Brest sea level record: a time series construction back to the early eighteenth century

Abstract

International audienceThe completeness and the accuracy of the Brest sea level time series dating from 1807 make it suitable for long-term sea level trend studies. New data sets were recently discovered in the form of handwritten tabulations, including several decades of the eighteenth century. Sea level observations have been made in Brest since 1679. This paper presents the historical data sets which have been assembled so far. These data sets span approximately 300 years and together constitute the longest, near-continuous set of sea level information in France. However, an important question arises: Can we relate the past and the present-day records? We partially provide an answer to this question by analysing the documents of several historical libraries with the tidal data using a ‘data archaeology’ approach advocated by Woodworth (Geophys Res Lett 26:1589–1592, 1999b). A second question arises concerning the accuracy of such records. Careful editing was undertaken by examining the residuals between tidal predictions and observations. It proved useful to remove the worst effects of timing errors, in particular the sundial correction to be applied prior to August 1, 1714. A refined correction based on sundial literature [Savoie, La gnomique, Editions Les Belles Lettres, Paris, 2001] is proposed, which eliminates the systematic offsets seen in the discrepancies in timing of the sea level measurements. The tidal analysis has also shown that shallow-water tidal harmonics at Brest causes a systematic difference of 0.023 m between mean sea level (MSL) and mean tide level (MTL). Thus, MTL should not be mixed with the time series of MSL because of this systematic offset. The study of the trends in MTL and MSL however indicates that MTL can be used as a proxy for MSL. Three linear trend periods are distinguished in the Brest MTL time series over the period 1807–2004. Our results support the recent findings of Holgate and Woodworth (Geophys Res Lett) of an enhanced coastal sea level rise during the last decade compared to the global estimations of about 1.8 mm/year over longer periods (Douglas, J Geophys Res 96:6981–6992, 1991). The onset of the relatively large global sea level trends observed in the twentieth century is an important question in the science of climate change. Our findings point out to an ‘inflexion point’ at around 1890, which is remarkably close to that in 1880 found in the Liverpool record by Woodworth (Geophys Res Lett 26:1589–1592, 1999b)

On analysing sea level rise in the German Bight since 1844

“…High rates are observed for the period around 1900 and the highest rates of about 5 mm a −1 are estimated for the year 2000. The inflection point at the end of the 19th century is remarkably close to observed inflection points around 1880 at the Liverpool tide gauge (Woodworth, 1999) and around 1890 at the Brest tide gauge (Wöppelmann et al, 2006).…”

“…Therefore, regional and global sea level rise are subjects to many recent scientific publications (e.g., White et al, 2005;Jevrejeva et al, 2006;Wöppelmann et al, 2006;Holgate, 2007;Church et al, 2008;Woodworth Correspondence to: T. Wahl (thomas.wahl@uni-siegen.de) et al, 2009a, b). In contrast, the mean sea level (MSL) and its variability over the last centuries in the German North Sea area have not been analysed in detail up to now.…”

“…The latter can be estimated by averaging high resolution sea level data consisting of at least hourly measurements. The difference between MSL and MTL is small in most areas (e.g., Pugh, 2004;Wöppelmann et al, 2006), but up to more than 20 cm along the German North Sea coastline due to shallow water effects (Lassen, 1989). This has to be considered when generating and analysing mean sea level time series.…”

Observed mean sea level changes around the North Sea coastline from 1800 to present

“…Araújo (2005), Haigh (2009) and Haigh et al (2009) analysed MSL records from tide gauge sites around the English Channel. Araújo et al (2008) and Wöppelmann et al (2006Wöppelmann et al ( , 2008 considered data sets from single stations (i.e. Newlyn and Brest) to assess long-term sea level changes in the same region.…”

Rescue of the historical sea level record of Marseille (France) from 1885 to 1988 and its extension back to 1849–1851

“…Marseille time series was compared with the longest sea level record of Brest updated by Wöppelmann et al (2006b) and made available through the French SONEL data bank (http://www.sonel.org). Some features of the comparison must be stressed (Fig.…”