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How Well Do We Know What Global Sea Level Is Doing?
Reference
Wunsch, C., Ponte, R.M. and Heimbach, P. 2007. Decadal trends in sea level patterns: 1993-2004. Journal of Climate 20: 5889-5911.

Background
Wunsch et al. write that "given the widespread and generally consistent reports of global warming, melting glaciers, shoreline retreat, and the clear trend of the last 20,000 years, a compelling inference is that global-mean sea level is rising," and in this regard they note that "the advent of high-accuracy satellite altimetry has led to estimates that, since about 1993, global average sea level has been rising at a rate of 2.8 0.4 mm/year." They thus suggest "it is desirable to buttress [this finding] through independent means," which is what they set out to do.

What was done
"Using about 2.1 x 109 observations of many different types, all individually weighted, during the period 1992-2004 and a 1 horizontal resolution, 23-layer general circulation model," as the three researchers describe it, they derived estimates of "regional trends in global sea level."

What was learned
Wunsch et al. report that their analyses produced "a global mean of about 1.6 mm/year, or about 60% of the pure altimetric estimate, of which about 70% is from the addition of freshwater." However, they note that there is "great regional variability in trend values, sometimes up to two orders of magnitude larger than the apparent spatial mean."

What it means
In light of their findings, the three researchers state that "at best, the determination and attribution of global-mean sea level change lies at the very edge of knowledge and technology," and that "it remains possible that the database is insufficient to compute mean sea level trends with the accuracy necessary to discuss the impact of global warming -- as disappointing as this conclusion may be." As a result, they conclude that the altimetry result is "currently untestable against in situ datasets." Nevertheless, it is of interest to note that the mass of data they analyzed provided a result that was only 60% as large as that suggested by the satellite altimetry data, which has always been larger than results obtained from nearly all prior in situ studies.

Reviewed 23 April 2008