The city of Venice, built upon a shallow lagoon, has long faced a challenge from the water that defines it. This fragility is evident in the recurring phenomenon known as acqua alta, or “high water,” where exceptionally high tides inundate the lowest parts of the city. Although Venetians have adapted to this periodic flooding for centuries, the frequency and severity of these events have dramatically increased in recent decades. The city’s vulnerability is the result of a dual threat: the ground beneath it is sinking, while the sea level around it is steadily rising.
Local Factors Contributing to Subsidence
The physical sinking of Venice, known as subsidence, is a long-standing geological reality independent of global climate shifts. The city is constructed on soft, unconsolidated layers of sand, silt, and clay deposited over millennia by rivers flowing into the Adriatic Sea. These sedimentary layers naturally compress under their own weight and the load of centuries-old buildings, causing the ground to settle slowly over time. This natural geological subsidence has historically caused the city to sink at a rate of approximately 1 to 2 millimeters each year.
Adding to this natural process was a period of intense human activity during the 20th century that significantly accelerated the descent. From the 1930s through the 1960s, industries on the nearby mainland, particularly in Marghera, extracted enormous volumes of groundwater from the deep aquifers beneath the lagoon. This removal of fluid reduced the pressure supporting the sediment layers, causing them to compact much faster than normal. This anthropogenic subsidence contributed to a loss of land elevation totaling about 15 centimeters in the 20th century alone.
Although the practice of groundwater extraction was banned by the 1970s, halting the accelerated sinking, the overall loss of ground elevation remains permanent. The city’s position is also influenced by slow-moving tectonic forces, as the Adriatic Plate shifts beneath the region. Historically, the combined effect of geological subsidence, human-induced compaction, and early sea level rise lowered Venice’s ground level by roughly 23 to 25 centimeters relative to the mean sea level of the late 19th century. This loss of elevation means the city starts every tidal event from a significantly lower baseline.
Global Warming and Accelerated Sea Level Rise
While local subsidence set the stage for vulnerability, contemporary global warming is the primary driver behind the accelerating crisis and the increased severity of acqua alta. Climate change raises the global sea level through two mechanisms. The first is the thermal expansion of seawater, where rising ocean temperatures increase the overall volume of the ocean basin. The second involves the addition of water from melting land-based ice, specifically glaciers and ice sheets, which pour meltwater into the sea.
The result of these global phenomena is a marked acceleration in the rate of sea level rise around Venice. Historically, the average global sea level rise was measured at about 1.23 millimeters per year between 1872 and 2019. However, in the last few decades, this rate has nearly doubled, with measurements indicating an average rise of approximately 2.76 millimeters annually between 1993 and 2019. This faster rate means that the water level is continuously pushing the high tide mark higher, increasing the frequency of flooding.
This continuous rise translates to more frequent and damaging acqua alta events. For example, the city’s lowest point, Piazza San Marco, now experiences flooding hundreds of times a year, a stark increase from the historical average. Future projections indicate that this trend will worsen, with relative sea level rise in the Venice lagoon by the year 2100 estimated to range from 21 to 52 centimeters, or up to 58 to 110 centimeters under high-emission scenarios. Such an increase would make the city permanently uninhabitable without major intervention.
Engineering Responses and Long-Term Protection
In response to the escalating threat, Venice implemented the MOSE system (Modulo Sperimentale Elettromeccanico). This infrastructure consists of 78 mobile steel gates positioned across the three inlets connecting the lagoon to the Adriatic Sea. Under normal conditions, these gates lie flat on the seabed, but when a high tide exceeding 110 centimeters is forecast, compressed air is pumped into the structures. This process causes them to rotate upward on hinges, forming a solid barrier that isolates the lagoon from the incoming sea surge.
The MOSE system is designed to protect the city from water levels up to three meters high and was built to withstand a sea level rise of 60 centimeters. Since its first deployment in 2020, the barriers have effectively prevented several major high-water events. However, the system faces limitations regarding its long-term viability and routine operation. The established activation threshold of 110 centimeters means that more frequent, lower-level flooding, which begins to affect Piazza San Marco at around 80 centimeters, is not prevented.
The frequent closing of the gates, which can cost hundreds of thousands of euros per activation, disrupts the natural exchange of water with the sea. This reduced circulation negatively impacts the lagoon ecosystem by altering water quality and potentially harming salt marsh habitats. Climate projections suggest that if the sea level rises by 50 centimeters, the barriers might need to close over 300 times a year, rendering the system impractical and causing severe environmental damage. Consequently, other strategies are also being employed, including raising quaysides, restoring the lagoon’s natural defenses, and using temporary glass barriers to protect the lowest-lying architectural landmarks.