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Floods may increase 300-fold on Atlantic, Gulf Coasts

Sea-level rise along the Atlantic and Gulf coasts combined with more frequent and violent storms could increase flooding from the Northeast to Texas by several-hundredfold, according to a new study out Monday.

Over the past century, the East Coast has seen sea-level rise far above the 8-inch global average - up to a foot in much of the Mid-Atlantic and Northeast, including New York City. It is expected to increase as much as four feet by 2100, mostly due to the melting ice sheets as well as the expansion of the seawater as the oceans warm.

At the same time, several studies have suggest the North Atlantic could see more intense storms, since warmer warmers contain more energy.

"When you look at hazards separately, it's bad enough, but when you consider the joint effects of two hazards together, you can get some surprises," said Radley Horton, a climate scientist at Columbia University's Earth Institute and study coauthor. "Sometimes, 1 plus 1 can equal 3."

The two factors have in the past mostly been considered independently. But this study, published in Nature Climate Change Monday, scientists from Columbia, Princeton, Rutgers and several other institutions for the first time looked at them together.

The authors analyzed 15 climate models at five locations: Atlantic City, N.J.; Charleston, S.C.; Key West, Fla.; Pensacola, Fla.; and Galveston, Tex. Among the things they looked at was the probability that the two factors could act together to lethal effect. Five models simulated both high local sea-level rises and increases in the strongest storms.

The authors also looked at the extent of the flooding if world leaders, meeting in Paris later this year, take action to cut emissions to stem global warming or if nothing is done. Frustratingly, the flooding is projected to increase even if the world works to cut emissions and keep temperatures from rising 2 degrees Celsius (3.6 degrees Fahrenheit) over what they were in preindustrial times.

The reduced-emissions calculations suggest a four- to 75-fold increase in the flood index - that is, the combined heights and durations of expected floods - across the five locations. If no action is taken, the flood index might go up 35 to 350 times.

"It's an aggregate number over a big area - not a specific prediction for any one place," said lead author Christopher Little of Atmospheric and Environmental Research, a company that performs weather and climate research, and related risk assessments. "But these projections help lay the groundwork for more specific research that will be valuable for adapting to climate change."

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