Venice – the City of Dreams – may have one less nightmare to deal with following a finding that the frequency of extreme storm surge events generated by Adriatic Sea tempests（大风暴） could fall by about 30 per cent by 2100. A team of international scientists led by CSIRO's Dr Alberto Troccoli studied atmospheric circulation in the Mediterranean region to assess climate impacts through changes in storm surge frequency in Venice – a World Heritage-listed city built on 117 small islands and considered vulnerable to high sea levels (locally known as Acqua Alta).

Dr Troccoli said predictions of such extreme and small-scale events are exceedingly challenging, even for relatively short time horizons.

"The survival of Venice and its lagoon（泻湖，环礁湖） is seriously questioned under the International Panel on Climate Change (IPCC) global sea level rise scenarios and the results of this study, published in the journal Climatic Change, emphasise the need for location-by-location studies to determine coastal flooding impacts.

"While some assessment of the vulnerability of Venice to extreme high-water events have been carried out in the past, possible future changes in storm surge occurrences critical to flooding events remain largely unexplored. It is important to understand how these events will evolve since a moderate to strong storm surge event is required to cause serious flooding.

"We found that the frequency of extreme storm surge events affecting Venice is projected to decrease by about 30 per cent by the end of the 21st century which would leave the pattern of flooding largely unaltered（不变的） under 21st Century climate simulations," Dr Troccoli said.

The research team – from CSIRO, the University of Padova and the University of Reading – used data from observations of storms, analysis of atmospheric and surface conditions, and climate scenario simulations. Storm surges in the northern part of the Adriatic Sea are driven by the passage of deep low-pressure systems, which cause sea level pressure gradients（气压梯度） and strong Sirocco (South-Easterly) winds along the Adriatic Sea. These forces combine to push water into the northern end of the basin where Venice is located.

Dr Troccoli said an implication of the finding is that alterations in extreme tidal levels under climate change must be considered on a location-by-location basis in spite of the projected increase in global sea level.

"Thus the projected change in the storm surge contribution, combined with the projected change of mean water levels in the Adriatic Sea indicates that tidal flooding events might not be exacerbated（加重，恶化） over the current century, with potential beneficial consequences for the conservation of the city.

"Our paper presents an alternative analysis for storm surges and we believe our findings could have important implications on the way future storm surge and high tide events are interpreted," he said.