A review of the trends observed in British Isles mean sea level data measured by tide gauges

This report presents an up-to-date review of the British Isles (BI) Mean Se a Level (MSL) data set obtained from tide gauges, and of the long-term secu lar trends in BI MSL. The data are of mixed quality, and not as copious as one would like for scientific analysis given the complexity of long-term se a and land level changes around BI coasts. Nevertheless, the small number o f very long records can be studied effectively, and indicate that twentieth century secular trends in BI MSL are consistent with those obtained from N W Europe as a whole and with the bottom range of estimates for global avera ge MSL change during the past 100 years. Century-timescale low-frequency 'a ccelerations' in MSL of the order of 0.4-0.8 mm yr(-1) century(-1) are obta ined from the three longest records, which are also similar to estimates fr om mainland Europe. 'Sea level indices' for the BI are constructed which ca n provide responsible agencies with a guide to the 'average state' of BI se a level. Combined ocean tide and storm surge ('tide + surge') numerical mod elling is demonstrated to be a valuable tool in understanding part of the v ariability of MSL around the BI, although modelling of long-term changes is limited at present by the lack of adequate meteorological data sets from b efore 1955. Nevertheless, it is shown that, even if a complete meteorologic al modelling data set were available, it would not be able to account for a ll of the MSL variability. Finally, recommendations are made for the develo pment of tide gauge and Global Positioning System (GPS) recording in the BI for the purpose of long-term MSL monitoring in the next century.