Relocating the city of Venice is among four potential options – including movable barriers, ring dikes and closing the Venetian Lagoon - that could help it adapt to future sea-level rise over the next 200 years, according a new study.

Scientists assessed existing and potential adaptation strategies for Venice against sea-level-rise projections from the Intergovernmental Panel on Climate Change’s (IPCC) Sixth Assessment Report.

Publishing their findings today in the journal Scientific Reports, the authors suggest they could inform long-term planning for the city, although rapid action is essential. They also say that Venice exemplifies the challenges that many low-lying coastal areas globally - such as the Maldives, the Netherlands, and coastal cities - will face due to sea-level rise over the coming centuries.

Venice is a UNESCO World Heritage Site within the Venetian Lagoon and has flooded increasingly often over the past 150 years. The city’s current flood defences include a trio of movable barriers at the lagoon’s edge.

The team, including lead author Prof Piero Lionello of the University of Salento and co-author Prof Robert Nicholls from the University of East Anglia (UEA), estimate that, if additional measures are implemented, the existing movable barriers may be effective against sea-level rise of up to 1.25 metres.
They say this benchmark is likely to be exceeded under a low-emissions scenario by 2300 due to climate change and ground subsidence.

Alternative options they explored include protecting the centre of Venice with dikes that would separate it from the rest of the lagoon; closing the lagoon with a “super levee”; or relocating the city, its residents and historic landmarks inland.

Prof Nicholls, Professor of Climate Adaptation at the Tyndall Centre for Climate Change Research at UEA, said: “Our analysis shows that there is no optimal adaptation strategy for Venice.

“Any approach taken must balance multiple factors including the wellbeing and safety of Venice’s residents, economic prosperity, the future of the lagoon’s ecosystems, heritage preservation, and the region’s traditions and culture.

“This study shows that all low-lying populated coastal areas should recognise the challenge of long-term sea-level rise and start considering adaptation implications now."

The authors estimate that dikes may be necessary beyond 0.5 metres of sea-level rise, which may occur by 2100 under a low-emissions scenario. The closed-lagoon strategy could also be viable beyond 0.5 metres of sea-level rise, and the authors estimate that this could protect the city against sea level rise of up to 10 metres.

They propose that relocating the city may be necessary beyond 4.5 metres of sea-level rise, which is projected to occur after 2300.

The researchers used the costs of previous engineering projects - adjusted for inflation to 2024 prices - to estimate the potential costs and feasibility for each adaptation strategy.

They report that the overall cost of building Venice’s existing flood defence system was €6 billion and estimate that construction of dikes could cost between €500 million and €4.5 billion. Closing the lagoon with a super levee could initially cost more than €30 billion, while relocating the city could cost up to €100 billion.

Prof Nicholls added: “Given the high cultural value of Venice, these costs are clearly incomplete and no adaptation measure can sustain the Venice that we see today in the long-term.”

The authors also caution that, as the construction of large-scale interventions such as permanent barriers can take between 30 and 50 years, early planning is essential.

‘Long-term adaptation pathways for Venice and its lagoon under sea level rise’, is published in Scientific Reports report on April 16.