title: UK Mean Sea Level Change from Analysis of Tide Gauge Records
Creators: name: Hogarth, Peter Ian
Publisher: name: University of Liverpool Repository
Year: 2021

description: Abstract This thesis is mainly concerned with changes in mean sea level as measured at coastal tide gauge sites around the coast of Great Britain, and addresses the question of whether a secular change in rate of rise of sea level can be discerned over the measurement period. This is an important question for the coastal and low-lying areas of not only the UK, but globally, with the IPCC estimating that 11% of the worlds population currently live in low elevation coastal zones, with sea level rise projected to impact available land area, coastal infrastructure, ecosystems and the viability of some island nations. The two main thrusts of this present work are concerned with 1) Extending the temporal extent of currently available tide gauge records using data archaeology, ideally to the point where long term climate related trends emerge. This is important as currently many of the records are only a few decades long, and analysis of trends can give inconsistent results in relation to ongoing climate change. The small number of records which stretch back into the 19th Century do appear to show a long term increase in rate of sea level rise. 2) investigating optimal methods of identifying and accounting for any non-climate related variability in the records, again allowing any underlying trends to be more easily discerned, and importantly, shortening the period over which a climate related signal might be detected. The results are conclusive. Long term sea level rise and acceleration are confirmed around the entire UK coastline. For sites where recent decadal scale sea level falls have been reported, these are shown to be due to a combination of datum control errors and insufficient record length. Datum shifts due primarily to instrumentation changes are shown to be a significant error source in many of the UK tide gauge records. Many of these shifts have not been differentiated from assumed inter-site variability due to other causes until now. Accounting for these shifts with recorded calibration data and knowledge of physical changes at the tide gauge allows these errors to be systematically reduced, to the point that inter-site variability and differences in global isostatic adjustment adjusted sea level rise are much smaller and records appear visually similar. A further result is that the similarity of tide gauge records from nearby locations along the same coastline coupled with low uncertainties in land based survey levelling over short distances allows the monthly or annual mean sea level time series from these locations to be combined into a single representative record with quantifiable uncertainties. Furthermore, the variation between records further apart is found to be mainly due to localised meteorological effects. We confirm that these effects are well described by a tide and surge model, and this allows this variability to be accounted for. This leaves a residual signal where the variability is shown to be largely common mode, and therefore likely to be a far field ocean related effect. The finding that remaining inter-site variability is small allows averaging of the tide gauge records to obtain a single representative UK sea level index. This methodology eases the extension of localised tide gauge records using even short sections of historical tide gauge data. A comprehensive data archaeology exercise was then carried out to recover as much of this data as possible, much of which was previously unpublished, and assimilate this data first into localised records and then into a single UK wide record stretching back over more than two centuries. Clear acceleration is evident, with upward changes in gradient around the end of the 19th and 20th Centuries. Over most of the 19th Century the rise in sea level appears to be close to zero, rising to an average of just over 2mm/yr during the 20th Century, and rising again to an average of 3.4 mm/yr in the 21st Century so far. The acceleration over the entire period is around 0.01 mm/yr2 which is consistent with observed century scale sea level change when averaged globally. These data processing methods can be applied to other coastlines around the world where observations from tide gauges have been recorded.