ESTIMATIONS OF A GLOBAL SEA-LEVEL TREND - LIMITATIONS FROM THE STRUCTURE OF THE PSMSL GLOBAL SEA-LEVEL DATA SET

Among the possible impacts on environmental conditions of a global warming expected as a consequence of the increasing release of CO2 and various other greenhouse gases into the atmosphere, a predicted rise in global sea level is considered to be of high importance. Thus, quite a number of recent studies have focused on detecting the ''global sea level rise'' or even an acceleration of this trend. A brief review of these studies is presented, showing, however, that the results are not conclusive, though most of the studies have been based on a single global data set of coastal tide gauge data provided by the Permanent Service for Mean Sea Level (PSMSL). A detailed discussion of a thoroughly revised subset reveals that the PSMSL data set suffers from three severe limitations: (1) the geographical distribution of reliable tide gauge stations is rather uneven with pronounced concentrations in some areas of the northern hemisphere (Europe, North America, Japan), and much fewer stations on the southern hemisphere where particularly few stations are located in Africa and in Antarctica; (2) the number of stations recording simultaneously at any time is far less than the total number of stations with the maximum within the interval between 1958 and 1988; (3) the number of long records is extremely small and almost all of them originate from a few regions of the northern hemisphere. The sensitivity of the median of the local trends to these temporal and spatial limitations is discussed by restricting the data set in both the spatial and temporal distribution. It is shown that the data base is insufficient for determining an integral value of the global rise in relative sea level. The effect of polar motion on sea level is modelled and it turns out to be locally of the order of 0.5 mm/yr, affecting regional trends to an order of 0.1 mm/yr. Thus, this effect can be neglected on time scale of decades to a hundred years. Though the data set is insufficient for determining an integral sea level rise, the data, nevertheless, are providing information concerning regional trends in sea level and even temporal variations in these regional patterns. Thus, the most comprehensive subset of the data reveals the existence of a significant interdecadal east-west fluctuation of the oceans possibly indicating some kind of teleconnection. Besides the limitations of the data set, the physical nature of the long term changes in relative sea level, which by no means are globally uniform, might render any globally averaged value of relative sea level rise useless for detecting climatic impacts or validating climate models.