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Winter 2012 Vol. 12 Number 2

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Rising Seas Along
America's West Coast

Coastline south of Ecola State Park, Oregon, ©Digital Vision/Thinkstock

A rise in sea levels is one of the most visible results of a warming planet. Higher temperatures cause ocean water to expand and cause glaciers and ice sheets to melt, adding water to the oceans. Ocean levels began to rise during the late 19th or early 20th century and are expected to continue to climb at an even higher rate during this century. Although the global trend is upward, seas don't rise evenly everywhere. Shifts in a particular place are influenced by factors such as patterns in ocean circulation, climate patterns such as El Niņo, and the rise or fall of coastal land itself.

Concerned about sea-level rise along their 1,400 miles of coastline, the states of California, Oregon, and Washington, together with several federal agencies, asked the National Research Council to evaluate how much the ocean will rise along America's West Coast by 2030, 2050, and 2100.

The committee's report finds that the sea level for most of California's coast -- the area south of Cape Mendocino -- will rise slightly higher than the global average. Relative to the land, the sea is expected to rise about 1 meter over the next century. In northern California, Oregon, and Washington, however, the sea will rise less -- about 60 centimeters -- during the same period of time. There's even a chance these northern areas may see a slight drop in sea level during the next few decades, the report says. Although the water is rising, the land is rising along with it; the plate that forms the ocean floor is descending below the continental plate, pushing the coast upward.

However, extreme events such as a major earthquake could raise sea levels much higher than this estimate, and far more rapidly. An earthquake magnitude 8 or greater north of Cape Mendocino -- which occurs in this area every several hundred to 1,000 years -- could cause parts of the coast to descend immediately and the relative sea level to rise suddenly by a meter or more. Even gradual increases in sea level are expected to magnify the impact of coastal flooding and erosion from storm surges and high waves. Such events are costly, given that much development on the West Coast -- such as airports, freeways, and housing developments -- has been built only a few feet above the highest tides. San Francisco International Airport, for example, could flood if the sea level rises even 40 centimeters, a number that could be reached within several decades.
-- Sara Frueh & Jennifer Walsh

Sea-Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future . Committee on Sea Level Rise in California, Oregon, and Washington; Board on Earth Sciences and Resources and Ocean Studies Board; Division on Earth and Life Studies (2012, 201 pp.; ISBN 0-309- 25594-5; available from National Academies Press, tel. 1-800-624-6242; $54.00 plus $5.00 shipping for single copies).

The study was chaired by Robert Dalrymple, Willard and Lillian Hackerman Professor of Civil Engineering, Whiting School of Engineering, Johns Hopkins University. The study was funded by the states of California, Washington, and Oregon; National Oceanic and Atmospheric Administration; U.S. Geological Survey; and U.S. Army Corps of Engineers.

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Copyright 2012 by the National Academy of Sciences