The Difficulties in Using Tide Gauges to Monitor Long-Term Sea Level Change

SUMMARY Climate change has a variety of important impacts, one of which is reflected in sea levels. Indeed, long term rising trends in global sea levels are often used to corroborate the assertion of long term climate change. When tide gauge records are examined in order to determine the long-term trends in sea level it is typical for a single number representing the derived trend, to be quoted. However, the problems in deriving such numbers are rarely, if ever, discussed. Indeed, there appears to be a widespread ignorance as to the fragility of tide gauge records and hence the accuracy of derived long-term sea level trends. This paper uses specific examples from New Zealand to illustrate and explain the problems that exist in deriving an accurate figure for the eustatic changes in sea-level at a tide gauge site. It highlights the importance of assessing accurately the influence of anthropological factors, changes in tide gauge datums, and geophysical effects. These factors, which can compromise or even completely invalidate a record, must be able to be assessed over the entire history of the tide gauge record (often 100+ years). This paper, after exploring these factors and their potential influence, concludes by making recommendations on procedures to be followed if we are to leave future generations better quality sea level data than is often available at present.