威尼斯正在沉没——我们分析了所有拯救它的计划，但没有一个能保留我们所知的这座城市

Venice has co-existed with the sea throughout its 1,500-year history, perhaps better than any other city on earth. Yet over the past century it has flooded increasingly often, as the sea rises and the city itself sinks under its own weight.

We recently published an academic analysis of the various options Venice has to ensure its long-term survival.

Our study compares a range of possible strategies against different degrees of sea-level rise. These include maintaining the current system of mobile barriers, building ring dykes to separate the city from the lagoon in which it sits, enclosing the whole lagoon within a much larger defence system, or – in the most extreme case – relocating much of the city and its population inland.

Each option becomes relevant at different points as sea levels rise. The city’s flood defences have already been upgraded substantially, at a cost of €6 billion (£5.2 billion). This involves a series of huge steel gates attached to the seafloor, known as the Mose barriers. When raised, these barriers effectively seal off the Venetian Lagoon from the wider Mediterranean Sea.

The Mose barriers mean the flood risks are currently manageable, but the frequency of their use is rising. In the first five years of use (between 2020 and 2025) the system was closed for 108 high waters, while in the first two months of 2026 it was activated 30 times. And as sea levels continue to climb, it would need to be closed more and more often – potentially for weeks at a time each year.

This creates a series of problems. Frequent closures would disrupt shipping and tourism, alter the lagoon’s ecology, and would require major new systems for sewage treatment and huge pumps to maintain lagoon water levels. A system designed for occasional protection risks becoming a semi-permanent barrier – something it was never intended to be.

With additional measures, such as raising the city by injecting sea water into the rocks deep underground, reversing the subsidence to some degree, these barriers could remain effective for some time – perhaps even after a metre of sea-level rise.

But even under relatively low levels of warming, the sea is projected to keep rising for centuries, eventually pushing beyond what the barriers can handle.

At that point, more radical measures may be necessary. Building a ring of dykes around the city would physically separate Venice from the lagoon, but may be necessary by the end of this century.

A fully enclosed lagoon – protected by a much larger “super levee” and supported by continuous pumping – could protect the city from up to 10m of sea level rise, but at severe cost to the living lagoon.

The only other option is to relocate the city to safer ground. This may be necessary beyond about 5m of sea-level rise, which is projected to occur after 2300.

Difficult choices ahead

The financial costs of these choices are substantial. We used the costs of Mose and other previous engineering projects (adjusted for inflation to 2024 prices) to estimate the cost of each adaptation strategy.

The dykes could cost between €500 million and €4.5 billion. Closing the lagoon with a super levee could initially cost more than €30 billion, and relocating the city could cost up to €100 billion.

But costs aren’t the only issue. How do you even put a price on the cultural value of Venice? Especially as none of these measures will be able to sustain the Venice we see today in the long-term. Adaptation can manage change up to a certain point – beyond that, we are no longer preserving the present. Rather, we are designing a fundamentally different future.

Our analysis shows there is no optimal adaptation strategy. Any approach involves trade-offs between 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. In addition, many of these measures can take decades to fully implement, so early planning is essential.

At least Venice is thinking about these things in a long-term way. Most vulnerable coastal areas are not. In fact, many continue to attract businesses and people, even as rising seas gradually narrow the range of viable long-term options.

With its long and unique history, Venice has particular challenges, but all low-lying coastal areas should recognise the danger of long-term sea-level rise and start preparing now.

Robert James Nicholls received funding from the Horizon 2020 program of the European Commission through the CoCliCo project (#101003598).

Piero Lionello has received funding from Italian Ministry for Education and Research (PNRR-HPC Center). Piero LIonello is a member of the Scientific Committee of the International Centre for Climate Change Research and Studies Co-coordinator of the MedECC network (Mediterranean Experts on Environmental and Climate Change)

Marjolijn Haasnoot does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.