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, as also indicated in the USGS paper, 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.
The NAS report indicates that extreme events could raise sea level much faster than the rates projected by the committee. For example, they say 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.
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The report indicates that 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.
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