Venice and sea-level rise

Venice is susceptible to episodic flooding, owing to a concurrence of high tides, storm surges, and excessive rainfall. Therefore, Venice serves as an excellent indicator of long-term changes in sea level, such as those due to global warming.

Background

On November 4, 1966, Venice, Italy, was inundated by a flood of 1.94 meters (6.36 feet) above mean high water. The extent of the flood reflected the combination of high lunar tides, heavy rainfall, and fierce, persistent winds. The storm surge overwhelmed the Lidi, destroying sea walls. Meanwhile, flooding rivers poured freshwater into the brackish lagoon, causing ecological harm. Bases of historic buildings were immersed in lagoon water for two days, producing considerable damage. This set a new record for high water (acqua alta) in the city, although a 2019 flood reached 1.87 meters (6.2 feet) and nearly broke the decades-long record. The 2019 flood killed two people and caused roughly $1 billion (USD) in damage.

Sensitivity to Flooding

Venice is a particularly sensitive indicator of long-term changes in sea level, because several factors causing the lagoon water to rise, or the land level to sink, are poised against human interventions, such as dike building and diversion of rivers. Sources of flood waters are rivers that originally emptied into the lagoon, including the Piave and the Sile in the east and the Brenta and Bacchiglione in the west. All these rivers have been diverted away from the lagoon, but during episodes of heavy rainfall they manage to overflow into the lagoon anyway. Heavy rains over the ocean can also augment storm surges in raising sea level.

Venice itself, meanwhile, has been sinking in a process called subsidence. Subsidence can be natural, as when sediment is compressed by the weight of soil and water above it, or artificial, caused, for example, by removal of water from an underlying aquifer for industrial purposes. Such removal was forbidden by statute in the 1970s, and Venetian subsidence was substantially reduced as a result, however, data from the 2000s revealed that Venice was still subsiding about 1–2 millimeters a year. By 2024, the city had sunk an estimated 23 centimeters (9 inches) over the last century.

Historical Remediation Efforts

Throughout its long history, Venice has been both threatened and protected by the proximity of its lagoon, and its citizens have acted to minimize threats while promoting protection. Thus, the lagoon has been kept intact but with efforts to prevent storm-related changes in sea level. One task has centered on the rivers that could otherwise transport sediment and gradually convert the lagoon into dry land with pockets of freshwater, which would harbor malaria-transmitting mosquitoes. The island town of Torcello succumbed in medieval times to a combination of silt from the Sile river and resulting malaria. Therefore, beginning in 1324, the courses of the several rivers in the area were progressively diverted to avoid the lagoon, with a canal receiving water from the Brenta. Additional links were completed in 1507 and 1613, the last one allowing the waters from several rivers to flow into the Adriatic Sea through a common mouth. Meanwhile, dikes and seawalls were built, including the great St. Mark’s Dike to the north of the lagoon (1534) and others protecting the barrier islands (Lidi).

Acqua alta episodes in the twentieth century led to demands for government intervention, resulting in Special Law 798. Earlier such special laws had been ineffectual, but this one was different. It called for creation of a committee (the Comitatone, or “big committee”) with the prime minister as chair. The committee proposed several interventions, the most dramatic of which was a system of movable sea gates in the three passages separating the barrier islands.

To investigate this strategy, a prototype gate, the modulo sperimentale elettromeccanico, or MOSE, was constructed and towed into place in one of the channels. It spent from 1988 until 1992 in place and, by most accounts, performed well. The principle of its operation was simple: Most of the time, the gates would be submerged owing to the weight of water in their tanks. When a flood threatened, the water would be expelled and the gates would rise to block any storm surge. The single MOSE gate cost the equivalent of about 13.3 million dollars, and a total of about eighty gates would be required for the entire system to function.

