The frequent and dramatic flooding events known as Acqua Alta have become a defining feature of life in Venice, threatening its historic architecture and cultural heritage. This phenomenon prompts a simple question: is the city sinking into the Adriatic Sea, or is the water itself rising? The reality is an interplay of both forces acting simultaneously, creating a complex and accelerating vulnerability for the lagoon city.
The Geological Reality of Subsidence
The foundation upon which Venice is built is naturally predisposed to downward movement. The city rests on a thick layer of soft, unconsolidated sediments—sand, silt, and clay—deposited by the Po and other rivers in the Po Plain. Over thousands of years, the sheer weight of the islands and their stone buildings has caused these underlying layers to compact, a process known as natural geological subsidence. This long-term sinking contributes approximately 1 to 2 millimeters of vertical land loss each year, a slow but relentless descent.
This natural process was accelerated during the 20th century by human activity. Extensive pumping of groundwater and methane gas from deep aquifers beneath the Venetian Lagoon created voids in the subsoil. This extraction caused the layers to compact much faster, inducing an accelerated, anthropogenic subsidence that peaked in the mid-1900s. Although groundwater pumping was halted in the 1970s, the historical damage resulted in a permanent loss of ground elevation of about 15 centimeters.
Eustatic Sea-Level Rise
Working against the sinking land is the global upward trend of the sea surface, known as eustatic sea-level rise. This phenomenon is a direct consequence of climate change, driven by two physical mechanisms. The first is the thermal expansion of seawater, as the ocean absorbs excess heat trapped by greenhouse gases, causing the water molecules to expand and occupy a greater volume.
The second contributor is the melting of land-based ice, primarily from continental ice sheets in Greenland and Antarctica. As this meltwater flows into the oceans, it adds mass and volume to the global sea level. Corrected for local land movement, the rate of eustatic rise in the northern Adriatic Sea has accelerated, increasing from a historical average of about 1.23 millimeters per year to a higher rate in recent decades.
This modern, climate-driven rise exceeds long-term averages. For the period between 1993 and 2019, the sea level in the Venice region, independent of land sinking, has been increasing by approximately 2.76 millimeters annually. This acceleration means the water is encroaching upon the city at a faster pace, compounding the vulnerability caused by geological instability.
Relative Sea-Level Rise: The Combined Threat
The true measure of Venice’s vulnerability is the relative sea-level rise, which represents the cumulative effect of land sinking (subsidence) and global sea level rising (eustatic rise). This combined rate determines the effective loss of elevation the city experiences relative to the sea surface. Historically, this has accounted for an average relative rise of approximately 2.5 millimeters per year.
Over the last century, the city has lost a total of about 23 to 24 centimeters in elevation relative to the sea. This change has increased the frequency and severity of Acqua Alta events, as the threshold for flooding is reached more easily. Projections suggest that under a high-emission climate scenario, Venice could experience a relative sea-level rise of 58 to 110 centimeters by the year 2100.
This combination of sinking land and rising water explains why Venice is disproportionately affected compared to other coastal cities. Even if the local subsidence rate is currently small, the compounding effect of an accelerating eustatic rise means the city is rapidly losing its protective buffer against storm surges and high tides. The continuous erosion of this safety margin highlights the effects of combined land-ocean processes.
Engineering Resilience: The MOSE Project
To counter this combined threat, Italy constructed the Modulo Sperimentale Elettromeccanico, better known as the MOSE Project. This civil engineering endeavor consists of 78 hinged steel gates installed across the three inlets that connect the Venetian Lagoon to the Adriatic Sea: Lido, Malamocco, and Chioggia. The system is designed to create a temporary, sealed barrier against exceptionally high tides and storm surges.
When forecasts predict a tide level of 110 centimeters or higher above the reference point, the gates are activated. Normally resting flat on the seabed, compressed air is pumped into the hollow structures, expelling the water and causing the gates to rise and block the incoming tide. The entire process takes less than 30 minutes to deploy and can protect the city from water levels of up to three meters.
While the system has successfully protected Venice from severe flooding since its first operational use in 2020, its operation presents challenges. Each activation of the barriers is estimated to cost around €300,000, contributing to the total project cost of approximately €7 billion. Closing the inlets disrupts the natural tidal circulation of the lagoon, which is necessary for flushing pollutants and maintaining the health of the wetland ecosystem.
The lowest parts of the city, such as St. Mark’s Square, can begin to flood at tide levels as low as 80 centimeters. Since the MOSE gates are only raised at the 110-centimeter threshold, the city still experiences frequent, moderate flooding events that impact daily life and tourism. With projected sea-level acceleration, the barriers may need to be closed more frequently, potentially sealing the lagoon off from the sea for a significant portion of the year.