From the Extreme to the Mean

Published: Friday, 03 April 2015 14:22

By 2050, most of the US coast can expect to see 30 or more days a year of floods up to two feet above high tide levels, says a new NOAA study.

The study, ‘From the Extreme to the Mean: Acceleration and Tipping Points for Coastal Inundation due to Sea Level Rise’, has been published in the American Geophysical Union’s online peer-reviewed journal Earth’s Future.

NOAA scientists Sweet and Joseph Park established a frequency-based benchmark for ‘tipping points,’: when so-called nuisance flooding, defined by NOAA’s National Weather Service as between one to two feet above local high tide, occurs more than 30 or more times a year.

Based on that standard, the NOAA team found that these tipping points will be met or exceeded by 2050 at most of the US coastal areas studied, regardless of sea level rise likely to occur this century. In their study, Sweet and Park used a 1.5 to 4 foot set of recent projections for global sea level rise by year 2100 similar to the rise projections of the Intergovernmental Panel for Climate Change, but also accounting for local factors such as the settlement of land, known as subsidence.

These regional tipping points will be surpassed in the coming decades in areas with more frequent storms, the report said. The tipping points will be also be exceeded in areas where local sea levels rise more than the global projection of one and half to four feet. This also includes coastal areas like Louisiana where subsidence, which is not a result of climate change, is causing land to sink below sea level.

NOAA tide gauges show the annual rate of daily floods reaching these levels has drastically increased – often accelerating – and they are now five to ten times more likely today than they were 50 years ago.

The NOAA team is projecting that Boston, New York City; Philadelphia; Baltimore; Washington, D.C.; Norfolk, Virginia; and Wilmington, North Carolina; all along the Mid-Atlantic coast, will soon make, or are already being forced to make, decisions on how to mitigate these nuisance floods earlier than planned.

In the Gulf, NOAA forecasts earlier than anticipated floods for Galveston Bay and Port Isabel, Texas. Along the Pacific coast the earlier impacts will be most visible in the San Diego/La Jolla and San Francisco Bay areas.

Mitigation decisions could range from retreating further inland to coastal fortification or to a combination of ‘green’ infrastructure using both natural resources such as dunes and wetland, along with ‘gray’ man-made infrastructure such as sea walls and redesigned storm water systems.