11.3.2.2 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
11.2.3.3. |
 |
| |
Minimum
(mm/yr)
|
Central value
(mm/yr)
|
Maximum
(mm/yr)
|
|
| 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.
|
