A semi-empirical approach to estimate the future frequency of extreme sea level events: The case study of Trieste (north Adriatic)

This paper describes a semi-empirical methodology to estimate the future evolution of the frequency of extreme sea level events, combining the outputs of climate models, the IPCC projections of the mean sea level in given scenarios, and the statistical distribution of extreme events deduced from the observations. The application is carried out to the case study of Trieste, in the north Adriatic, for which over 100 y of observed sea level data are available, in an extreme and in a mild climate scenario. The climate models runs cover the period from 1987 to 2100. The extreme sea level events are quantified by the daily mean sea level, that mainly accounts for the role played by the atmospheric forcing, and by the daily Highest High Waters, that allow to directly assess the impact of the extreme sea level events on the urban environment. An increase in the frequency of those events is evident mainly in connection with the mean sea level rise, while the atmospheric forcing appears to play a minor role. By the end of the 21st century the extreme sea level events are expected to become more than one order of magnitude more frequent than in the current climate. Although large uncertainties affect the estimated frequencies of future events, there is indication of a remarkably fast increase during the second half of the century, which raises the problem of a rapid response to adapt the urban environment to the expected changes.