Relocating Venice to a new site has been mooted as one possible option to protect it from rising sea levels as the increasing number of flooding events threaten to overwhelm the city’s infrastructure.

Scientists have been assessing potential adaptation strategies for Venice as part of the Intergovernmental Panel on Climate Change’s Sixth assessment report. Other possibilities include the installation of movable barriers, ring dikes and closing the Venetian Lagoon entirely.

Venice is a particularly stark example of the challenges that many low-lying coastal areas face globally – with other areas such as the Maldives, the Netherlands and coastal cities also facing challenges in the future.

Venice’s current flood defences include a trio of movable barriers at the lagoon’s edge, but the city has still flooded increasingly often over the past 150 years.

The research team estimate that, if additional measures are implemented, the existing movable barriers may be effective against sea-level rise of up to 1.25 metres, although this benchmark is likely to be exceeded under a low-emissions scenario by 2300 due to climate change and ground subsidence.

Alternative options 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.

“Our analysis shows that there is no optimal adaptation strategy for Venice,” said Professor Robert Nicholls from the University of East Anglia. “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 even 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 €6bn and estimate that construction of dikes could cost between €500m and €4.5bn. Closing the lagoon with a super levee could initially cost more than €30bn, while relocating the city could cost up to €100bn.

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.