Scientists report that many cities near the coasts of the US should prepare for daily flooding at high tide by mid-century because of rising sea levels

by Tim Radford, Climate News Network, January 5, 2015

LONDON − Oceanographers have just identified the US coastal regions likely to experience 30 days or more of “nuisance” flooding every year. And the answer is that most of the American coast will experience high waters that are 30-60 cm above local high tides, at least 30 times a year.

Nuisance flooding means just that − somewhere between an inconvenience and modest damage. But climate change, and its attendant sea-level rise, will make them much more frequent, and possibly more damaging.

William Sweet and Joseph Park, scientists at the National Oceanic and Atmospheric Administration (NOAA), report in the journal Earth’s Future that sea level rise has accelerated from 1.7 mm a year in the last century to 3.2 mm a year in the last two decades, and flooding events that were once extreme could become the mean.

The oceanographers wanted to establish what they call “regional tipping points” – places where extra high waters would wash across streets and promenades normally above water and start to do so frequently.

Detailed picture

New York was inundated when Superstorm Sandy hit the city in 2012, and studies have repeatedly warned that coastal inundations will cost communities colossal sums each year by 2050, and even more by 2100. Nor is the US alone in this respect. There have been ominous calculations for the UK as well.

The NOAA scientists add detail to the big picture. They started with the projections for global sea level rise delivered by the Intergovernmental Panel on Climate Change and then included the more local factors such as land subsidence or settlement, and cyclic weather patterns that exacerbate the tidal highs. Such floods have already increased, and are now 5-10 times more likely than 50 years ago.

They looked at all those tidal stations with a continuous 50-year record of measurement. This does not include the city of Miami, where the tide stations were destroyed in 1992 by Hurricane Andrew.

Coastal changes

And they warn that Boston, New York City, Philadelphia, Baltimore and many other places along the Atlantic Coast, Galveston and Port Isabel in the Gulf of Mexico, and San Francisco Bay and San Diego along the Pacific Coast will all see a lot more seawater in city streets.

“Coastal communities are beginning to experience sunny-day nuisance flooding, much more so than in decades past,” said Dr Sweet. “This is due to sea level rise.

“Unfortunately, once impacts are noticed, they will become commonplace rather quickly. We find that in 30 to 40 years, even modest projections of global sea level rise – 1.5 feet by the year 2100 – will increase instances of daily high tide flooding to a point requiring an active and potentially costly response.

“And by the end of the century, our projections show that there will be near-daily nuisance flooding in most of the locations that we reviewed.”