American Cities and the Rising Sea

Rudee Heights, Virginia Beach, pictured above, is at major risk when it comes to sea level rise. More than 95% of the area sits below 6 meters of elevation. Credit: Bruce A Stockwell/flickr

Sea level rise gives climate change an address.

That’s why I think it’s a crucial topic for really connecting with people about the risks posed by warming. Information registers better when it’s local. Scientists can project that fires, storms, heat waves and droughts will increase globally and regionally, but no other impact can be pinpointed quite like sea level rise.

And that’s why I approached Jeremy Weiss and Jonathan Overpeck, scientists at the University of Arizona, in autumn 2009. They had already developed a detailed map of low-lying coastal areas in the contiguous U.S. I proposed intersecting their map with the Census boundaries of towns and cities. Our initial results will be published this week in the scientific journal Climatic Change Letters. We believe these are the first estimates of vulnerability to sea level rise covering each and every major coastal city in the Lower 48 – the places where Americans live in our highest concentrations. (Click here for a slide show of the 20 biggest cities in the U.S. to be affected by sea level rise.)

For each city, we reported the percentage of its land under 1-6 meters in elevation (about 3-20 feet). We included only areas also connected to the sea by equal or lower areas – not isolated depressions. We did not factor in complications like erosion or building defensive levees.

What did we find? For coastal American cities with populations above 50,000 people, about nine percent of the land lies below one meter in elevation – and scientists expect about one meter or more of sea level rise this century. About 36 percent of the land lies below six meters – and scientists expect enough global warming this century to potentially commit us to a six-meter sea level rise in the longer run. (The great ice sheets of Greenland and Antarctica take time to respond to warming temperatures.)

Not surprisingly, places like New Orleans and Miami look highly vulnerable. But we also uncovered less familiar pockets of risk, from Virginia Beach to the New Jersey Shore, from Tampa Bay to the San Francisco Bay Area.

Further, a key factor tilts our evaluation toward underestimating the area susceptible to rising seas. It’s this: the average high tide line differs from place to place, and is above zero elevation for most of the country. In other words, a spot one meter “high” is not necessarily one meter above the water – it could be much less, or even below the local high tide line. For example, the average high tide at the Battery, at the southern tip of Manhattan, today reaches an elevation near 0.9 meters; and Seattle, WA high tides can reach about 2.5 meters. At these locations, calculating the area beneath one meter of elevation, as our study does, clearly does not capture the full area susceptible to one meter of sea level rise.

Climate Central is leading research on a follow-up paper that takes today’s local high tide levels into account. We’re also developing a detailed interactive map and database with information on every coastal town or city, small or large, in the lower 48 states, plus short- and long-term timelines of risk from rising waters. As we publish additional studies this summer and beyond, look for a lot more news and tools from Climate Central on how your community may be affected by sea level rise.