Why We Try to Extend Sea Level Records

BODC have been involved in a number of previous projects aimed at extending historic sea level records. I thought we could use this post to discuss one of the scientific studies that have come about from using these data.

In 2006, Ivan Haigh contacted BODC. He was working on a Ph.D at the University of Southampton with the topic ‘Extreme sea levels in the English Channel’. The study relied on analysing high-frequency sea level data (for data to be ‘high-frequency’, the readings
need to be an hour or less apart). He would then analyse the data and attempt to calculate

  • Mean Sea Level (MSL)
  • The probability of future extreme sea level events

This kind of research requires long time series. As Ivan says on his blog:

“The study of extremes is more difficult than research into MSL changes owing to the general lack of access to long, high frequency and quality-controlled datasets. Records of about 35 years (i.e. about two 18.6 nodal cycles) are needed to accurately determine changes in extreme levels, as the influence of the nodal cycle can significantly bias trend estimates in shorter datasets. At the start of the study, there were only two tide gauge stations on the UK south coast (Newlyn and Dover) with sea level records matching these requirements and a further seven sites on the northern French coastline.”

For background on the 18.6-year nodal cycle, see this Wikipedia entry on tidal analysis.

In order to increase the timespan of data series available for study, Ivan carried out a data archaeology exercise. The first step was to identify what data existed in analogue formats. He asked BODC about sea level data for Southampton that he knew we held in chart form. He then took these charts and had them digitised, converting the data into a useable format.

Mean sea-level trends from English Channel tide gauge records and their wider context

Taken from Haigh, I.D., Nicholls, R.J., Wells, N.C., 2009a. Mean sea-level trends from English Channel tide gauge records and their wider context. Continental Shelf Research, 29, 2083-2098

The data archaeology exercise was very successful and in total 173 years worth of digital data for the South Coast were created. The record for Southampton was extended back to 1937.

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