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4 OVERVIEW AND RECOMMENDATIONS glaciers. One hundred years from now it is likely that sea level will be 0.5 to 1 m higher than it is at present. There are two principal uncertainties at present about global sea level. (1) What if any is the value of the long-term trend over the next few centuries? (2) If a secular trend exists, what proportion of this trend results from changes in the specific volume of sea water (called steric changes) and what from changes in the total mass of water in the oceans? The apparent trend of sea level at any particular place as measured by a tide gauge is the sum of the trend in motion of the gauge itself as the land on which it is mounted moves vertically, the trend of change in steric sea level, and the trend of change in water mass under the tide gauge. To understand what is happening, one needs to be able to make measurements that will separate these three components of the observed sea level. In principle, a combination of inverted echo sounders (which in effect measure thermal expansion or contraction of the water column) or systematic observations of ocean tem- perature as a function of depth with conductivity-temperature-depth recorders to determine the steric component, plus one or more of three methods (very-long-baseline interferom- etry, global positioning system, and absolute gravimetry) for measuring the vertical mo- tions of the tide gauge, plus tide-gauge measurements at the sea surface should allow us to separate the three major components of changes in RSL. Within the next few years, it should be possible to measure accurately the combined effects on global sea level of steric changes and changes in the mass of sea water from laser or radar altimeters mounted on Earth-orbiting satellites. Recommendation 1. Long-term sea-level measurements of sufficient accuracy over the world's oceans could provide one of the most significant data sets for understanding global change, particularly climatic change resulting from greenhouse warming. It is for this reason that the planning committees for the World Climate Research Program and the Intergovernmental Oceanographic Commission of UNESCO have given a very high prior- ity to extending the global sea-level network in the Indian, South Atlantic, and South Pacific oceans. We strongly recommend that national oceanographic and meteoro- logical communities-lend moral and intellectual support to this sea-level program. Recommendation 2. Possible changes in the mass balance of the Antarctic and Greenland ice sheets are fundamental gaps in our understanding and are crucial to the quantification and refinement of sea-level forecasts (the probable contribution from ice wastage makes up more than half of various forecasts). A polar-orbiting satellite altimeter would be invaluable in monitoring the mass balance of these ice sheets. Recommendation 3. To refine estimates of sea-level change related to greenhouse warming, it is necessary to develop and improve coupled atmosphere-ocean-cyrosphere global circulation models in which greenhouse gas concentrations in the atmosphere are gradually increased. Recommendation 4. The Cretaceous period offers special opportunities to understand global processes and their variations, in particular, large, long-term changes in sea level. One of the major projects of the Global Sedimentary Geology Program, under the auspices of the International Union of Geological Sciences, is entitled "Cretaceous Resources, Events, and Rhythms." We urge national and international support of this and similar programs that will improve our understanding of past sea-level changes and the processes that produced them. Recommendation 5. To separate epeirogeny from eustasy and steric components, it is important to measure repeatedly the absolute heights of tide gauges. We recommend that global measurements of absolute heights of these gauges be undertaken using abso- lute gravimetry and space-based techniques.
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