There have been documented long-term changes of climate in the last several millennia comparable in magnitude to what might occur in the near future. Thus the question arises as to how the current (i.e., over the last 100+ years) rate of sea level rise compares to that over the previous few millennia. This is an important issue; determining the past response of sea level to climate change could provide critical insights as to what might happen in the future.
There is a convincing body of evidence that the sea level rise value of the last 100+ years has not pertained to the last 2 millennia. Flemming [1978], and Flemming and Webb [1986], report on the results of examining hundreds of coastal archeological sites in the Mediterranean area. In the latter paper it is noted that the average of detailed studies published since 1969 give for the eustatic change of sea level in the Mediterranean about 0.4 m for the last 2000 years, or 0.2 mm per year. The effect of PGR in the area is small and variable, according to the results of Tushingham and Peltier [1991], so that there is some confidence in the conclusion that the modern rate of eustatic sea level rise approaching 2 mm per year is a relatively recent development. In other words, the present-day elevation of Mediterranean archeological sites in relation to sea level is grossly inconsistent with the notion of a near 2 mm per year rate of sea level rise maintained over the past 2000 years.
Flemming [1982] has also reported on vertical crustal movements and eustatic rise for the United Kingdom since the onset of the last deglaciation. His results there are consistent with those for the Mediterranean. Shennan and Woodworth [1992] have also considered tide gauge and geological data in a larger region consisting of the UK and the North Sea boundary. They concluded that the rate of sea level rise during the time of tide gauge measurements is about 1 mm per year higher in the area than during the late Holocene, further evidence that in the recent past (order 100+ years) sea level is rising faster than in earlier millennia.
Additional evidence exists outside of Europe for a low rate of sea level rise in the last several thousand years. Varekamp et al. [1992] have dated marsh cores in Connecticut by several methods and obtained consistent results. They found an average value of about 1.1 mm per year for sea level rise in Connecticut for the last 1500 years. The ICE-3G model of Tushingham and Peltier [1991] gives for subsidence due to PGR in this area about 1.5 mm per year, implying a fall of eustatic sea level of 0.4 mm per year over the period. Given the uncertainties in both the PGR and their sea level estimates, this result should be regarded as evidence for an insignificant average eustatic change during the 1500 year interval.
Varekamp et al. [1992] did find significant periodic changes
of sea level in their investigation, and concluded that their
``
data show no unequivocal correlation between warm periods (on
a decadal to centennial time-scale) and accelerated sea level
rise.'' This is an obviously intriguing and challenging result
which deserves more research, and verification at other sites.
The Chesapeake Bay provides yet more strong evidence that the average value of sea level rise was lower before the 19th century than since that time. Kearney and Stevenson [1990] studied marsh vertical accretion rates by independent (pollen and radionuclide) methods, and by reconstruction of land loss in the Bay since colonial times from historical records. They document that rates of sea level rise were relatively slow during the 17th and 18th centuries, and that the current rapid rate of sea level rise (about 3.5 mm per year) dates only from the early 19th century. Prior to that sea level was apparently stable in the region, possibly, they remark, as a response to the Little Ice Age. They also speculate that groundwater withdrawal for commercial operations could have played a role in the late 19th century. However, the documented increased rise of sea level in the entire mid-Atlantic region over the historical tide gauge record does not require an appeal to land subsidence anthropogenic in origin.