently issued an “urgent” call for more study of West Antarctica to determine how rapidly the gigantic Thwaites glacier, which is nestled deep below sea level and exposed to flows of warm water, could actually become destabilized.

“The potential magnitude of sea-level rise is staggering,” said Michael Oppenheimer, a Princeton University climate researcher who is on the board of Climate Central and says he offered comment on a version of the study. “In the short term, it risks serious disruption of life along the coast while in the long term, it could lead to obliteration of a large and priceless amount of our cultural heritage, worldwide.”

The authors do not stop at calculating the current sea-level-rise commitment; they translate different scenarios for specific places in the United States (regional sea-level changes will vary because of gravitational and other geophysical factors). And this is how they reach some central — and very striking — numbers.

Namely, in a high­-emissions scenario, which would represent “business as usual” if no actions are taken on climate change, the current locations of over 26 million Americans’ homes might be inundated, and more than 1,500 U.S. cities and municipalities could find the areas where half of the residents live inundated.


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