A study assessed how sea level rise at the hand of global warming will affect coastal populations in the United States. The analysis made by Climate Central,  a nonprofit news organization that analyzes and reports on climate science, found 20 million Americans’ homes might be flooded, and more than 1,500 U.S. cities and municipalities could have at least half of their residential area under water if the world emits under a ‘business-as-usual’ scenario. Unfortunately, there’s a lot of damage that’s already been done. Carbon emitted today will continue to warm the planet for hundreds of years and its effects on the climate are already locked in. Cities like Miami and New Orleans are ‘already lost in the long run,’ said Ben Strauss, vice president for sea level and climate impacts at Climate Central.

According to Strauss, “future emissions will determine which areas we can continue to occupy or may have to abandon.” Numerous studies have been conducted by climate scientists attempting to predict sea level rise, with findings ranging from as little as 1.6 meters, to six meters, to nine meters (when adding the ice lost from the West Antarctic Ice Sheet). The extent isn’t certain, but the fact that the rise will be substantial is well agreed upon. “Just think of a pile of ice in a warm room. You know it is going to melt, but it is harder to say how quickly,” said Strauss for AFP.

After looking at studies that link carbon emissions to rising sea levels up to the year 2100, ranging from 4.3 to 9.9 m, and based on “topographic and population data, local high tide lines, and regional long-term sea-level commitment for different carbon emission”, the researchers made a map where they chart the populations most at risk in the United States. They found “1,185–1,825 municipalities where land that is home to more than half of the current population would be affected, among them at least 21 cities exceeding 100,000 residents”.

According to the report, the world has already committed to  1.6 meters of long-term sea-level rise, and the damages felt extend far beyond the US – it’s a global matter with coastal areas under most serious threat, and islands more so. For the sake of simplicity and since it takes far too many resources than they can access, Strauss and colleagues settled for an US only risk assessment.

“In our analysis, a lot of cities have futures that depend on our carbon choices but some appear to be already lost,” Strauss said.

“And it is hard to imagine how we could defend Miami in the long run.”

Miami is one of these cities, under any scenario. An unabated emissions scenario sees Florida as the most vulnerable state, holding 40% or more of the population living in soon-to-be flooded areas. Next up in running order are  California, Louisiana and New York. New Orleans seems to be worse even than Miami.

image: climate.org

Of course, that’s not to say that all of these cities will be lost. Most of these municipalities, along with outside help, can secure resources that they can use to build huge sea walls. That’s if conditions allow. Miami for instance, sits on a porous limestone foundation. Not even walls and levees might work. Even now the City of Miami Beach is investing hundreds of millions of dollars for a series of pumps that have already been put to work during the annual King Tide to keep the streets dry. These pumps might prove useless only a couple decades from now.

Then there’s the fact that no dike can be built to stop seawater from penetrating far inland, contaminating drinking water supplies and lifting the water table to the point of inundation along a very flat and low landscape. When this happens, a strong rainstorm would be enough to flash flood urban centers.

While much of the damage is irreversible, some of it can avoided by lowering emissions. A total of 14 cities with more than 100,000 residents could avoid locking in this century if global average emissions are set back to 1950 levels by 2050.

“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.”

An interactive two-view map lets you see how the city you live in might become affected by sea level rise under different carbon emission scenarios.