NRC report projects sea level off most of California will rise at slightly higher rate than global average; lower rate for Oregon, Washington, NorCal; variety of factors affect outcome
23 June 2012
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, according to 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.
Tide gauges show that global sea level has risen about 7 inches (18 cm) during the 20th century, and recent satellite data shows that the rate of sea-level rise is accelerating. As Earth warms, sea levels are rising mainly because: (1) ocean water expands as it warms; and (2) water from melting glaciers and ice sheets is flowing into the ocean. Melting of land ice is now the largest component of global sea-level rise (about 65%), largely because ice loss rates are increasing, according to the report. However, sea-level rise is uneven and varies from place to place.
Sea-level rise off the west coast of the United States is influenced by a variety of local factors; therefore, sea-level projections for California, Oregon, and Washington differ from global projections. The factors that affect local sea-level projections include steric variations; wind-driven differences in ocean heights; gravitational and deformational effects (sea-level fingerprints) of melting of ice from Alaska, Greenland, and Antarctica; and vertical land motions along the coast. The local steric and wind-driven components were estimated by extracting northeast Pacific data from the same ocean models used for the global projections. The cryosphere component was adjusted for gravitational and deformational effects and then extrapolated forward. Finally, vertical land motion was projected using continuous GPS measurements for two tectonically distinct areas: Cascadia, where the coastline is generally rising, and the San Andreas region, where the coastline is generally subsiding.—“Sea-Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future”
Along the US west coast sea-level rise 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.
For its projections, the committee producing the report chose a combination of approaches for its projections. The output of GCMs (global climate models) was used to project the steric contribution (primarily thermal expansion) to global sea-level rise over the three time frames. For the land ice projections, the committee extrapolated mass balance estimates. Like the IPCC (2007), the committee did not project land hydrology contributions because uncertainties are too large, and the latest comprehensive assessment found that the primary sources (groundwater depletion) and sinks (reservoir storage) appear to effectively cancel out. The individual components were then summed and compared with results from semi-empirical methods. The projections are for individual years (2030, 2050, and 2100, relative to 2000), and were derived using single-year values from low-order curves, except for the steric values.
Key findings of the report include:
Global sea level is projected to rise 8-23 cm (3-9 in) by 2030, relative to 2000 levels, 18-48 cm (7-19 in) by 2050, and 50–140 cm (20-55 in) by 2100. The 2100 estimate is 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.
Vertical land motions caused by plate tectonics and the ongoing response of the Earth to the disappearance of North American ice sheets have a significant impact on sea-level rise along the Washington, Oregon, and California coasts.
Sea level along the California coast south of Cape Mendocino is projected to rise 4-30 cm (2-12 in) by 2030, relative to 2000 levels, 12-61 cm (5-24 in) by 2050, and 42-167 cm (17-66 in) by 2100. 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.
For the Washington, Oregon, and California coasts north of Cape Mendocino, sea level is projected to change between -4 cm (-2 in) (sea-level fall) and +23 cm (9 in) by 2030, -3 cm (-1 in) and +48 cm (19 in) by 2050, and 10-143 cm (4-56 in) by 2100. These values are lower than projections further north. 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.
An earthquake magnitude 8 or greater along the Cascadia Subduction Zone—which occurs in this area every several hundred to 1,000 years with the most recent in 1700—would suddenly raise sea level along parts of the coast by an additional 1-2 meters (3-7 feet) over projected levels north of Cape Mendocino.
Uncertainties grow as the projection period lengthens. Confidence in the projections is high for 2030 and perhaps 2050. By 2100, the authors are confident only that the value will fall within the uncertainty bounds.
Most coastal damage is caused by the confluence of large waves, storm surges, and high astronomical tides during a strong El Niño.
Some models predict a northward shift in North Pacific storm tracks, and some observational studies report that largest waves are getting higher and winds are getting stronger. Observational records are not long enough to confirm whether these are long-term trends.
Even if storminess does not increase in the future, sea-level rise will magnify the adverse impact of storm surges and high waves on the coast.
Storms and sea-level rise are causing coastal cliffs, beaches, and dunes to retreat at rates from a few cm/yr to several m/yr. Cliffs could retreat more than 30 m (about 100 feet) by 2100.
Wetlands are likely to keep pace with sea level until 2050. Their survival until 2100 depends on maintaining elevation through high sedimentation, room to move inland, or uplift.
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.—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 US Army Corps of Engineers.
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