Historical Documents as Proxy Data in Venice and Its Marine Environment

Abstract

The environmental history of Venice over the last millennium has been reconstructed from written, pictorial, and architectural documentary sources, used in a synergistic way. The method of transforming a document into an index and then into calibrated numerical values according to an international system of units has been applied in the case of Venice and its geographical and climate peculiarities. Because frost constituted a dramatic challenge for the city, a series of severe winters is well documented: The city was sieged by ice, meaning Venetians had to cross the ice transporting food, beverages, and wood for burning in carts, as recorded in written reports and visual representations. The sea level in the 18th century has been reconstructed based on paintings by Canaletto and Bellotto, who took advantage of a camera obscura to precisely draw the views of the city and its canals.. These paintings accurately represent the green algae belt that corresponds to the level of soaking created by marine waters at high tide. This has made it possible to measure how much the green algae (and therefore the seawater) has risen since the 18th century. Similarly, a painting by Veronese has enabled the reconstruction of sea level rise (SLR) since 1571. Another useful proxy is the water stairs of the Venetian palaces. These were originally built to access boats and are now (almost) totally submerged and covered with algae. As the sea level rose, these steps became submerged underwater. The depth of the lowest step is therefore representative of how much the sea level rose after the stair was built. This proxy has allowed the relative sea level since 1350 to be reconstructed, and an exponential trend in the rising of the sea level has been identified. Venice has at times been flooded by seawater, including tsunamis at the beginning of the second millennium. A long series of sea floods due to storm surges triggered by particular meteorological situations shows that the flooding frequency is related to the exponential SLR. In the 1960s, there was a sharp increase in frequency of flooding, which coincided with the digging of deep and wide canals, excavated to allow the passage of tankers. This increased the exchange of water between the sea and the lagoon. Proxies based on archaeological remains, as well as geological-biological cores extracted from the coastal area and dated with isotopic methods, cover long time periods; the longest record reaching 13 ka BP. However, the time resolution is reduced, thus providing good data for physical geography purposes.