WASHINGTON, D.C. — The sea level off most of California is expected to rise about one meter during the next century, an amount slightly higher than projected for global sea levels, and will likely increase damage to the state’s coast from storm surges and high waves, says a new report from the National Research Council. Sea levels off Washington, Oregon, and northern California will likely rise less, about 60 centimeters over the same period of time. However, an earthquake magnitude 8 or larger in this region could cause sea level to rise suddenly by an additional meter or more.
Global sea level rose during the 20th century, and projections suggest it will rise at a higher rate during the 21st century. A warming climate causes sea level to rise primarily by warming the oceans — which causes the water to expand — and melting land ice, which transfers water to the ocean. However, sea-level rise is uneven and varies from place to place. Along the U.S. West Coast it depends on the global mean sea-level rise and regional factors, such as ocean and atmospheric circulation patterns, melting of modern and ancient ice sheets, and tectonic plate movements.
California Executive Order S-13-08 directed state agencies to plan for sea-level rise and coastal impacts and asked the Research Council to establish a committee to assess sea-level rise. Oregon, Washington, and several federal agencies joined California to sponsor the study. The report estimates sea-level rise both globally and for those three states for the years 2030, 2050, and 2100.
The committee that wrote the report projected that global sea level will rise 8 to 23 centimeters by 2030, relative to the 2000 level, 18 to 48 centimeters by 2050, and 50 to 140 centimeters by 2100. The 2100 estimate is substantially higher than the United Nation’s Intergovernmental Panel on Climate Change’s projection made in 2007 of 18 to 59 centimeters with a possible additional 17 centimeters if rapid changes in ice flow are included.
For the California coast south of Cape Mendocino, the committee projected that sea level will rise 4 to 30 centimeters by 2030, 12 to 61 centimeters by 2050, and 42 to 167 centimeters by 2100. For the Washington, Oregon, and California coast north of Cape Mendocino, sea level is projected to change between falling 4 centimeters to rising 23 centimeters by 2030, falling 3 centimeters to rising 48 centimeters by 2050, and rising between 10 to 143 centimeters by 2100. The committee noted that as the projection period lengthens, uncertainties, and thus ranges, increase.
The committee’s projections for the California coast south of Cape Mendocino are slightly higher than its global projections because much of the coastline is subsiding. The lower sea levels projected for northern California, Washington, and Oregon coasts are because the land is rising largely due to plate tectonics. In this region, the ocean plate is descending below the continental plate at the Cascadia Subduction Zone, pushing up the coast.
Extreme events could raise sea level much faster than the rates projected by the committee. For example, an earthquake magnitude 8 or greater north of Cape Mendocino, which occurs in this area every several hundred to 1,000 years with the most recent in 1700, could cause parts of the coast to subside immediately and the relative sea level to rise suddenly by a meter or more.
"As the average sea level rises, the number and duration of extreme storm surges and high waves are expected to escalate, and this increases the risk of flooding, coastal erosion, and wetland loss," said Robert Dalrymple, committee chair and Willard and Lillian Hackerman Professor of Civil Engineering at Johns Hopkins University.
Most of the damage along the west coast is caused by storms, particularly the confluence of large waves, storm surges, and high tides during El Niño events. Significant development along the coast — such as airports, naval air stations, freeways, sports stadiums, and housing developments — has been built only a few feet above the highest tides. For example, the San Francisco International Airport could flood with as little as 40 centimeters of sea-level rise, a value that could be reached in several decades. The committee also ran a simulation that suggested sea-level rise could cause the incidence of extreme water heights in the San Francisco Bay area to increase from about 9 hours per decade, to hundreds of hours per decade by 2050, and to several thousand hours per decade by 2100.
The study was sponsored by the states of California, Washington, and Oregon; National Oceanic and Atmospheric Administration; U.S. Geological Survey; and U.S. Army Corps of Engineers. The National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council make up the National Academies. They are independent, nonprofit institutions that provide science, technology, and health policy advice under an 1863 congressional charter. Panel members, who serve pro bono as volunteers, are chosen by the Academies for each study based on their expertise and experience and must satisfy the Academies’ conflict-of-interest standards. The resulting consensus reports undergo external peer review before completion.