Global changes in mean tidal high water, low water and range

Impacts of extreme sea levels are increasing as the population and infrastructure in coastal zones increases. Extreme high sea levels generally increased at a similar rate to mean sea level through the twentieth century at most sites around the world, suggesting that the same mechanisms are driving both increases. However, the simplicity of this conclusion belies the fact that many mechanisms known to act on the different components of sea level, have been observed to change in local and regional studies. Using a 'quasi-global' dataset of sea level records, this paper investigates changes in the tidal component of sea level and shows that changes in mean high and low waters, and hence tidal range, are occurring over long-time scales at many sites around the world. Over half of the selected sites show significant trends in tidal range datums, but no clear spatial patterns of change exist, suggesting that mechanisms are affecting the tide on local scales. Trends are dependent on the tidal datum selected which has wide-ranging practical applications given the variety of uses of tidal datums.