11.3.2.3 Mean sea level change from satellite altimeter observations
In contrast to the sparse network of coastal and mid-ocean island tide gauges,
measurements of sea level from space by satellite radar altimetry provide near
global and homogenous coverage of the world’s oceans, thereby allowing
the determination of regional sea level change. Satellite altimeters also measure
sea level with respect to the centre of the earth. While the results must be
corrected for isostatic adjustment (Peltier, 1998), satellite altimetry avoids
other vertical land movements (tectonic motions, subsidence) that affect local
determinations of sea level trends measured by tide gauges. However, achieving
the required sub-millimetre accuracy is demanding and requires satellite orbit
information, geophysical and environmental corrections and altimeter range measurements
of the highest accuracy. It also requires continuous satellite operations over
many years and careful control of biases.
To date, the TOPEX/POSEIDON satellite-altimeter mission, with its (near) global
coverage from 66°N to 66°S (almost all of the ice-free oceans) from
late 1992 to the present, has proved to be of most value to direct estimates
of sea level change. The current accuracy of TOPEX/POSEIDON data allows global
average sea level to be estimated to a precision of several millimetres every
10 days, with the absolute accuracy limited by systematic errors.
Careful comparison of TOPEX/POSEIDON data with tide gauge data reveals a difference
in the rate of change of local sea level of -2.3 ± 1.2 mm/yr (Mitchum,
1998) or -2 ± 1.5 mm/yr (Cazenave et al., 1999). This discrepancy is
caused by a combination of instrumental drift, especially in the TOPEX Microwave
Radiometer (TMR) (Haines and Bar-Sever, 1998), and vertical land motions which
have not been allowed for in the tide gauge data. The most recent estimates
of global average sea level rise from the six years of TOPEX/POSEIDON data (using
corrections from tide gauge comparisons) are 2.1 ± 1.2 mm/yr (Nerem et
al., 1997), 1.4 ± 0.2 mm/yr (Cazenave et al., 1998; Figure 11.8), 3.1
± 1.3 mm/yr (Nerem, 1999) and 2.5 ± 1.3 mm/yr (Nerem, 1999), of
which the last assumes that all instrumental drift can be attributed to the
TMR. When Cazenave et al. allow for the TMR drift, they compute a sea level
rise of 2.6 mm/yr. Their uncertainty of ± 0.2 mm/yr does not include
allowance for uncertainty in instrumental drift, but only reflects the variations
in measured global sea level. Such variations correlate with global average
sea surface temperature, perhaps indicating the importance of steric effects
through ocean heat storage. Cazenave et al. (1998) and Nerem et al. (1999) argue
that ENSO events cause a rise and a subsequent fall in global averaged sea level
of about 20 mm (Figure 11.8). These findings indicate
that the major 1997/98 El Niño-Southern Oscillation (ENSO) event could
bias the above estimates of sea level rise and also indicate the difficulty
of separating long-term trends from climatic variability.
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Figure 11.10: Estimated sea level rise from 1910 to 1990. (a)
The thermal expansion, glacier and ice cap, Greenland and Antarctic contributions
resulting from climate change in the 20th century calculated from a range
of AOGCMs. Note that uncertainties in land ice calculations have not been
included. (b) The mid-range and upper and lower bounds for the computed
response of sea level to climate change (the sum of the terms in (a) plus
the contribution from permafrost). These curves represent our estimate
of the impact of anthropogenic climate change on sea level during the
20th century. (c) The mid-range and upper and lower bounds for the computed
sea level change (the sum of all terms in (a) with the addition of changes
in permafrost, the effect of sediment deposition, the long-term adjustment
of the ice-sheets to past climate change and the terrestrial storage terms).
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After upgrading many of the geophysical corrections on the original European
Remote Sensing (ERS) data stream, Cazenave et al. (1998) find little evidence
of sea level rise over the period April 1992 to May 1996. However, over the
time span of overlap between the ERS-1 and TOPEX/POSEIDON data, similar rates
of sea level change (about 0.5 mm/yr) are calculated. For the period April 1992
to April 1995, Anzenhofer and Gruber (1998) find a sea level rise of 2.2 ±
1.6 mm/yr.
In summary, analysis of TOPEX/POSEIDON data suggest a rate of sea level rise
during the 1990s greater than the mean rate of rise for much of the 20th century.
It is not yet clear whether this is the result of a recent acceleration, of
systematic differences between the two measurement techniques, or of the shortness
of the record (6 years).
