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Is Sea Level Rise Accelerating?
Reference
Cazenave, A. and Nerem, R.S.  2004.  Present-day sea level change: observations and causes.  Reviews of Geophysics 42: 10.1029/2003RG000139.

What was done
The authors of this review paper summarize what is known about past and current rates of sea level rise because, in their words, "determining the rate of sea level change over the last century is critically important in order to determine if the present-day rate of sea level change has changed appreciably," which is something that climate models suggest should have occurred as a consequence of 20th-century global warming.

What was learned
Cazenave and Nerem report that "the geocentric rate of global mean sea level rise over the last decade (1993-2003) is now known to be very accurate, +2.8 0.4 mm/yr, as determined from TOPEX/Poseidon and Jason altimeter measurements," and that "this rate is significantly larger than the historical rate of sea level change measured by tide gauges during the past decades (in the range of 1-2 mm/yr)."  However, as they continue, "the altimetric rate could still be influenced by decadal variations of sea level unrelated to long-term climate change, such as the Pacific Decadal Oscillation, and thus a longer time series is needed to rule this out."  In addition, they say that satellite altimetry has revealed a "nonuniform geographical distribution of sea level change, with some regions exhibiting trends about 10 times the global mean [our italics]."  Also, they note that "for the past 50 years, sea level trends caused by change in ocean heat storage also show high regional variability," which fact "has led to questions about whether the rate of 20th century sea level rise, based on poorly distributed historical tide gauges, is really representative of the true global mean."

What it means
In spite of the many new instruments and techniques that have been developed and used to search for a global warming signal in global sea level data, Cazenave and Nerem report that "these tools seem to have raised more questions than they have answered."  Hence, we still do not know if model predictions of CO2-induced global warming will ultimately be confirmed or rejected by this ancillary field of study that Cazenave and Nerem call "sea level change science."  Indeed, we may well need another decade or two before we can place much confidence in any result obtained in the interim.

Reviewed 1 June 2005