5.5.2.2 Sea Level Change during the Last Decade from Satellite Altimetry

Since 1992, global mean sea level can be computed at 10-day intervals by averaging the altimetric measurements from the TOPEX/Poseidon (T/P) and Jason satellites over the area of coverage (66°S to 66°N) (Nerem and Mitchum, 2001). Each 10-day estimate of global mean sea level has an accuracy of approximately 5 mm. Numerous papers on the altimetry results (see Cazenave and Nerem, 2004, for a review) show a current rate of sea level rise of 3.1 ± 0.7 mm yr^–1 over 1993 to 2003 (Cazenave and Nerem, 2004; Leuliette et al., 2004; Figure 5.14). A significant fraction of the 3 mm yr^–1 rate of change has been shown to arise from changes in the Southern Ocean (Cabanes et al., 2001).

Figure 5.14. Variations in global mean sea level (difference to the mean 1993 to mid-2001) computed from satellite altimetry from January 1993 to October 2005, averaged over 65°S to 65°N. Dots are 10-day estimates (from the TOPEX/Poseidon satellite in red and from the Jason satellite in green). The blue solid curve corresponds to 60-day smoothing. Updated from Cazenave and Nerem (2004) and Leuliette et al. (2004).

The accuracy needed to compute mean sea level change pushes the altimeter measurement system to its performance limits, and thus care must be taken to ensure that the instrument is precisely calibrated (see Appendix 5.A.4.1). The tide gauge calibration method (Mitchum, 2000) provides diagnoses of problems in the altimeter instrument, the orbits, the measurement corrections and ultimately the final sea level data. Errors in determining the altimeter instrument drift using the tide gauge calibration, currently estimated to be about 0.4 mm yr^–1, are almost entirely driven by errors in knowledge of vertical land motion at the gauges (Mitchum, 2000).

Altimetry-based sea level measurements include variations in the global ocean basin volume due to GIA. Averaged over the oceanic regions sampled by the altimeter satellites, this effect yields a value close to –0.3 mm yr^–1 in sea level (Peltier, 2001), with possible uncertainty of 0.15 mm yr^–1. This number is subtracted from altimetry-derived global mean sea level in order to obtain the contribution due to ocean (water) volume change.

Altimetry from T/P allows the mapping of the geographical distribution of sea level change (Figure 5.15a). Although regional variability in coastal sea level change had been reported from tide gauge analyses (e.g., Douglas, 1992; Lambeck, 2002), the global coverage of satellite altimetry provides unambiguous evidence of non-uniform sea level change in open oceans, with some regions exhibiting rates of sea level change about five times the global mean. For the past decade, sea level rise shows the highest magnitude in the western Pacific and eastern Indian oceans, regions that exhibit large interannual variability associated with ENSO. Except for the Gulf Stream region, most of the Atlantic Ocean shows sea level rise during the past decade. Despite the global mean rise, Figure 5.15a shows that sea level has been dropping in some regions (eastern Pacific and western Indian Oceans). These spatial patterns likely reflect decadal fluctuations rather than long-term trends. Empirical Orthogonal Functions (EOF) analyses of altimetry-based sea level maps over 1993 to 2003 show a strong influence of the 1997–1998 El Niño, with the geographical patterns of the dominant mode being very similar to those of the sea level trend map (e.g., Nerem et al., 1999).

Figure 5.15. (a) Geographic distribution of short-term linear trends in mean sea level (mm yr^–1) for 1993 to 2003 based on TOPEX/Poseidon satellite altimetry (updated from Cazenave and Nerem, 2004) and (b) geographic distribution of linear trends in thermal expansion (mm yr^–1) for 1993 to 2003 (based on temperature data down to 700 m from Ishii et al., 2006).