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Uncertainties about Temporal Changes in Ocean Properties, their Effects on Sea Level, and What they Imply about Global Warming
Volume 6, Number 28: 9 July 2003

The first of the two Science articles we consider in this Editorial is the Perspective piece of Munk (2003), who notes that "surveys of glaciers, ice sheets, and other continental water storage can place only very broad limits of -1 to +1 mm/year on sea level rise from freshwater export," which essentially means that we haven't a clue as to what is really happening in the area of overall cryospheric response to putative global warming. He also notes that "polar melting would result in movement of water mass toward the equator, causing a decrease in the rate of Earth's rotation," but he reports that "observations show a (nontidal) increase in Earth's rotation (attributed to a movement of mass toward the poles in response to the unloading of ice mass since the last glacial maximum)," which again leaves us guessing as to the net effect of the modest global warming of the past century on sea level trends.

Against this backdrop, Munk goes on to consider other approaches to assessing contemporary sea level behavior, focusing on the observation of Antonov et al. (2002) that the mean salinity of the global ocean decreased slightly between 1954 and 1997. Here too, however, there are knotty problems that stand in the way of a clear exposition of what may be happening. In addition, Munk notes that "the jury is still out on the interpretation of the tide gauge records," and that "the large discrepancy between the sea ice thinning estimates from the sonar method and the wave method leaves the interpretation of freshening in limbo."

With respect to the future, Munk notes the great promise of "global coverage by satellite altimetry (which is replacing tidal estimates)," but he cautions that "it will take several decades [our italics] to obtain good estimates of the role of global warming in sea level rise." In the meantime, as he puts it, "20th-century sea level remains an enigma - we do not know whether warming or melting was dominant, and the budget is far from closed."

The second of the two Science articles germane to the subject of ocean property shifts and their climate change implications is the Report by Bryden et al. (2003), who note that "with so few repeat observations, there has been a tendency to treat any observed change in water mass properties or circulation as an indicator of ocean climate change." Hence, in a compelling exercise that demonstrates the great danger of this almost irresistible urge, they compare five Indian Ocean hydrographic sections across 32S that were made in 1936, 1965, 1987, 1995 and 2002.

The results of their study, in the words of the scientists, clearly indicate "there has been an oscillation in the water mass properties of the upper thermocline waters with freshening from 1965 to 1987 and then an increase in salinity from 1987 to 2002, with the properties observed in 2002 close to those observed in 1936 and 1965." They further note that these changes demonstrate "there can be substantial oscillations over decadal time scales," and that "without regular observations, oceanographers have little understanding of the scales of variability in water mass properties." Indeed, they note that the most recent change "almost entirely reverses the observed freshening of mode waters from the 1960s to 1987 that has been interpreted to be the result of anthropogenic climate change on the basis of coupled climate models."

Clearly, many people have been much too quick to jump on the CO2-induced global warming/rising sea level bandwagon. The existing oceanic data pertaining to this problem are just too few and far between to answer the climate change questions on everybody's minds. We must remember that patience is a virtue; and we are going to need a whole lot of it throughout the future decades of data acquisition that will be required to definitively resolve this issue.

Sherwood, Keith and Craig Idso

Antonov, J.I., Levitus, S. and Boyer, T.P. 2002. Steric sea level variations during 1957-1994: Importance of salinity. Journal of Geophysical Research 107: 8013-8021.

Bryden, H.L., McDonagh, E.L. and King B.A. 2003. Changes in ocean water mass properties: Oscillations or trends? Science 300: 2086-2088.

Munk, W. 2003. Ocean freshening, sea level rising. Science 300: 2041-2043.