Until recently, studies of long-term sea level change were of necessity done with tide gauge data, which are poorly distributed spatially and suffer from uncertainties due to unknown and widely varying rates of vertical land motion. With the advent of precise satellite altimetry, however, it is now possible to measure global sea surface height variability with a precision of a few centimeters within a 10-20 day window. For studies of long-term sea level change, though, the satellite measurements suffer from relatively short time series, and serious questions remain about the long-term stability of these data, especially when multiple satellite missions must be joined together. These two data types, tide gauge sea levels and satellite sea surface heights are argued to be highly complementary, and we will review both the present state of and future prospects for both tide gauge observations and satellite altimetric measurements.
The primary problem is how to optimally use both datasets to obtain
descriptions of long-term regional and global sea level change. Progress in this area will be reviewed in some detail, and falls into three categories. First, the use of continuous GPS measurements at tide gauges is reducing the uncertainties due to vertical land motion to manageable levels. Second, the long-term stability of the altimeters and the errors due to patching together data from different satellite missions is being addressed by using
the tide gauges to monitor the stability of the altimeters, and also to provide calibrations for the low frequency drift in the satellite data. In a sense, one can use the altimeter data to spatially interpolate the sparse tide gauge series to obtain better estimates of the global and regional averages. The third problem, the relative brevity of the satellite time series, is more difficult, and present efforts are mainly focused on attempting to simply estimate the magnitude of this error by determining how representative the time period spanned by altimetry is relative to a longer time period as observed by the tide gauges. With the launch of a global gravity mission, however, it may be possible to address this problem
in a more significant way, and a proposal for attempting this will be
outlined.