A novel proxy and the sea level rise in Venice, Italy, from 1350 to 2014

Abstract

The evolution of relative sea level (RSL) in Venice, Italy, is crucial for the safeguarding of the city and it is now possible to extend our knowledge back to 1350, including the whole Little Ice Age and modern global warming. The existing tide gauge record is extensive, going back to 1871, but it is affected by the superposition of multidecadal swings and short-term fluctuations, including both natural and manmade forcing factors. A biological proxy, i.e. the green algae belt reported on paintings made with the help of a camera obscura by the Venetian painters Canaletto and Bellotto (eighteenth century) and Veronese (sixteenth century), helps us to go back to 1571. This paper presents an exceptionally long series (i.e. 664 years) and adds a novel proxy: the submersion of water stairs of the historic palaces facing the Grand Canal. Originally, the bottom step of the water stairs was built in relation to the sea level and the slippery algae belt, while today, the water stairs are mostly submerged. An underwater survey of 78 water stairs has provided new data about the RSL since 1350. The results show that RSL in Venice was always rising at an increasingly fast rate. By subtracting local land subsidence (LLS) from RSL, absolute (eustatic) sea level (ASL) has been calculated. For both RSL and ASL, the apparent acceleration is +0.0030 ± 0.0004 mm year−2. This figure becomes unstable when the record length is reduced. A discussion is made about the interpolation functions, i.e. the second-order polynomial and the exponential that provide almost the same best-fit over the common period. The RSL and ASL trend lines and the possibility of turning points are also discussed and compared with other scholarly studies. A eustatic turning point is suggested for the mid fifteenth century, consistent with the literature. However, the comparison between scholarly papers is difficult due to geographic and geological differences between sites and record durations.