Acqua alta results from several physical processes combining, not from any single cause. The baseline is the ordinary astronomical tide, which in the Venice lagoon can contribute an anomaly of roughly 50 cm during spring tides. Layered on top of that is the meteorological surge: sirocco winds blowing up the length of the Adriatic from the southeast pile water into the enclosed, shallow northern basin where Venice sits, while low atmospheric pressure allows the sea surface to rise further. These surges arise from planetary-scale atmospheric waves lasting 10 to 100 days, from individual cyclones, and from smaller mesoscale weather systems (Lionello et al., 2021, p.2637). A separate mechanism, the seiche, is a standing oscillation of the entire Adriatic basin with a period of roughly 23 hours; seiches can raise Venice's water level even without any storm at all. When astronomical tide, meteorological surge, and seiche peaks coincide, water levels spike sharply.
Two long-term trends compound this variability. First, Venice has subsided relative to the sea. Industrial-era groundwater extraction caused rapid land subsidence through the mid-20th century, driving relative sea level rise to about 5 mm per year between 1950 and 1970; this cause has been substantially controlled since the 1970s through regulation of groundwater pumping (Lionello et al., 2021, p.2636). Second, eustatic sea level, the rise of the ocean itself, continues independent of local subsidence. Over the past 150 years, Venice's relative sea level rose at an average of 2.5 mm per year, but the most recent 30-year record (1991-2020) shows a rate of 4.9 mm per year, nearly double the long-term average (Ferrarin et al., 2024). The consequence is a marked acceleration in flooding frequency: events exceeding 120 cm above the reference datum rose from fewer than 2 per decade in the early 20th century to 40 events in the 2010s, and events exceeding 110 cm rose from 4.2 to 95 per decade over the same span (Lionello et al., 2021, p.2635). The catastrophic flood of November 4, 1966 reached 194 cm and stayed above 110 cm for 22 consecutive hours, remaining the highest level ever recorded; November 12, 2019 produced the second-highest level, 189 cm, in a month that saw 15 separate events above 110 cm (Lionello et al., 2021, p.2634).
The MOSE barrier system, 78 mobile gates across the lagoon's three inlets, became operational in 2020 and is raised ahead of forecast tides above roughly 110 cm to seal the lagoon from the Adriatic. It has prevented dozens of floods since becoming active, but it has real limits. Because sea levels keep rising, the barrier must close more often over time: under all emission scenarios, 3-week annual closures are considered virtually certain before the end of the century, and under high-emission scenarios closures of up to 6 months a year become likely (Lionello et al., 2021, p.2638). Frequent, prolonged closures create a secondary problem, since they cut the lagoon off from the tidal exchange that flushes it and moderates its temperature; under severe climate scenarios, gates could need to stay shut over 20% of autumn and early winter, intensifying ecological stress on the lagoon and raising the risk of marine heat waves reaching four times open-sea intensity in shallow tidal flats by 2050 (Ferrarin et al., 2024). Under the high-emission scenario, the magnitude of a 100-year flood event is projected to increase 65% by 2050 and 160% by 2100, and relative sea level itself could rise between 30 and 110 cm by 2100, with a plausible high-end scenario exceeding 180 cm (Lionello et al., 2021, p.2638-2639). MOSE buys time; it does not remove the underlying trend.
