American Cities Face Growing Flood Risks

By Tim Radford / Climate News Network

Sea level rise—driven by global warming and climate change—will bring new flood risks to America’s coastal cities.

Paradoxically, those conurbations already at risk of catastrophic floods driven by hurricanes can expect a greater number of “moderate” floods. And those cities that have little or no history of severe flooding can expect a greater level of risk from historically unprecedented inundation, according to a new study in the journal Environmental Research Letters.

This is another step in what might be called prepare-for-the-future studies. Coastal flooding already costs cities on both east and west coasts an estimated $27bn a year. Researchers have been doing the arithmetic and so far forecast that – globally at least – sea level rise is going to cost $1 trillion by 2050, and $100 trillion by 2100.

Sea level rise is happening everywhere, as ice caps and glaciers melt and the seas rise in response to global warming driven by prodigal human combustion of fossil fuels, and researchers have advanced from general warning to the kind of detail that could help city and state planners prepare to cope with flood risks.

European ports and estuary cities are at risk with projections of sea level rise of half a metre, and US coastal cities could one day face almost daily challenges at high tide.

Some of these risks are simply the inevitable consequence of living by the sea: a kind of littoral home truth. Others are more serious. One study has warned that the inundation levels may in decades be such as to create a new class of American climate refugee.

Notoriously, the US president, Donald Trump, has in the past called climate change “a hoax” and withdrawn from the Paris Agreement in which the world’s nations agreed to work together to contain global warming. But the research goes on.

In the latest study, scientists from two great US universities began to look at the fine detail of local risk. They calculated the “amplification factor” of hazard as the high tide mark: they assumed that the frequency of storms would remain unchanged and then they factored in projections of sea level rise.

And then they figured the effect of this rise on the frequency of floods that historically tend to happen once in a century, in places like Charleston – in the path of seasonal hurricanes – and Seattle, where catastrophic floods are rare.

They found that by 2050 the moderate floods – the kind that tend to happen once every 10 years – would recur 173 times more often in Charleston but only 36 times more often in Seattle.

The really bad invasions of the sea, the kind that happen once every 500 years, would sweep over Charleston six times as often. Seattle would, on the other hand, see what had once been very rare events 273 times more often.

That is, of course, because even a modest rise in sea level makes a dangerous storm surge much more dangerous. “For example, to produce a six-foot flood, if the ocean is a foot higher, you only need as much storm surge as you would have previously needed to produce a five-foot flood,” said Robert Kopp, professor of earth and planetary science at Rutgers university, and one of the authors.

And his research colleague Michael Oppenheimer, professor of geosciences and international affairs at Princeton, said: “We hope that this study provides additional information that cities and municipalities can use to start planning the defence against climate change and sea-level rise.

“This is especially important as federal programmes for planning for climate adaptation are on the chopping block.”