Monday, June 25, 2012

Two Reports On U.S. Atlantic & Pacific Sea Level Rises

Jun 24: Two separate reports, one by the U.S. Geological Survey and the other by the National Academy of Sciences, warn of rising sea levels along the Atlantic and Pacific coasts and the fact that levels are rising at faster than average rates.
    The report by the U.S. Geological Survey (USGS), published in Nature Climate Change, indicates that the rates of sea level rise are increasing three-to-four times faster along portions of the U.S. Atlantic Coast than globally. Since about 1990, sea-level rise in the 600-mile stretch of coastal zone from Cape Hatteras, NC to north of Boston, MA -- coined a "hotspot" by scientists -- has increased 2 - 3.7 millimeters per year; the global increase over the same period was 0.6 – 1.0 millimeter per year. Based on data and analyses included in the report, if global temperatures continue to rise, rates of sea level rise in this area are expected to continue increasing.
    USGS indicated that the report shows that the sea-level rise hotspot is consistent with the slowing of Atlantic Ocean circulation. Models show this change in circulation may be tied to changes in water temperature, salinity and density in the subpolar north Atlantic. USGS Director Marcia McNutt said, "Many people mistakenly think that the rate of sea level rise is the same everywhere as glaciers and ice caps melt, increasing the volume of ocean water, but other effects can be as large or larger than the so-called 'eustatic' rise. As demonstrated in this study, regional oceanographic contributions must be taken into account in planning for what happens to coastal property."
    USGS said that though global sea level has been projected to rise roughly two-to-three feet or more by the end of the 21st century, it will not climb at the same rate at every location. Differences in land movements, strength of ocean currents, water temperatures, and salinity can cause regional and local highs and lows in sea level.
    A separate report from the National Academy of Sciences' (NAS) National Research Council (NRC), released on June 22, indicates that the sea level off most of California is expected to rise about one meter over 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.  Sea levels off Washington, Oregon, and northern California will likely rise less, about 60 centimeters over the same period of time.  However, the report warns that 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, 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. 


    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 Ni├▒o 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.


    Access a release on the USGS report and link to the complete report (click here). Access a release on the NAS report and link to the complete report (click here). [#Climate, #Water]


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