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

Millions of Americans live on land destined to be reclaimed by rising sea levels, and that number rises dramatically if carbon dioxide emissions continue unchecked or if West Antarctica’s ice sheet is as unstable as recent studies suggest, according to a new report.

“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, New Jersey. 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 turns on a critical number: For every 1 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,” says Stefan Rahmstorf of the Potsdam Institute for Climate Impacts Research, 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 2.3 meters estimate plays out over a 2,000-year window, 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 likely already committed to 1.6 meters of very long term sea level rise - more than 5 feet. And if you take into account existing, carbon-spewing infrastructure and the added emissions that it implies in the future, that rises to more than 7 feet.

How much more sea level rise takes place, in their analysis, depends upon how much more carbon we emit and – critically – whether or not 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 says: “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, in order to determine just how rapidly the gigantic Thwaites glacier, which is perched 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 don’t stop at calculating the current sea level rise commitment – they then translate different sea level scenarios for specific places in the U.S. (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 very 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 might find the areas where half of their residents live inundated.

The world is moving toward cutting emissions – so this extreme sea level scenario probably won’t happen. But what’s striking about the study is how much very long-term sea level rise we’ve already committed to. For instance, the research finds that current 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 under water - and might flood the homes of more than 9 million people.

“If we don’t cut emissions,” says Strauss, “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 reports. It also finds that decisions made in this century will determine whether or not 14 U.S. cities with populations over 100,000 will also be locked in for at least half of their populated areas - such cities include Jacksonville, 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 can’t be stopped, then current emissions alone go from implying 600 at least half flooded municipalities to over 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,” says the study.

There are several gaps in the study worth noting – 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, say, New Orleans is currently protected by a 26-foot-high sea wall, and that the state of 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 if rise is more rapid.

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.

“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, and so long as emissions continue, the tally will continually increase,” the researchers conclude.

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