This chapter has shown that tide gauges record a (perhaps frustratingly) rich array of geophysical and oceanographic information. Vertical crustal movements and especially interannual and longer sea level variations make it difficult to obtain sea level trend estimates truly indicative of global sea level rise. The instrumental sea level record may not be stationary, nor does it consist of a simple trend plus measurement noise. These are problems that bedevil all climate records, as is well documented in National Research Council (1995). But even if these problems can be overcome to the satisfaction of researchers, major issues remain. What is needed for an understanding of global sea level and its relation to climate is an accurate budget of the contributors to sea level rise including thermal expansion and the various sources and sinks of water. As a practical matter for those hundreds of millions of persons living in threatened coastal regions and islands, the current rates of local (relative) sea level rise are already causing severe problems. Climate research and new observations should enable accurate predictions of future sea level rise to be made so that mitigation activities can be planned well in advance of disasters exacerbated by increased sea level, rather than in response to them.
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