Sea level rise in Venice

Aerial image of Venice and adjacent Lagoon

The future impacts of sea level rise on Venice, Italy, represent a significant concern for the city, which is renowned for its cultural heritage and unique lagoon ecosystem. Due to climate change-induced rising sea levels, Venice faces increasing risks of flooding and structural damage. Historically, the city's intricate network of canals and low-lying land have made it particularly vulnerable to changes in sea levels, affecting both its infrastructure and cultural landmarks. Current trends and scientific projections indicate that the frequency and severity of flooding events are likely to increase, posing substantial economic, social, and environmental impacts. Various adaptation and mitigation strategies, such as the MOSE project and other flood prevention systems, are being implemented to safeguard the city.

Historical context of sea level changes in Venice

Over the last three decades, Venice has experienced significant changes in relative sea level which has been driven by a combination of global sea-level rise and local subsidence. Between 1993 and 2019, tide-gauge measurements in Venice, after correcting for subsidence, indicate an average sea-level rise of approximately 2.76 ± 1.75 mm per year. This rate is notably higher than the historical trend observed between 1872 and 2019, which was estimated at 1.23 ± 0.13 mm per year.

Venetian sea level is closely linked to broader Mediterranean and Adriatic Sea trends, which, in turn, are influenced by complex oceanic and atmospheric dynamics. The exchange of water through the Strait of Gibraltar is a major factor contributing to uncertainties in regional sea-level rise projections. Also, short-term fluctuations due to atmospheric pressure variations, storm surges, and tidal cycles have exacerbated the flooding events experienced in the city.

Sea-level rise has already impacted Venice and will bring growing challenges. The frequency and intensity of acqua alta (high water events) have increased, leading to disruptions in daily life, damage to infrastructure, and a greater need for adaptive measures. In response, large-scale engineering projects like the MOSE (Modulo Sperimentale Elettromeccanico) barrier system have been developed to protect the city from extreme flooding. While these efforts have provided temporary relief, continued sea-level rise and ongoing subsidence remain pressing concerns for the long-term sustainability of Venice.

Projected sea level rise scenarios

Future sea level rise poses a significant threat to the Venice Lagoon and its complex salt marsh ecosystems. A 2018 study by Ivajnšič et al. modeled the impacts of various sea level rise (SLR) scenarios on the Venice Lagoon using high-resolution geospatial data and habitat-based statistical modeling. The research evaluated four distinct SLR scenarios—three model-based (GFDL P50, RCP4.5, RCP8.5) and one based on a historical linear trend—across multiple time frames up to the year 2100.

The findings revealed stark differences between the northern and southern sectors of the lagoon. Under all scenarios, the northern part is expected to face earlier and more severe submergence due to its lower sediment accretion rates. By 2075, between 37% and 51% of the lagoon’s salt marshes are projected to be underwater, depending on the scenario. By 2100, model-based scenarios GFDL P50 and RCP8.5 predict near-total inundation of marsh habitats (up to 99.8%), while more moderate projections like RCP4.5 and the linear trend scenario estimate losses of 41% to 55%.

The study also emphasized the variability in habitat resilience. Lower-elevation marshes and pioneer vegetation zones, such as those dominated by Spartina maritima, are most vulnerable and could disappear from the northern lagoon as early as 2075. In contrast, some habitat types in the southern lagoon may persist longer due to relatively higher sedimentation rates, which help offset rising sea levels.

These projected scenarios underscore the urgent need for adaptive management strategies in Venice. Without significant intervention, much of the lagoon’s ecological diversity and flood-buffering capacity could be lost within decades.

Economic consequences

The MOSE system (Modulo Sperimentale Elettromeccanico), a series of mobile gates designed to protect the Venetian lagoon from high tides, represents a significant financial undertaking for Venice. The cost of the MOSE system has already exceeded €6 billion, reflecting the substantial investment required to safeguard the city from rising sea levels. Despite its high cost, the benefits of the MOSE system are expected to outweigh the expenses, as it provides crucial protection against flooding. However, the operation and maintenance of MOSE are expensive, adding to the financial burden on the city.

The implementation of the MOSE system also incurs direct costs on ship traffic, ranging from €348,000 to €1.3 million, due to longer wait times for harbor access. While these costs impact port activities, they are considered minor compared to the broader benefits the system provides to the city.

