5.5.2 Observations of Sea Level Changes

5.5.2.1 20th-Century Sea Level Rise from Tide Gauges

Table 11.9 of the TAR listed several estimates for global and regional 20th-century sea level trends based on the Permanent Service for Mean Sea Level (PSMSL) data set (Woodworth and Player, 2003). The concerns about geographical bias in the PSMSL data set remain, with most long sea level records stemming from the NH, and most from continental coastlines rather than ocean interiors. Based on a small number (~25) of high-quality tide gauge records from stable land regions, the rate of sea level rise has been estimated as 1.8 mm yr^–1 for the past 70 years (Douglas, 2001; Peltier, 2001), and Miller and Douglas (2004) find a range of 1.5 to 2.0 mm yr^–1 for the 20th century from 9 stable tide gauge sites. Holgate and Woodworth (2004) estimated a rate of 1.7 ± 0.4 mm yr^–1 sea level change averaged along the global coastline during the period 1948 to 2002, based on data from 177 stations divided into 13 regions. Church et al. (2004) (discussed further below) determined a global rise of 1.8 ± 0.3 mm yr^–1 during 1950 to 2000, and Church and White (2006) determined a change of 1.7 ± 0.3 mm yr^–1 for the 20th century. Changes in global sea level as derived from analyses of tide gauges are displayed in Figure 5.13. Considering the above results, and allowing for the ongoing higher trend in recent years shown by altimetry (see Section 5.5.2.2), we assess the rate for 1961 to 2003 as 1.8 ± 0.5 mm yr^–1 and for the 20th century as 1.7 ± 0.5 mm yr^–1.

Figure 5.13. Annual averages of the global mean sea level (mm). The red curve shows reconstructed sea level fields since 1870 (updated from Church and White, 2006); the blue curve shows coastal tide gauge measurements since 1950 (from Holgate and Woodworth, 2004) and the black curve is based on satellite altimetry (Leuliette et al., 2004). The red and blue curves are deviations from their averages for 1961 to 1990, and the black curve is the deviation from the average of the red curve for the period 1993 to 2001. Error bars show 90% confidence intervals.

While the recently published estimates of sea level rise over the last decades remain within the range of the TAR values (i.e., 1–2 mm yr^–1), there is an increasing opinion that the best estimate lies closer to 2 mm yr^–1 than to 1 mm yr^–1. The lower bound reported in the TAR resulted from local and regional studies; local and regional rates may differ from the global mean, as discussed below (see Section 5.5.2.5).

A critical issue concerns how the records are adjusted for vertical movements of the land upon which the tide gauges are located and of the oceans. Trends in tide gauge records are corrected for GIA using models, but not for other land motions. The GIA correction ranges from about 1 mm yr^–1 (or more) near to former ice sheets to a few tenths of a millimetre per year in the far field (e.g., Peltier, 2001); the error in tide-gauge based global average sea level change resulting from GIA is assessed as 0.15 mm yr^–1. The TAR mentioned the developing geodetic technologies (especially the Global Positioning System; GPS) that hold the promise of measuring rates of vertical land movement at tide gauges, no matter if those movements are due to GIA or to other geological processes. Although there has been some model validation, especially for GIA models, systematic problems with such techniques, including short data spans, have yet to be fully resolved.