In addition to the great cost of the project, critics questioned its adequacy, especially in light of the rate of sea-level rise from global warming. They feared that, once operational, the gate system would need to be deployed so often that the lagoon would have insufficient time to flush itself, seriously hindering shipping activity. Significant work on the MOSE project began in 2003; after multiple delays and extensions of the timeline, the first full successful test of the system was conducted in July 2020. Upon completion, MOSE consisted of seventy-eight mobile barriers and cost roughly $8 billion to construct. In October 2020 MOSE was used to combat flooding and was used dozens of times in subsequent years. However, if global emissions continued to increase, the Intergovernmental Panel on Climate Change predicted an increase of sea level rise in Venice of between 24 and 43 inches (60 and 110 centimeters). MOSE would not be able to handle this increase.

Changing Frequency of acqua alta Events

In the early 1900s, St. Mark’s Square on the Venetian island was flooded seven times per year; in the 1950s, the frequency was twenty times per year. In 1996 alone, it was flooded ninety-nine times. By the 2020s, St. Mark's Square flooded nearly every day, about 250 times a year. This progression reflected two aspects of climate change: a general rise of sea level and a related increase in the number and severity of storms. In all events, the frequency of acqua alta occurrences seems to provide a measure of climate warming and to support the notion of Venice as a useful indicator of its extent. The deadly 2019 flood also demonstrated the continued possibility of catastrophic floods and the risk these events posed to both human life and the city's economic activity.

Context

Venice is not the only location in the world vulnerable to sea-level rise. Many cities are located on estuaries and coastlines and thus are affected by even modest sea-level rise. For instance, London is situated on the Thames estuary and has experienced repeated floods throughout its history. Because of this, a storm-surge barrier has been under discussion through most of the twentieth century, was begun in 1973, and was completed in 1983. As climate-related sea-level rise proceeds, more and more coastal cities will become at risk, and many will not have the option of an estuarine barrier. For many of these cities, moving a sizable population might be the only remedy, with great cost and social dislocation. Much worse will be flooding of low-lying agricultural lands in countries such as Bangladesh, where extreme poverty and cultural dependence on rice culture will give rise to widespread suffering. Finally, coral-based Pacific island nations will simply vanish beneath the waves. Climate scientists, government officials, and experts across virtually every scientific discipline argue that the best, and perhaps only, solution is a worldwide reduction of greenhouse gas emissions.

Key Concepts

• acqua alta: Italian for high water—flooding
• Lidi: the chain of barrier islands protecting the Venetian lagoon
• MOSE: abbreviation for modulo sperimentale elettromeccanico, a system of sea gates protecting Venice from flooding; the name also alludes to Moses, punning on his ability to part the seas
• storm surge: high water associated with a storm, especially that caused by wind action
• subsidence: lowering of land surface owing, for instance, to compression by the weight of overlying land or depletion of an aquifer
• Venetian lagoon: the body of water in which Venice, formerly an island, is situated

Bibliography

Bubola, Emma, and Jeff Horowitz. “Venice Is Saved! Woe Is Venice.” The New York Times, 1 Apr. 2023, www.nytimes.com/2023/04/01/world/europe/venice-mose-flooding.html. Accessed 20 Dec. 2024.

Harlan, Chico and Stefano Pitrelli. "Venice's Plan to Protect Itself from Flooding Because a Disaster in Itself." The Washington Post, 19 Nov. 2019, www.washingtonpost.com/world/europe/how-venices-plan-to-protect-itself-from-flooding-became-a-disaster-in-itself/2019/11/19/7e1fe494-09a8-11ea-8054-289aef6e38a3‗story.html. Accessed 20 Dec. 2024.

Masters, Jeff. “Venice Has Its Worst Flood in 53 Years.” Scientific American, 14 Nov. 2019, blogs.scientificamerican.com/eye-of-the-storm/venice-has-its-worst-flood-in-53-years/. Accessed 20 Dec. 2024.

“MOSE System.” MOSE Venezia, www.mosevenezia.eu/project/?lang=en. Accessed 10 Dec. 2024.

Norwich, John Julius. A History of Venice. Vintage Books, 1989.

Phelan, Jeff. “Italy's Plan to Save Venice from Sinking.” BBC Future Planet, 27 Sep. 2022, www.bbc.com/future/article/20220927-italys-plan-to-save-venice-from-sinking. Accessed 10 Dec. 2024.