Increased flooding has a negative impact on tourism demand, a vital component of Venice's economy. The frequency and intensity of "acqua alta" (high water) events lead to direct economic losses for businesses due to closures and damage to goods. Indirect losses arise from decreased tourism and disruptions to daily life. For instance, the November 2019 flood was particularly devastating, causing extensive damage and highlighting the need for frequent and costly interventions. As a result, hotel prices tend to decrease when there is a greater chance of flooding, further affecting the tourism sector.

The damage to historic buildings and cultural heritage is another significant concern. Venice's unique architectural and cultural heritage is extremely vulnerable to saltwater intrusion and rising damp. Preservation and restoration efforts are not only expensive but also increasingly challenging with more frequent and severe flooding. This vulnerability impacts tourism, which is a cornerstone of Venice's economy, as the city's cultural sites are a major draw for visitors.

Threats to historical sites and cultural landmarks in Venice

The city of Venice, built around a network of canals, features many historic buildings with stairways that once provided access to waterways. These structures often included waterproofed basements designed to mitigate water intrusion. However, recent increases in sea levels have rendered many of these traditional flood-prevention methods insufficient. Numerous historical sites have already suffered damage due to the rising tides.

Significant deterioration has affected cultural landmarks such as the Teatro La Fenice opera house and the Cathedral of Murano. In addition to water damage, the accumulation of salt has accelerated the degradation of building materials. Indoor monuments not directly exposed to flooding are experiencing deterioration due to increased moisture and repeated wetting. While local building codes continue to be updated to ensure new constructions are more resilient, existing historical structures remain particularly vulnerable to environmental threats.

Impact on Venice's infrastructure

Impact on sewage systems

Rising sea levels pose significant challenges to Venice's sewage systems, primarily through the exacerbation of saltwater intrusion and backflow issues. As sea levels rise, saltwater can penetrate further into the sewage infrastructure, disrupting its functionality and increasing the risk of contaminating freshwater supplies. During high tide events, known locally as "acqua alta," the sewage system can become overwhelmed, leading to overflows. These overflows contribute to pollution in both the lagoon and the city streets, compounding the environmental and public health challenges faced by Venice.

Impact on building foundations

The constant exposure to saltwater significantly affects the structural integrity of Venice's buildings, leading to long-term challenges in preservation and restoration. Saltwater intrusion weakens building foundations over time, contributing to structural instability. This is particularly problematic in Venice, where many buildings are constructed using porous materials such as brick and limestone, which readily absorb saltwater. The absorption of saltwater accelerates the deterioration of these materials, causing them to crumble and erode. As a result, the costs associated with maintaining and restoring these historic structures are increasing. The ongoing rise in sea levels exacerbates these issues, as more frequent and severe flooding events lead to greater saltwater exposure.

Adaptation and mitigation strategies: current and proposed measures to protect Venice

Venice faces increasing threats from rising sea levels and storm surges, prompting the implementation of several protective measures. One of the most significant projects is the MOSE (MOdulo Sperimentale Elettromeccanico) system, which consists of 78 retractable gates located at the three inlets of the Venetian Lagoon—Lido, Malamocco, and Chioggia. These gates are connected between artificial islands housing the handling systems and are designed to rise the day of storm surges, which are identified 1 to 5 days before a high tide event. MOSE can protect the city from tides of up to three meters; however, due to accelerating climate change, it is estimated to be effective for only about three more decades before requiring replacement. The project has been controversial due to its high cost and concerns over its environmental impact, particularly the potential for water stagnation in Venice’s canals during extended periods of deployment. This stagnant water could lead to a loss of biodiversity due to the build up of waste water.

Acqua Alta in Piazza San Marco 2004

In addition to MOSE, other protective measures have been implemented. Notably, transparent glass barriers have been installed around St. Mark's Basilica to shield it from flooding. These 1.2-meter-high barriers protect the basilica against flood levels up to two meters, helping to preserve the historic landmark. Piazza San Marco, one of the lowest points in Venice, has been raised multiple times to prevent flood damage but has now reached the maximum elevation possible without altering its architectural stylie. Additionally, upgraded drainage systems have been installed to mitigate flooding; while they have reduced water accumulation, they have not fully prevented it.

Furthermore, coastal reinforcement efforts and the elevation of urban areas are underway to mitigate flooding risks. These combined strategies aim to prolong the city's resilience against rising waters, though long-term solutions will still be necessary.