These US Cities Have Already Passed a Climate Change 'Tipping Point'

Rising sea levels are most often associated with human-driven climate change — and for good reason. As global temperatures increase, melting glaciers and ice caps are driving up the seas at a rate of about three millimeters, or 0.12 inches, per year.

But greenhouse gas warming isn't the only factor pushing the seas farther ashore. Sinking land and compressed river sediment can compound the problem, meaning that even with seas rising globally, the effects will be vastly different locally.

A new study from the National Oceanic and Atmospheric Administration (NOAA) that incorporated these local factors with global sea-level projections found that US cities on the East, West, and Gulf coasts are already experiencing increased minor flooding and are likely to experience at least 30 days of flooding each year by the middle of the century.

"Projecting into the future, it's important to recognize that as sea level rises it's not a uniform bathtub," William Sweet, a NOAA oceanographer and co-author of the study, told VICE News.

In the Mid-Atlantic, for example, land is slowly sinking below sea level as a remnant effect of the last ice age. This means the Chesapeake Bay region will experience greater sea-level rise than the Florida coast, Sweet said. Withdrawing groundwater for irrigation and tapping oil and gas reserves can also cause land to compact, as can settling sediments in river deltas.

Using data from NOAA tidal stations that had a consistent 50-year record, Sweet established a "tipping point" when minor, or "nuisance," flooding occurs on more than 30 days of the year. He then created projections accounting for land movement and other local factors to look at flooding risk if global sea levels rise by 0.5 meters, 0.6 meters, 0.8 meters, and 1.2 meters (1.6 feet, 2 feet, 2.6 feet, and 3.9 feet).

Wilmington, North Carolina; Annapolis, Maryland; and Washington, D.C. have already passed their tipping points. From 2006 through 2010, for example, Annapolis experienced an average of 34.4 nuisance days a year, compared to an average 2.8 such days from 1956 through 1960. Of the 23 other cities surveyed, 17 are expected to pass their tipping points by 2050.

Because Hurricane Andrew wiped out NOAA's Miami monitoring stations in 1992, interrupting the data record, the study did not include Miami — one of the world's most threatened coastal cities.

Sweet noted an increase in "sunny-day" flooding, when floods occur in the absence of a storm. This can happen as high tides, which cycle throughout the year, get higher, or when wind blowing over the water creates a small surge over already increased sea levels.

"Decades ago, the winds were still blowing, the tides were still high," Sweet told VICE News. "But the mean sea level was just lower, so the overall event just wasn't sufficient to cause a real problem and be noticed."

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Because sea-level rise is a complex and slow process, scientists often focus on time scales of centuries when trying to pin down just how much rise we're in for. A 2013 study that surveyed 90 sea-level experts found that if warming is kept to two degrees Celsius (about 3.6 degrees Fahrenheit), seas are likely to rise between 0.4 and 0.6 meters (1.6 and 2 feet) by 2100, reaching a meter (3.2 feet) by 2300. But those broad global ranges and long time scales aren't useful for planners and engineers trying to make sense of sea levels now.

"It drives you crazy that the scientists will say, 'Oh, we could expect between six inches and a foot of global sea-level rise,'" Andrew Holland, a senior fellow for energy and climate at the American Security Project, told VICE News. "But what does that mean? That doesn't mean six inches to a foot of sea-level rise everywhere around the world."

"One of the things that we need to do better is to make projections for more specific places," Andrew Kemp, a coastal scientist at Tufts University, told VICE News. "The global mean is an interesting and helpful thing for climate scientists, but very few places are going to truly experience the global mean. They're going to experience local change."

That variance in local change means different cities and states will need different approaches to dealing with flooding. In Florida, Sweet said, some communities are installing pump systems to alleviate stress on storm water drains, which can't flow down to the ocean if ocean levels are higher.

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In Norfolk, Virginia, which is home to the the largest naval base in the world and which the study predicted will reach its tipping point between 2021 and 2040, a pilot project based at Old Dominion University roped together a slew of partners including commanders from the Navy, Coast Guard, and Army Corps of Engineers, as well as representatives from local and national government and the private sector, to plan for and protect against permanent sea-level rise and increased flooding. By one Old Dominion projection, the road to the naval base will be flooded every high tide — two to three hours a day — within the next two and a half decades.

"The data sort of speaks for itself," Sweet told VICE News. "As the problems become more obvious and the data support people's observations of what they're seeing, I think they will make the decisions to adapt accordingly."

Follow Laura Dattaro on Twitter: @ldattaro

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