The case has been argued in this paper that global sea level rise over the last 100-150 years has averaged nearly 2 mm per year, with no evidence of acceleration, and that this value is possibly an order of magnitude greater than the average over the last two millennia. Assuming for the moment that the arguments have been convincing, what precisely is it that can be learned from these conclusions?
We have seen that it is difficult to interpret any value for global sea level rise because there are so many contributors to it. Ice accumulation at high latitudes, ocean thermal expansion and changes of circulation, melting of small glaciers, and water storage in large and small artificial reservoirs can offset one another to a greater or lesser extent. One must also account for the relative stability of sea level for the several millennia prior to 19th century, and the subsequent sudden (in historic terms) increased rate of rise beginning well before there was a significant increase of anthropogenic atmospheric greenhouse gases. Finally, the strongly-held and not totally implausible view that the global sea level rise value reported by recent authors is only an artifact resulting from coastal subsidence or poor data distribution, must be addressed. (I do not anticipate that this review will result in the sudden disappearance of those objections.) In spite of all of the publication activity over the last 20 years, it seems very clear that much remains to be accomplished in the area of sea level determinations and analyses. On a positive note, at least the questions that need to be answered are now well understood.
The prospects for rapid progress in resolving the controversies and issues in the matter of global sea level rise are excellent. The areas that need to be explored are mostly known, and new technologies offer solutions to many of the problems. Research into the problem of global sea level rise must in the future include at least the following elements, and perhaps others. These are recovery of old and unpublished data (sometimes called data archeology), monitoring of crustal movements and earth angular momentum, repeated observations of water column height, ocean modeling, artificial sources and sinks of water on land, retreat of small glaciers, satellite and other surveys of polar ice sheet elevation, and satellite altimetric determinations of global sea level. The length of this list certainly underscores the interdisciplinary essence of research into global sea level.
In the matter of data archeology, Hannah [1990] and Maul
and Martin [1993] have made important contributions. The former
author has provided new, very long records in New Zealand, in the
southern hemisphere where long records are sparse. The latter
authors were able to construct a summary geodetic datum and connect
several tide gauge sites at Key West, Florida. This extends the sea
level record there to 1846. Although not yielding a continuous
record in the 19th century, and so not as strictly useful as the
complete record at San Francisco that reaches to 1854, the series
was adequate to demonstrate that no significant acceleration of sea
level occurred over the record. They estimated the value of
relative sea level rise to be 1.9 
0.1 mm per year, very much in
line with estimates elsewhere in Florida. In addition, Maul
and Martin [1993] found that this long record provides estimates
of absolute sea level rise of 1.7-2.4 mm per year, depending on
which recent correction for PGR that is applied. One can only hope
that other old tide gauge records can be found and made available.
More sea level records over the last several millennia derived from dated sedimentary cores are also needed. There are enough historical records of climate change, particularly since the middle ages, to make comparisons of these records and sea level change a valuable activity for climate modelers. Those who would predict the future must first be able to explain the past.
The single most contentious issue concerning studies of global sea level rise has been the matter of vertical crustal movements. These definitely play a major role, and no reliable estimate of global sea level rise can be obtained without careful consideration of them. But for sea level records shorter than about 50 years, low frequency variations of sea level are an equally important source of geophysical noise. Both vertical crustal movements and water column density changes must be evaluated, and filtered from sea level series for rapid, accurate measurement of global sea level rise and acceleration.
Concerning vertical crustal movements, the technology required to observe them exists, and is reasonable in cost. Carter et al. [1989] describe a monitoring system that uses Very Long Baseline Interferometry (VLBI) [ Carter et al., 1986] for establishment of a global absolute geodetic reference system to which tide gauge sites can be connected. Baker [1993] provides a good summary of the Carter et al. [1989] report, and Eden [1990] includes oceanographic and meteorological considerations.
Movements of individual tide gauge sites can be monitored with respect to the absolute global geodetic datum using Global Positioning System (GPS) receivers. The elements of this system are in place, and accuracy in the vertical direction of order 1 cm or better demonstrated [ Carter et al., 1988]. Thus areas of unusual subsidence can be evaluated relatively quickly, and model results for vertical changes due to PGR can be tested in a few years time. A joint effort involving the National Ocean Service of NOAA, the NASA Goddard Space Flight Center, and the Laboratory for Coastal Research at the University of Maryland has been set up to monitor subsidence in the Chesapeake Bay region. Continuously operating receivers are operating at Solomons Island, Greenbelt, Annapolis, and Gaithersburg in Maryland. These are also observing simultaneously with receivers at Bermuda (an important sea level station), and at VLBI sites in Massachusetts and Florida. More stations will be added in the next few years, so that both local and regional scale vertical movements will be revealed. Since GPS receivers with the features needed for monitoring at the sub-cm level cost only about $20K, monitoring of tide gauge sites for vertical movements at a global network of locations is practical now.
