Venice, a UNESCO World Heritage site, faces an environmental challenge driven by the slow but persistent sinking of its landmass. This process, known as subsidence, threatens the city’s future by reducing its elevation relative to the surrounding sea. Understanding the speed of this vertical movement is important for scientists and engineers working to preserve the historic city’s structures and unique cultural heritage. The crisis is a gradual reduction in the city’s margin against the tides, making precise measurement of the sinking rate a scientific priority.
Quantifying Vertical Movement
The city of Venice is currently sinking, averaging between 1 and 2 millimeters per year. This rate represents the land’s downward motion, resulting from natural geological processes and tectonic forces. Areas within the lagoon, such as surrounding islands and marshlands, sometimes exhibit higher rates, reaching 3 to 4 millimeters annually.
Scientists track this movement using an integrated monitoring system combining two advanced technologies. Continuous Global Positioning System (GPS) receivers provide absolute elevation data. This data is combined with space-borne radar interferometry (InSAR), which uses satellite radar signals to measure ground deformation with millimetric accuracy. This approach confirms the city is also tilting slightly eastward as the land subsides.
Geological and Anthropogenic Causes
The land beneath Venice rests on deep layers of soft, compressible Holocene sediments. The weight of the city’s buildings naturally compacts these underlying layers, causing slow, ongoing geological subsidence. This natural sinking is compounded by the city’s location on the Adriatic tectonic plate, which is slowly subducting beneath the Apennine Mountains, contributing to the overall drop in elevation.
Human activity accelerated the sinking in the mid-20th century. Industrial operations extracted vast quantities of groundwater from the subsurface aquifer system in the nearby mainland. This fluid removal caused sediment pores to collapse, leading to rapid, human-induced subsidence that peaked at 6 millimeters per year in the 1960s. Groundwater extraction has since been halted, successfully slowing the anthropogenic contribution, though geological factors continue their slow work.
The Role of Rising Adriatic Sea Levels
The problem of Venice sinking is significantly worsened by the simultaneous rise in sea level. Global sea-level rise, driven by thermal expansion of warming ocean water and the melting of glaciers, contributes to a higher baseline water level in the Mediterranean. Tide-gauge measurements in Venice, after accounting for land sinking, show the local sea level rising at approximately 2.76 millimeters per year in recent decades.
This global effect is exacerbated by local factors within the Adriatic Sea. The shallowness and enclosed shape of the Adriatic basin amplify water movement, making the area highly susceptible to storm surges. Strong, persistent Sirocco winds blowing across the sea can push large volumes of water into the Venetian Lagoon. This combination of astronomical high tides, low atmospheric pressure, and wind-driven water stacking triggers the damaging high-water events, locally known as Acqua Alta.
Current Protective Measures
The engineering response to the combined threat of subsidence and rising sea levels is the MOSE system. This project is an array of 78 mobile steel floodgates installed across the three inlets connecting the Venetian Lagoon to the Adriatic Sea. The gates are designed to isolate the lagoon from extreme high tides and storm surges.
The hollow barriers rest on the seabed during normal conditions. When a high tide is forecast to exceed a specific threshold, compressed air is pumped into the gates, expelling the water they contain. This buoyancy causes the gates to rotate upward on their hinges, creating a temporary sea barrier. The MOSE system became fully operational in 2020 and has successfully prevented several severe Acqua Alta events from flooding the historic city center.