20.03.2012 - UNESCO Office in Venice/UNESCO Natural Sciences Sector

Workshop Report From Global to Regional: Local Sea Level Rise Scenarios makes a story in A World of SCIENCE

©JøMa - High waters in Venice in September 2009 -Saint Mark's Square

The Quarterly Natural Sciences Newsletter A World of SCIENCE, Vol. 10, No. 2, April–June 2012 includes an article on the Workshop Report 1. From Global to Regional: Local Sea Level Rise Scenarios - Focus on the Mediterranean Sea and the Adriatic Sea entitled "Venice will succumb to sea-level rise, the question is when".

The newly published report of a workshop run by UNESCO’s Venice Office on 22–23 November 2010 has concluded that ‘the planned mobile barriers (MOSE) might be able to avoid flooding [of the World Heritage site of Venice and its Lagoon] for the next few decades but the sea will eventually rise to a level where even continuous closures will not be able to protect the city from flooding. The question is not if this will happen, but only when it will happen.’

In order to avoid flooding, the Italian authorities have authorized the construction of an underwater barrier system, referred to as the MOSE Project, which should be operational by 2014. During the project planning phase, three scenarios for sea-level rise by 2100 were considered: 16.4 cm, 22.0 cm – the scenario recommended for the MOSE project – and a pessimistic scenario of 31.4 cm. Today, even the pessimistic scenario is considered overoptimistic.

The Intergovernmental Panel on Climate Change (2007) forecast global sea-level rise of 18–59 cm to 2100 but excluded ice melt from its calculations, as this parameter could not be modelled. Observed global sea-level rise actually exceeded the model projections for the period 1961–2003 by 50% and for 1990–2008 by 80%.

Other uncertainties stem from the insufficiently understood dynamics of heat uptake by the oceans, which causes oceans to swell and thus sea level to rise, as well as from the variety of possible scenarios for future carbon emissions and the consequential heating of the atmosphere.

Some recently published papers give higher estimates of global sea-level rise: Vermeer and Rahmstorf (2009) give a range of 75–190 cm, Horton et al. (2008) a potential lower limit of 54– 89 cm and Jevrejeva et al. (2010) a range of 60–160 cm. Looking farther ahead, the Delta Committee (2008) gives a range of 1.5–3.5 m for the year 2200, while the German Advisory Council on Global Change (2006) estimates sea-level rise of 2.5–5.1 m by 2300. ‘This means that sea-level rise will be governed in the coming centuries by a delayed response to 21st century anthropogenic (human-induced) warming.’

As for sea level in the Mediterranean, it has shown strong variability over the past century, rising by approximately 1.2 mm/ year, which is ‘significantly lower than the global average.’ Based on measurements from tide gauges, it even dropped a few centimetres between 1960 and 1993 before rising 4–5 cm between 1993 and 2000, after which there was no change.

One factor affecting regional sea level is atmospheric pressure: a drop in pressure of 1 millibar (mbar) is equal to about a 1 cm rise in sea level. A rise in atmospheric pressure linked to the North Atlantic Oscillation was responsible for the drop in sea level in the Mediterranean between 1960 and 1993. Climate models indicate that atmospheric pressure could rise in future in the Mediterranean, causing a drop of 2 cm by 2100, or -0.2 mm/ year on average.

Another factor controlling sea-level change is the steric effect, by which higher temperatures raise sea level, whereas higher salinity lowers it. Scientists conclude from this that, although there has been a rise in both temperature and salinity, the latter could dominate in the Mediterranean.

As the Mediterranean Sea is linked to the Atlantic Ocean through the Strait of Gibraltar, one crucial uncertainty concerns how exchanges through the strait will influence sea level in the Mediterranean. The latest findings indicate that the difference between both basins should not be greater than 10 cm, with an adjustment process that should not take longer than a few months. ‘Sea-level rise in the Mediterranean will thus be dominated by the global trend, even if some local differences might continue to exist,’ states the report. ‘The fact that the steric change in the level of the Mediterranean Sea could be much less (or even negative) simply indicates that the contribution of the Mediterranean to global sea-level rise will be much smaller than that of the other oceans. However, in the long run, the Mediterranean will follow the global ocean.’

As the exchanges between the Adriatic and Mediterranean Seas are not restricted by a narrow strait, it is ‘conceivable that the Adriatic Sea should follow very closely the trends in the Mediterranean.’

Mean sea level is basically identical between the Adriatic Sea and the Venice lagoon, despite the strong hydraulic control exerted by the inlets. The city begins to flood when the water level reaches 110 cm. Over the past century, the lagoon has been gradually sinking, owing to natural subsidence and sea-level rise, combined with industrial extraction of groundwater. In the 1980s and 1990s, the average water level was about 23 cm above the zero datum. This level is now closer to 30 cm above the datum. This means that sea-level rise of 80 cm would bring the mean water level to the critical threshold of 110 cm. In this case, Venice would experience regular flooding twice a day at high tide. Over the past three years, mean sea level in the lagoon has risen by about 10 cm during the summer months and by as much as 20 cm during the winter months. This rise correlates with a drop in atmospheric pressure from 2020 mbar to 2013 mbar in the past three years. It is doubtful that these trends will continue but mean sea level in the Adriatic Sea and close to the Venice lagoon will likely be extremely variable.

The workshop was organized by UNESCO in partnership with Georg Umgiesser from the Italian Institute of Marine Sciences of the National Research Council, lead author on the report. UNESCO has since organized three other workshops to evaluate environmental, cultural and socio-economic challenges faced by Venice and its Lagoon in relation to global change.

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