It is also possible to monitor vertical crustal movements at certain sites by measuring changes of the force of gravity. Repeatability equivalent to height change of a few mm has been achieved in field operations [ Klopping et al., 1991]. Thus GPS results for elevation change can be calibrated against measurements from absolute gravity meters.
The GPS has implications for global change research far beyond monitoring of vertical crustal movements. Mader and Lucas [1989], Brozena et al. [1993], and others have demonstrated that moving platforms such as airplanes can be navigated at the few-cm level of accuracy. Thus airborne laser profiling of small and large glaciers is practical. This technology also enables photogrammetric surveys of shorelines, ice floes, etc., to be made repeatedly and compared for changes without in situ geodetic control. GPS is also being used to supplement the global VLBI system for monitoring earth angular momentum. As noted earlier, changes in the distribution of water alter the angular momentum vector of the earth, both in angular velocity and direction. Angular momentum of the earth is a unique integrator over many effects and provides an important constraint for observational and model results.
The other factor that plagues determination of sea level rise is low-frequency ``noise'' from interdecadal water column density fluctuations. A solution to this problem is needed if an acceleration of sea level is to be detected soon enough to be useful as an indicator of climate change [ Woodworth, 1990; Douglas, 1992]. Roemmich [in NRC, 1990] showed that the decimeter-level interdecadal fluctuations of sea level at Bermuda were explained entirely by changes in dynamic height above 2000 m. Roemmich [1992] was able to account for similar fluctuations in the ocean offshore of the southern half of California, at least in part, from upper ocean temperature variations. The dilemma for this approach is that there are very few sites whose water column properties are measured frequently over extended periods, as at Bermuda and now Hawaii.
Another approach taken by Sturges and Hong [1994] may provide
an alternative solution for the sea level problem. They ``
made
a simple model of wind forcing of the Atlantic
and
find sea
level variability
estimated this way in remarkably good agreement
with observations
'' Figure 2, taken from their paper,
illustrates the efficacy of their approach. The interdecadal
fluctuations of sea level are accurately modeled, and the removal
of this ``noise'' will make it far easier to determine an
underlying trend. Clearly, if crustal movements can be monitored
with space geodetic techniques, and low frequency fluctuations of
water column height measured or modeled even in part for a globally
distributed set of sea level observing stations, the eustatic rise
and any change in its rate can be observed relatively quickly. The
extrapolation study in Woodworth [1990] indicates that an
acceleration of sea level could be determined under these
conditions very early in the next century.
Finally, satellite altimetry cannot be overlooked as a means of obtaining a direct, global measurement of sea level change. Wagner and Cheney [1992] examined altimeter data from the Seasat and Geosat oceanographic satellites and found that the principle errors to overcome for this application were those associated with the ionospheric and water vapor corrections, and absolute calibration of the altimeter, including the orbit. All of these matters have been explicitly addressed by the TOPEX/Poseidon oceanographic satellite which began its 5-year mission in 1992 [ Fu et al., 1994]. The TOPEX/Poseidon satellite uses a dual frequency altimeter to determine the ionospheric correction, a water vapor radiometer for direct measurement of the precipitable water in the path of the altimeter beam, and advanced Global Positioning System (GPS), laser, and DORIS dual frequency doppler tracking systems for orbit determination [ Fu et al., 1994]. Cheney et al. [1994] conclude that TOPEX/Poseidon data are accurate at the level of 2 cm for monthly mean changes on scales of a few hundred km, and by inference far more accurate on a global, annual scale. In support of this view, Wagner et al., [1994] report a robust determination of global sea level rise over two years of the TOPEX/Poseidon mission of 5 mm per year, which when corrected for instrument drift [ Hayne et al., 1994] gives about 3 mm per year. This satellite altimeter result is not far from recent estimates of global sea level rise for the last 100+ years derived from tide gauge data. The agreement could be fortuitous, but the result is interesting, and demonstrates the potential of the method.
Thus it appears that the problem of obtaining an accurate estimate of global sea level rise and acceleration is tractable. If the supporting measurements of ice volume, water storage, and earth angular momentum are also continued and expanded, the scientific problem of interpreting the sea level results will also become tractable, and the entire field of global change will greatly benefit.