BRYAN: Do you suppose that the basic reason we don't see sea-level changes corresponding to the temperature changes in the climatic record is that the main signal, which reflects recovery from the Little Ice Age, is so large as to mask any other trends?
DOUGLAS: Well, the current rate of rise is close to 2 mm/yr, half of which is unaccounted for by glaciers and upper-layer warming. It's true that it may reflect a short-term readjustment, since we know archeologically that we haven't had a 4-m rise in the last 2000 years. But I think the only thing we can conclude from the tide-gauge record is that there hasn't been any acceleration in the last 100 or 150 years.
GHIL: Some of the high-latitude records suggest that the internal variability in sea level substantially exceeds the warming trend of about 20 mm in 10 years.
LEVITUS: The interannual sea-level changes at Bermuda are highly correlated with the steric changes from the Panulirus station. We've found large-scale, basically mechanical displacements of the whole pycnocline in the subtropical gyre—but no simple explanation for them.
RASMUSSON: One of the great advantages we have in studying the ENSO cycle is that we know where to index it to get good results. Some of the things you were showing, Bruce, suggested that you could pick out index stations too. I think it's very important in maximizing results, and also for setting priorities on which stations should be maintained.
MUNK: I'd like to point out that a lot can be learned from seasonal variations in sea level—a 1-year period—which seems not to have been done adequately. To a first order, the seasonal variation is steric; you get a significant agreement between sea level and volumetric changes. As far as I know, no one has attempted to account for the observed seasonal variation in terms of the seasonal variation in wind-stress curl, which I think would be worthwhile.
My other point is that tide-gauge data will not really be useful until it is corrected for the structural variability. It's unacceptable to have the geologic noise be about the same magnitude as the oceanic noise. The technology for getting accuracy of a few millimeters in the movement of the ground to which the tide gauge is attached is available. Also, it seems to me that you'd learn more from a few gauges from which the crustal movement could be removed than from many observation points subject to unknown geologic variability.
DOUGLAS: I agree completely. In fact, my original proposal called for a dozen or so super-stations, where we could do density and geodetic measurements, but unfortunately they were too expensive to get funded.
KARL: Could you elaborate a little on the acceleration you didn't find and how that relates to global warming?
DOUGLAS: Well, we have nearly 150 years of good records with reasonable distribution, and overall they don't show any global acceleration of sea-level rise. That says that the 25 percent increase in CO2 over the last 100 years has not caused anything that would accelerate sea-level rise—yet.
LEHMAN: I'd just like to point out that there is no consensus on the mass-balance status of the Antarctic ice sheet. Recent Greenland ice-sheet data seem to support Zwaly's controversial estimates, which suggest that it might actually have a positive mass balance. There are large uncertainties about the effect of ice volume on sea level; if it's offsetting a steric or thermally induced rise, it could be an important indicator that the ocean is absorbing greenhouse heat.
DOUGLAS: We really need a satellite-mounted laser to monitor the Arctic and Antarctic.
DESER: The large accelerations in sea level at Charleston and Bermuda between about 1920 and 1940 correspond to a time when there was warming along the Gulf Stream. I suspect that any differences between them might reflect changes in transport in the Gulf Stream.
GHIL: Ingemar Holmström has performed the data reduction you suggest, applying empirical orthogonal functions. He found very little independent information in the Baltic tide gauges; the first EOF contains some 90 percent of the variance.
I think we could sum all this up by saying that tide gauges may have relatively little to say about global warming, but they have a lot to say about air-sea interaction on interdecadal time scales.