About 3.7 million Americans live within a few feet of high tide and risk being hit by more frequent coastal flooding in coming decades because of the sea level rise caused by global warming, according to new research.
If the pace of the rise accelerates as much as expected, researchers found, coastal flooding at levels that were once exceedingly rare could become an every-few-years occurrence by the middle of this century.
By far the most vulnerable state is Florida, the new analysis found, with roughly half of the nation’s at-risk population living near the coast on the porous, low-lying limestone shelf that constitutes much of that state. But Louisiana, California, New York and New Jersey are also particularly vulnerable, researchers found, and virtually the entire American coastline is at some degree of risk.
“Sea level rise is like an invisible tsunami, building force while we do almost nothing,” said Benjamin H. Strauss, an author, with other scientists, of two new papers outlining the research. “We have a closing window of time to prevent the worst by preparing for higher seas.”
The project on sea level rise led by Dr. Strauss for the nonprofit organization Climate Central appears to be the most elaborate effort in decades to estimate the proportion of the national population at risk from the rising sea. The papers are scheduled for publication on Wednesday by the journal Environmental Research Letters. The work is based on the 2010 census and on improved estimates, compiled by federal agencies, of the land elevation near coastlines and of tidal levels throughout the country.
Climate Central, of Princeton, N.J., was started in 2008 with foundation money to conduct original climate research and also to inform the public about the work of other scientists. For the sea level project, financed entirely by foundations, the group is using the Internet to publish an extensive package of material that goes beyond the scientific papers, specifying risks by community. People can search by ZIP code to get some idea of their own exposure.
While some coastal governments have previously assessed their risk, most have not, and national-level analyses have also been rare. The new package of material may therefore give some communities and some citizens their first solid sense of the threat.
Dr. Strauss said he hoped this would spur fresh efforts to prepare for the ocean’s rise, and help make the public more aware of the risks society is running by pumping greenhouse gases into the air. Scientists say those gases are causing the planet to warm and its land ice to melt into the sea. The sea itself is absorbing most of the extra heat, which causes the water to expand and thus contributes to the rise.
The ocean has been rising slowly and relentlessly since the late 19th century, one of the hallmark indicators that the climate of the earth is changing. The average global rise has been about eight inches since 1880, but the local rise has been higher in some places where the land is also sinking, as in Louisiana and the Chesapeake Bay region.
The rise appears to have accelerated lately, to a rate of about a foot per century, and many scientists expect a further acceleration as the warming of the planet continues. One estimate that communities are starting to use for planning purposes suggests the ocean could rise a foot over the next 40 years, though that calculation is not universally accepted among climate scientists.
The handful of climate researchers who question the scientific consensus about global warming do not deny that the ocean is rising. But they often assert that the rise is a result of natural climate variability, they dispute that the pace is likely to accelerate, and they say that society will be able to adjust to a continuing slow rise.
Myron Ebell, a climate change skeptic at the Competitive Enterprise Institute, a Washington research group, said that “as a society, we could waste a fair amount of money on preparing for sea level rise if we put our faith in models that have no forecasting ability.”
Experts say a few inches of sea level rise can translate to a large incursion by the ocean onto shallow coastlines. Sea level rise has already cost governments and private landowners billions of dollars as they have pumped sand onto eroding beaches and repaired the damage from storm surges.
Insurance companies got out of the business of writing flood insurance decades ago, so much of the risk from sea level rise is expected to fall on the financially troubled National Flood Insurance Program, set up by Congress, or on state insurance pools.
Federal taxpayers also heavily subsidize coastal development when the government pays to rebuild infrastructure destroyed in storm surges and picks up much of the bill for private losses not covered by insurance.
For decades, coastal scientists have argued that these policies are foolhardy, and that the nation must begin planning an orderly retreat from large portions of its coasts, but few politicians have been willing to embrace that message or to warn the public of the rising risks.
Organizations like Mr. Ebell’s, even as they express skepticism about climate science, have sided with the coastal researchers on one issue. They argue that Congress should stop subsidizing coastal development, regarding it as a waste of taxpayers’ money regardless of what the ocean might do in the future.
“If people want to build an expensive beach house on the Florida or Carolina coast, they should take their own risk and pay for their own insurance,” Mr. Ebell said.
The new research calculates the size of the population living within one meter, or 3.3 feet, of the mean high tide level, as estimated in a new tidal data set from the National Oceanic and Atmospheric Administration. In the lower 48 states, that zone contains 3.7 million people today, the papers estimate, a figure exceeding 1 percent of the nation’s population.
Under current coastal policies, the population and the value of property at risk in that zone are expected to continue rising.
The land below the 3.3-foot line is expected to be permanently inundated someday, possibly as early as 2100, except in places where extensive fortifications are built to hold back the sea. One of the new papers calculates that long before inundation occurs, life will become more difficult in the low-lying zone because the rising sea will make big storm surges more likely.
Only in a handful of places have modest steps been taken to prepare. New York City is one: Pumps at some sewage stations have been raised to higher elevations, and the city government has undertaken extensive planning. But the city — including substantial sections of Brooklyn, Queens and Staten Island — remains vulnerable, as do large parts of Long Island, Connecticut and New Jersey.