It is a well known fact that global sea levels change with the climate. About 20,000 years ago, during the last Ice Age, sea levels were about 120 m lower than today. The top of the current Adriatic Sea was completely dry. Then, about 6,000 years ago, once the icecaps invading North America and the Finish-Scandinavian peninsula had finished melting, global sea levels had near enough settled at their present position and the Adriatic lagoons were formed.
In the 20th Century, estimates of variations in sea level were generally based on marigraph recordings. However these have the failing of not being able to distinguish between movements of the earth and those of the sea surface.
Thus, in the Adriatic where subsidence is greater in Venice than it is in Trieste, the rise in the mean sea level observed last century was higher in Venice (c. 24 cm) than in Trieste (12 cm or so). According to the 2001 report by the IPCC (Intergovernmental Panel on Climate Change), the mean rise during this same period on a global scale was between 1 and 2 mm a year, i.e. ranging between 10 and 20 cm over the course of the century.
In 1992, with the advent of altimeter satellites, there was a leap in quality in how global sea levels were measured, thanks to the fact that these satellites were able to measure the height of the global surface of the oceans (between 66° North and 66° South) with respect to the centre of the Earth with 4-5 mm precision thanks to their repeating the same trajectory every ten days.
The maps produced every 10 days bearing the measurements of these altimeter satellites have revolutionised all the studies , showing the variations in sea levels from area to area, the trends being for a rise in certain oceanic regions and a fall in others.
These maps also show that the trends along the coastline (where most marigraphs are installed) may differ from the trends seen offshore. Similar trends are true for the Mediterranean : sea levels in the West Mediterranean and Adriatic have risen more than the global mean over the past ten years, while the mean sea level of the Ionian Sea is slightly lower.
These regional variations are mainly due to the differences in thermal expansion of the sea water, following the global trend for an increase in mean temperature. Variations in thermal expansion are not stable in these regions, moving from one area to another, with a tendency for a global equilibrium with 10-year fluctuations.
Despite these variations and their shifting nature, the trend for global sea level indicated by satellite readings (having filtered out the seasonal variations) is essentially that of a rise in sea level at a rate estimated at +3,3 ±0,4 mm a year in the period 1993-2006. This is clearly faster than all the estimates made in the last century. There has thus been a recent acceleration in the rise of global sea levels, in line with the recent rise in global temperature.
According to the last IPCC report, this speeding-up in rising sea levels is simply a consequence of the recent acceleration of thermal expansion in oceanic waters (this has gone from an average +0,4 ±0,1 mm/year in the period 1950-2000 to +1,6 ±0,3 mm/year during the ten years 1993-2003, according to four independent oceanographic studies) and an increase in the rate at which continental glaciers are melting.
The latter (in terms of equivalent sea level) has gone from a contribution of +0,50 ±0,18 mm/year during the period 1961-2004 to +0,77 ±0,22 mm/year during the period 1991-2004 in the case of mountain glaciers and from +0,12 ±0,05 mm/year during the period 1961-2003 to +0,21 ±0,07 mm/year during the period 1993-2003 in the case of the Greenland icecap.
Signs of this recent speeding-up in the rise of the sea level can be seen in the Adriatic marigraphs. During the period 1993-2006 the mean sea level grew by +4,2 ±1,5 mm/year in Venice (Punta della Salute), by +3,2 ±1,4 mm/year in Trieste and by even more for Spalato and Ragusa .
In 2001, the IPCC made a forecast for the 21st Century based on six scenarios for greenhouse gas emissions. It forecast a rise in sea level of between 9 and 88 cm . In its most recent report (2007), the IPCC estimates a smaller rise based on climatic models of between 18 and 59 cm. However this estimate is probably an underestimation, as it does not take into account recent quickening in the ice flow observed in Greenland and the Antarctic. If these increases continue at the same rate, the IPCC estimates that the global seal level could well rise by a further 10-20 cm.
What is more, the model used to get the 18-59 cm forecast is based on long term physical laws, independent of the climatic data observed since 1990. Nevertheless, as Rahmstorf & Co. observed in 2007 in a note published on the Science journal, global rise in sea levels since 1990 has been faster than forecast by the models, being close to the upper limit rather than in the middle of the range foreseen by the IPCC in 2001 , which forecast a rise of 88 cm in 2100.
In the specific case of Venice, subsidence needs to be taken into account alongside the eustatic rise. Subsidence has been estimated at a rate of 1.2 mm/year according to geological data provided by E. Carminati and G. Di Donato and at a rate of 1.3 mm/year on the basis of archaeological data from A.J. Ammermann. In other words, by the end of the 21st Century the relative sea level in Venice could be at least 70-80 cm higher than the current level.
So what defences can we find for Venice and its lagoon?
It’s obvious that the MOSE flood barrier system, even if it were completed, would offer insufficient protection.
In fact, the planned sluice gates are designed to oscillate independently in the presence of waves. This means that gaps are foreseen between one sluice gate and the nest to allow the sea penetrate into the lagoon even when the gates are closed. It is recognised that in the event of rough seas the resonance phenomenon risks amplifying these oscillations, thus greatly increasing the size of the gaps. While these gates should be closed for only a few hours at present sea levels, the they would need to be closed for far longer periods should sea levels rise. P.A. Pirazzoli and G. Umgiesser have proved in the Journal of Marine Environment Engineering (2006) that in the event of a half metre rise in the current sea level accompanied by prolonged atmospheric depressions, the MOSE would not manage to avoid flooding of the lower sections of Venice during the ten hours immediately afterwards.
Therefore, before sea levels rise, the MOSE would need to be dismantled (and the project is not reversible nor can it be adapted to a fixed structure) in order to replace it with a fixed impermeable dam capable of cutting the lagoon completely off from the sea.
Of course, this would only be possible after a complete clean-up of the lagoon and would imply a gradual conversion of the lagoon environment from seawater/brackish water to brackish/fresh water, not to mention considerable changes for many lagoon-based activities.
Paolo Pirazzoli
