Working Group I: The Scientific Basis

Other reports in this collection Long-term mean sea level accelerations

Comparison of the rate of sea level rise over the last 100 years (1.0 to 2.0 mm/yr) with the geological rate over the last two millennia (0.1 to 0.2 mm/yr; Section 11.3.1) implies a comparatively recent acceleration in the rate of sea level rise. The few very long tide gauge records are especially important in the search for “accelerations” in sea level rise. Using four of the longest (about two centuries) records from north-west Europe (Amsterdam, Brest, Sheerness, Stockholm), Woodworth (1990) found long-term accelerations of 0.4 to 0.9 mm/yr/century (Figure 11.7). Woodworth (1999a) found an acceleration of order 0.3 mm/yr/century in the very long quasi-mean sea level (or ‘Adjusted Mean High Water’) record from Liverpool. From these records, one can infer that the onset of the acceleration occurred during the 19th century, a suggestion consistent with separate analysis of the long Stockholm record (Ekman, 1988, 1999; see also Mörner, 1973). It is also consistent with some geological evidence from north-west Europe (e.g., Allen and Rae, 1988). In North America, the longest records are from Key West, Florida, which commenced in 1846 and which suggest an acceleration of order 0.4 mm/year/century (Maul and Martin, 1993), and from New York which commenced in 1856 and which has a similar acceleration. Coastal evolution evidence from parts of eastern North America suggest an increased rate of rise between one and two centuries before the 20th century (Kearney and Stevenson, 1991; Varekamp et al., 1992; Kearney, 1996; Van de Plassche et al., 1998; Varekamp and Thomas, 1998; Shaw and Ceman, 2000).

Table 11.10: Estimated rates of sea level rise components from observations and models (mm/yr) averaged over the period 1910 to 1990. (Note that the model uncertainties may be underestimates because of possible systematic errors in the models.) The 20th century terms for Greenland and Antarctica are derived from ice sheet models because observations cannot distinguish between 20th century and long-term effects. See Section
Central value
Thermal expansion 0.3 0.5 0.7
Glaciers and ice caps 0.2 0.3 0.4
Greenland – 20th century effects 0.0 0.05 0.1
Antarctica – 20th century effects – 0.2 – 0.1 0.0
Ice sheets – adjustment since LGM 0.0 0.25 0.5
Permafrost 0.00 0.025 0.05
Sediment deposition 0.00 0.025 0.05
Terrestrial storage (not directly from climate change) – 1.1 – 0.35 0.4
Total – 0.8 0.7 2.2
Estimated from observations 1.0 1.5 2.0

There is no evidence for any acceleration of sea level rise in data from the 20th century data alone (Woodworth, 1990; Gornitz and Solow, 1991; Douglas, 1992). Mediterranean records show decelerations, and even decreases in sea level in the latter part of the 20th century, which may be caused by increases in the density of Mediterranean Deep Water and air pressure changes connected to the North Atlantic Oscillation (NAO) (Tsimplis and Baker, 2000), suggesting the Mediterranean might not be the best area for monitoring secular trends. Models of ocean thermal expansion indicate an acceleration through the 20th century but when the model is subsampled at the locations of the tide gauges no significant acceleration can be detected because of the greater level of variability (Gregory et al., 2001). Thus the absence of an acceleration in the observations is not necessarily inconsistent with the model results.

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