Background
The authors, who are employed at the U.S. Geological Survey's National Center in Reston, Virginia, note that "the concept of proportionality between CO2 and sea level implies that the rate of sea-level rise accelerates in tandem with an exponential increase in CO2 and atmospheric temperature," and that "this simple concept has been used to argue that the rate of sea-level rise was low over the past 6000 years, began to increase during the 19th century, and will continue to increase during the next century."

What was done
In an analysis of this concept, which is essentially the climate-alarmist view of the matter, Larsen and Clark considered data bases that represent three different time scales: (1) the last 6000 years, based on radiocarbon-dated organics from basal peat deposits below salt marshes and estuarine sediments along stable or apparently subsiding coasts, (2) the last 1000 years, based on peat samples and benthic foraminifers from coastal salt marshes, and (3) the historic record of the last few hundred years, based on actual tide gauge data.

What was learned
The researchers report that when the three scales of sea-level variation are integrated, adjusted for postglacial isostatic movement, and replotted in terms of depth relative to present mean high water, they are able to "view the historic record as a continuation of the past rather than as a perturbation." In addition, they thereby demonstrated the strong linearity in the historic rate of sea-level rise over the past century and a half, which, as they describe it, "shows no indication of the pronounced mid-20th-century increase in temperature indicated by Mann et al. (1999)," noting further that "neither is there a relationship to the atmospheric CO2 record."

What it means
In contrast to the oft-stated claim that the rate of sea-level rise has been accelerating in tandem with the rate of rise in the air's CO2 concentration and/or its temperature, Larsen and Clark could find no evidence that supports that contention.

Reference
Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.