Surging Seas

Description: Sea level rise, storms, and global warming’s threat to the U.S. coast
Date:  March 14, 2012

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Executive Summary

Global warming has raised sea level about eight inches since 1880, and the rate of rise is accelerating. Scientists expect 20 to 80 more inches this century, a lot depending upon how much more heat-trapping pollution humanity puts into the sky. This study makes mid-range projections of 1 to 8 inches by 2030, and 4 to 19 inches by 2050, depending upon location across the contiguous 48 states.

Rising seas dramatically increase the odds of damaging floods from storm surges. For more than two-thirds of the locations analyzed (and for 85% of sites outside the Gulf of Mexico), past and future global warming more than doubles the estimated odds of “century” or worse floods occurring within the next 18 years — meaning floods so high they would historically be expected just once per century. For more than half the locations analyzed, warming at least triples the odds of century-plus floods over the same period. And for two-thirds of the locations, sea level rise from warming has already more than doubled the odds of such a flood even this year.

These increases are likely to cause an enormous amount of damage. At three quarters of the 55 sites analyzed, century levels are higher than 4 feet above the high tide line. Yet across the country, nearly 5 million people live in 2.6 million homes at less than 4 feet above high tide. In 285 cities and towns, more than half the population lives on land below this line, potential victims of increasingly likely climate-induced coastal flooding. 3.7 million live less than 1 meter above the tide.

About half of this exposed population, and eight of the top 10 cities, are in the state of Florida. A preliminary independent analysis suggests about $30 billion in taxable property is vulnerable below the three-foot line in just three counties in southeast Florida, not including the county with the most homes at risk in the state and the nation, Miami-Dade. Small pockets or wide areas of vulnerability, however, exist in almost every other coastal state.

The population and homes exposed are just part of the story. Flooding to four feet would reach higher than a huge amount of dry land, covering some 3 million acres of roads, bridges, commercial buildings, military bases, agricultural lands, toxic waste dumps, schools, hospitals, and more. Coastal flooding made worse by global warming and rising seas promises to cause many billions of dollars of damage over the coming decades.

This report and its associated materials, based on two just-published peer-reviewed studies, is the first major national analysis of sea level rise in 20 years, and the first one ever to include:

• Estimates of land, population and housing at risk;
• Evaluations of every low-lying coastal town, city, county and state in the contiguous U.S.;
• Localized timelines of storm surge threats integrating local sea level rise projections; and
• A freely available interactive map and data to download online (see SurgingSeas.org).

Summaries of these findings at a state-by-state level are available in fact sheets and the original peer-reviewed studies are available as well. All findings reflect best estimates from the research; actual values may vary.

This report focuses on new research and analysis, not recommendations; but it is clear from the findings here that in order to avoid the worst impacts, the United States must work to slow sea level rise by reducing emissions of heat-trapping gases, and work to diminish the remaining danger by preparing for higher seas in coastal cities and counties everywhere.  SurgingSeas.org/responses/plans lists a selection of existing resources, plans and efforts to prepare, from local to national levels.