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 future emissions that it implies, that rises to more than seven feet.

How much more sea-level rise takes place, in their analysis, then 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?”

Timelapse imagery from NASA shows a massive iceberg breaking away from a glacier in Antarctica that could potentially pose a danger to ships. (Reuters)

Indeed, Antarctic scientists recently issued an “urgent” call for more study of West Antarctica to determine how rapidly the gigantic Thwaites glacier, which is rooted 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.

The world is moving toward cutting emissions, so this extreme scenario probably will not happen. But what is striking about the study is how much we have already committed to long-term sea-level rise. For instance, the research finds that current emissions and expected emissions from existing infrastructure lead to more than 600 cities or municipalities in that same situation — where land that is home to 50 percent of current population is underwater — and might flood the current location of the homes of more than 9 million people.

Climate Central has created an interactive that lets you see the different scenarios for different parts of the United States.

“If we don’t cut emissions,” Strauss said, “we’re talking about losing American land [that’s] home to more people than live in any state, except for California and Texas. Home to more people than the state of Florida and New York.”

Granted, the key question is how fast this would play out; sea-level rise in 50 years is a much bigger deal to people living today than sea-level rise in 1,500 years.

“Under all scenarios, Florida has the plurality or majority of committed cities with total population greater than 100,000,” the study said. It also finds that decisions made in this century will determine whether 14 U.S. cities with populations greater than 100,000 will also be locked in for at least half of their populated areas — such cities include Jacksonville, Fla.; Norfolk; and Sacramento.

In some ways, the most troubling thing about the analysis is how the wild card — West Antarctica — makes the best case scenario a lot closer to the worst case. For instance, if West Antarctica is melting and cannot be stopped, then current emissions alone go from implying 600 at least half-flooded municipalities to more than 1,100, and from 9 million people’s current places of residence flooded to nearly 20 million.

Nonetheless, sharply cutting emissions still has a very significant impact — it “can lead to the avoidance of commitment for nearly 900 US municipalities, and, more broadly, for land that is home to 15.8 million people in the baseline case, using central estimates, and for nearly 400 municipalities and land that is home for 6.6 million people assuming WAIS collapse,” the study said.

There are several gaps in the study — many of which the authors directly acknowledge. For instance, the research only calculates what the sea level will be in the future, and compares that with the current elevation of cities. But it does not take into account the fact that New Orleans is currently protected by a 26-foot-high sea wall, and that Louisiana is contemplating diversions of the Mississippi River that would create new wetlands that might keep pace, at least to some extent, with rising seas.

In other words, there are a variety of admittedly expensive ways in which some cities may be able to “adapt” and partly ward off rising seas. And they will certainly be easier to adapt to if the oceans rise more slowly than more rapidly.

Also, the populations living in the places committed to rising seas may be considerably larger — or considerably smaller — by the time those rising seas actually arrive.

Finally, the fact remains that the most important question of all about sea-level rise — how fast it will happen — is still inadequately understood.

Still, the research underscores that ultimately, rising seas may be the most consequential of all implications of climate change. And as the authors note, these results are only for the United States — but rising seas will not distinguish between nations. All will be affected, albeit with relatively slight differences depending upon gravitational and other factors.

“Historic carbon emissions appear already to have put in motion long-term [sea-level rise] that will endanger the continuity and legacy of hundreds more municipalities,” the researchers conclude, “and so long as emissions continue, the tally will continually increase.”

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