Global Sea-Level Rise

Global sea level (i.e., the mean sea level averaged across the globe) has risen by 1.6 ± 0.2 mm/yr. (millimeters per year) in the period 1961 to 2003 and by a similar amount (1.7 ± 0.3 mm/yr.) when calculated over the entire 20th century. These rates are based on tide gauge measurements and, since the early 1990s, also on satellite altimetry. Satellite measurements have shown that global sea-level rise has not been uniform, either in time or in space.

The average global sea-level curve shows distinct temporal fluctuations. In the 1920s and 1930s, sea-level rise speeded up, while in the 1960s, sea-level rise slowed down. Since the 1970s, the maximum decadal rate of sea-level rise has increased from about 3 mm/yr. to more than 5 mm/yr. Averaged over the past 15 yrs., the global rate is about 3 mm/yr. The spatial pattern of sea-level rise shows large variability. Between 1993 and 2003, parts of the North Atlantic and Western Pacific oceans experienced rates of sea-level rise in excess of 10 mm/yr., but in the Eastern Pacific and Western Indian oceans, sea level fell by similar amounts. When averaged over the past 50 yrs. the pattern is also complex (Figure 1). Large parts of the world's oceans have been subject to sea-level rise, but there are places, most notably in the Indian Ocean and the tropical Pacific, where sea level has fallen, albeit by a small amount.

Spatial variability in patterns of sea-level rise is strongly correlated with changes in oceanic heat content that affect the density of ocean water and produce so-called steric sea-level change. Thermal expansion is the predominant contributor to steric sea-level change; salinity variations are less important. Thermal expansion was responsible for about 45% of global sea-level rise in the period 1961 to 2003, with glaciers and ice caps contributing about 30% and ice sheets in Greenland and Antarctica a little over 10% each. Building of dams and mining of groundwater may be significant additional factors in the global sea-level budget but are believed to cancel each other out.

The melting of ice sheets, ice caps, and glaciers also produces regional sea-level variability, albeit on a larger wavelength than do the steric effects. The diminishing gravitational attraction exercised by a shrinking polar ice mass on the ocean surface causes sea-level rise to be greater in the “far field” (i.e., equatorial regions and the oceans in the opposite hemisphere) than nearby. This effect means that coastlines on both sides of the North Atlantic Ocean, for example, are more vulnerable to the melting of the Antarctic Ice Sheet than to melting of the Greenland Ice Sheet. The sloping pattern of sea-level rise produced by melting ice is known as the sea-level fingerprint of the melt source.

Coastal sea-level change is measured by tide gauges, but these instruments measure simultaneously the movement of the coast to which the [Page 1345]tide gauge is attached. Globally averaged rates of sea-level rise based on tide gauge records should therefore be corrected for isostatic and tectonic effects. The global glacial isostatic adjustment (GIA) process has been extensively analyzed with the help of sophisticated Earth models. These studies have revealed that GIA produces not only land-level changes but also a slow increase in the global ocean volume in areas where the ocean floor is subsiding due to glacial forebulge collapse and hydroisostatic loading. The global sea-level fall due to this “ocean-syphoning” effect is about 0.3 mm/yr. and is important because it partly offsets the global acceleration of sea-level rise that has occurred between the 19th and the 20th centuries.

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