There is an on-going controversy in studies of global sea level rise. The latest entry is a paper in the Proceedings of the National Academy of Sciences, which has become, unfortunately, a “pal-reviewed” journal rather than a peer-reviewed journal.
The paper in question is titled, “Climate related sea-level variations over the past two millennia.” One of the co-authors is Michael Mann of “hide the decline” fame and author of the now debunked hockey stick. (see full paper) These researchers used samples from just two sites is North Carolina to conclude: “A second increase in the rate of sea-level rise occurred around AD 1880–1920; in North Carolina the mean rate of rise was 2.1 mm/y in response to 20th century warming. This historical rate of rise was greater than any other persistent, century-scale trend during the past 2100y.” In other words, they say that the rate of sea level rise is accelerating.
The problem I see with this research is that there are only two sample sites and these sites are within an unstable geological environment: barrier islands/estuaries. The apparent sea level is subject to frequent change due to reconfiguration of the coast by storms and coastal currents. In fact, the two closest long-term tidal gauge records, Wilmington, N.C. and Hampton Roads, Va., show widely varying rates of sea level rise: 2.0 mm/yr and 4.5 mm/yr respectively. The sample size used by these researchers was too small and not representative of global sea level rise.
In another paper published in May, 2011, in the Journal of Coastal Research, the researchers analyzed the records of 57 U.S. tidal gauges and found that the rate of sea level rise was decreasing during the 20th Century:
Our analyses do not indicate acceleration in sea level in U.S. tide gauge records during the 20th century. Instead, for each time period we consider, the records show small decelerations that are consistent with a number of earlier studies of worldwide-gauge records. The decelerations that we obtain are opposite in sign and one to two orders of magnitude less than the +0.07 to +0.28mm/y2
accelerations that are required to reach sea levels predicted for 2100 by Vermeer and Rahmsdorf
(2009), Jevrejeva, Moore, and Grinsted (2010), and Grinsted, Moore, and Jevrejeva (2010). Bindoff et al. (2007) note an increase in worldwide temperature from1906 to 2005 of 0.74uC. It is essential that investigations continue to address why this worldwide-temperature increase has not produced acceleration of global sea level over the past 100 years, and indeed why global sea level has possibly decelerated for at least the last 80 years.
An earlier study Holgate (2007), using data from worldwide coastal tidal gauge records, shows that the rate of sea level rise is decreasing. Specifically, the mean rate of global sea level rise was “larger in the early part of the last century (2.03 ± 0.35 mm/yr 1904-1953), in comparison with the latter part (1.45 ± 0.34 mm/yr 1954-2003).” (Citation: Holgate, S.J. 2007. On the decadal rates of sea level change during the twentieth century. Geophysical Research Letters 34: 10.1029/2006GL028492)
NASA satellite data show that sea level rise has been steady, not accelerating, and has in fact been decelerating since 2006.
For more background on sea level since the end of the last glacial epoch 15,000 years ago, see my post “Sea Level Rising?” That post presents graphs and shows that the rate of sea level rise is cyclical. It also gives more references.
It seems that both sample size and location matter when trying to determine what is happening with global sea level. And, it appears that researchers with an agenda can cherry-pick data to suit their needs.