Sea-Level Change in the Northern Mediterranean Sea from Long-Period Tide Gauge Time Series
DIFA Authors: Susanna Zerbini, Sara Bruni, Maddalena Errico, Efisio Santi
The oldest tide gauge observations date back to the 18th century. Knowledge of the past sea level behavior can contribute key insights to the understanding of climate change impacts. We highlight the greatest importance of monitoring sea-level changes at all spatial scales, from global to local, using terrestrial and space techniques and outline the physical processes, natural and man-induced, responsible for such changes. Tide gauges measure local sea-level relative to a benchmark on land, hence, correctly interpreting these observations is challenging since it demands, among other requirements, a proper knowledge of vertical land motions at the stations. For more than two decades, space geodetic techniques, such as GPS and InSAR (Interferometric Synthetic Aperture Radar), have provided the opportunity to accurately position points in the surroundings of tide gauge sites, potentially giving rise to a large amount of information. In the Mediterranean area, there are a few centennial tide gauge records. Our study focuses on the time series of Alicante, in Spain, Marseille, in France, Genoa, Marina di Ravenna, Venice and Trieste, in Italy. Two Italian stations, Marina di Ravenna and Venice, are affected by both natural and anthropogenic subsidence, the latter was particularly intense during a few decades in the 20th century because of ground fluid withdrawal. For these two stations, we have retrieved levelling data of benchmarks close to the tide gauges from the end of the 19th century and, for the last couple of decades, we have evaluated GPS and InSAR heights in close proximity to the stations. Modelling of the long-period non-linear behavior of subsidence was successfully accomplished. After removing the vertical land motions in Venice and Marina di Ravenna, and the inverted barometer effect at all the sites, the linear long period sea-level rates were estimated. The results are in excellent agreement ranging between +1,2 and +1,3 mm/yr for the overall period from the last decades of the 19th century till 2012. The associated errors, computed by accounting for serial autocorrelation, are of the order of 0,2-0,5 mm/yr.
Our estimated rates for the northern Mediterranean, a relatively small regional sea, are slightly lower than the global mean rate, +1,7±0,2 mm/yr, recently published in the IPCC AR5 (Intergovernmental Panel on Climate Change 5th Assessment Report ), but close enough, if uncertainties are taken into account. It is known that Mediterranean stations had always had lower trends than the global-average ones. Our regional results, however, are in close agreement with the global mean rate, +1,2 mm/yr, published by Hay et al. (2015) which is currently being discussed by the oceanographic community.