BOX S.1
Committee Charge

The committee will provide an evaluation of sea-level rise for California, Oregon, and Washington for the years 2030, 2050, and 2100. The evaluation will cover both global and local sea-level rise. In particular, the committee will

1.   Evaluate each of the major contributors to global sea-level rise (e.g., ocean thermal expansion, melting of glaciers and ice sheets); combine the contributions to provide values or a range of values of global sea-level rise for the years 2030, 2050, and 2100; and evaluate the uncertainties associated with these values for each timeframe.

2.   Characterize and, where possible, provide specific values for the regional and local contributions to sea-level rise (e.g., atmospheric changes influencing ocean winds, ENSO [El Niño-Southern Oscillation] effects on ocean surface height, coastal upwelling and currents, storminess, coastal land motion caused by tectonics, sediment loading, or aquifer withdrawal) for the years 2030, 2050 and 2100. Different types of coastal settings will be examined, taking into account factors such as landform (e.g., estuaries, wetlands, beaches, lagoons, cliffs), geologic substrate (e.g., unconsolidated sediments, bedrock), and rates of geologic deformation. For inputs that can be quantified, the study will also provide related uncertainties. The study will also summarize what is known about

a.   climate-induced increases in storm frequency and magnitude and related changes to regional and local sea-level rise estimations (e.g., more frequent and severe storm surges);

b.   the response of coastal habitats and geomorphic environments (including restored environments) to future sea-level rise and storminess along the west coast;

c.   the role of coastal habitats, natural environments, and restored tidal wetlands and beaches in providing protection from future inundation and waves.

records. More recent tide gage and altimetry data confirm that the higher rate of sea-level rise is continuing. However, because of natural climate variability, which affects sea level on decadal and longer timescales, more data are needed to determine whether the higher rates since the 1990s mark an acceleration in the long-term sea-level trend.

Components of Global Sea-Level Rise

A warming climate causes global sea level to rise by (1) warming the oceans, which causes sea water to expand, increasing ocean volume, and (2) melting land ice, which transfers water to the ocean. Human activities that transfer water between the land and ocean also affect global sea-level change. In particular, water withdrawn from aquifers eventually reaches the ocean, raising global sea level, whereas water stored behind dams effectively lowers global sea level.

The IPCC (2007) estimated that ice melt from glaciers, ice caps, and ice sheets contributed about 40 percent of the observed sea-level rise for 1961–2003 and that thermal expansion of ocean water contributed one-quarter of the observed rate for 1961–2003 and one-half for 1993–2003. Contributions from groundwater extraction and reservoir storage were poorly quantified but were thought to account for less than 10 percent of the observed rise. More recent data have changed these estimates. After the IPCC (2007) report was published, a bias was discovered in some ocean temperature measurements, which gave systematically warmer temperatures than the true values. Data sets corrected for this bias yield significantly lower rates of thermal expansion for the 1993–2003 period than were found by the IPCC (2007).

New research results also indicate that the relative contribution of land ice to global sea-level rise is increasing. Since 2006, the ice loss rate from the Greenland Ice Sheet has increased, and, according to most analyses, the contribution of Antarctic ice to sea-level change has shifted from negative (lowering sea level by accumulating ice) to positive (raising sea level). Ice loss rates from glaciers and ice caps have declined over the same period, but not enough to offset the increases in ice sheet melt. As a result of higher observed ice loss rates and a lower (corrected) contribution from thermal expansion, land ice is currently the largest contributor to global sea-level rise. In the most recent published estimate, land ice accounted for about 65 percent of the total sea-level rise from 1993 to 2008.

The contributions of groundwater withdrawal and reservoir storage to sea-level change remain poorly



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