Approximately one third of these sites are located in Italy, followed by Croatia, Greece, and Tunisia. 47 of these 49 sites may be at risk from at least one of these hazards, flooding or erosion, by the end of the century. Until 2100, flood risk may increase by 50% and erosion risk by 13% across the region, with considerably higher increases at individual sites (21).

On the one hand, these risk estimates may be overestimated for certain sites, as no coastal protection measures were assumed in the assessment. Venice is a clear example of a World Heritage site that will be well protected by flood barriers in future decades. On the other hand, risk estimates may be underestimated in certain locations, as the assessment also did not account for human-induced land subsidence. Subsidence due to ground water extraction can be high in cities such as Venice (23) and Istanbul, and in river deltas such as those of the Nile, Po and Rhone (24).

In Italy 18.6% and 19.4% of the total number of cultural assets is at risk of landslides and floods, respectively. These risks are directly related to climate change. While for landslides the risk is well distributed in the country with prevalence in the Centre of Italy, for floods the risk is concentrated above all in the North (26).

Tourism is Venice’s most important economic resource, but also a major source of pollution, and negatively influences the quality of life of the inhabitants. At present the mean ratio between the number of tourist presences and that of the inhabitants is about 160 (1).

The lagoon of Venice is the largest Mediterranean lagoon. It is one of only two tidal coastal wetlands in the Mediterranean region (2).

The lagoon of Venice - Permanently unstable

The Lagoon of Venice, like most lagoons, is liable to major and sudden variations and, consequently, its equilibrium is permanently unstable. This is the cause of its great changes, evolving either into a marine bay or, by a natural succession, through a salt-marsh environment, to dry land. As a consequence, on the geological scale, the life of a lagoon is always rather short.

Indeed, the load of solid material discharged into the lagoon from its tributaries would bury it or, if the violence of the sea exceeded the solid load from the watershed, the sandbars protecting the lagoon would be demolished and the lagoon transformed into a marine bay. This would be the fate of the Lagoon of Venice without the uninterrupted work undertaken by the Serenissima Republic of Venice to arrest the processes of the natural succession. Thanks to this work, the lagoon has maintained its identity through the centuries. The Lagoon of Venice is protected from the Adriatic Sea by sandbars (Cavallino, Lido and Pellestrina) which are interrupted by three inlets-opening (bocche di porto): Lido, Malamocco and Chioggia (1).

About a quarter (140 km2) of the total lagoon surface is occupied by mud flats which form marshy areas (barene), emerging from the water only during low tide. Another characteristic of the Lagoon of Venice consists in its numerous islands, many of them located in the central basin of the lagoon. The total area occupied by the islands amounts to 40 km2 (1).

At present, the most crucial problems of Venice and its lagoon are: the lowering of the level of the city in relation to that of the sea, the continuous decline in the number of inhabitants, the increasing frequency of the “high water” phenomenon, air and water pollution, and the increased erosion and salinity of the lagoon (1).

The lagoon of Venice - Subsidence and sea level rise in the past

During the 20th century, the mean relative sea level in Venice has increased by 0.3 m (13). Previous reports mention a lower relative sea level rise, however, from 13 to 15 cm in the 20th century (4), and 2.5 mm/year averaged over the last 150 years (27). Approximately half of it is due to vertical land motion (27), with two main contributions: local anthropogenic subsidence, mostly caused by the extraction of groundwater in the past, and long-term tectonic vertical motion. While the extraction of ground water has stopped (14), tectonic motion is expected to continue, though eventually not at a regular pace. Furthermore, sea level is expected to rise in the future at a pace that, as it depends on the action of many regional factors, is uncertain and cannot be easily related to global sea level rise. Thermosteric expansion, air pressure and wind forcing have been proven to be important for the Mediterranean sea level over the last century and will continue to be relevant for what is going to happen over the 21st century (13).

The current best estimate of the average rate of absolute sea level rise during the observational period from 1872 to 2019 based on tide-gauge data after removal of subsidence effects is 1.23 ± 0.13 mm/year. A higher – but more uncertain – rate of sea level rise is observed for more recent years: between 1993 and 2019, an average rate of sea level rise of about 2.76 ± 1.75 mm/year is estimated from tide-gauge data after removal of subsidence (27).

As an effect of subsidence, the Adriatic coasts are slowly but steadily sinking. The loads of solid material from the rivers flowing into the Adriatic Sea are distributed along the coast by the currents and amply counterbalance the coastal lowering rate. As a result, the Adriatic littoral zone is continuously advancing towards the sea. Conversely, in the Lagoon of Venice the subsidence effect is not offset by sediment accumulation because of the anthropogenic diversion outside the lagoon of its most important tributaries (1).

The natural subsidence seems to be aggravated by groundwater and natural gas extractions from the lagoon area; this exploitation was at its height from 1930 to 1970 (16,27). In addition, during the last five centuries the sea level has been rising slowly, but continuously (eustatism). The subsidence and eustatism processes combine to lower the level of Venice in relation to that of the sea; this is a source of permanent risk for Venice and the other urban centers of the lagoon, which are flooded with increasing frequency (3).

Venice is considered the world test-case city for sea level rise. Even if the journalistic description is occasionally exaggerated, the frequency of flooding increased dramatically in the second half of last century, both due to land subsidence and SLR. The present natural subsidence is limited to less than 0,4 mm/yr, while in the period 1950-1970 the actual rate of subsidence was as high as 7 mm/yr due to freshwater extraction from aquifers (2).

The load of solid material from the tributaries could, in the course of time, fill the lagoon; this process has been progressively accelerated by the increased erosion of the watershed caused by the clearing of forest land given over to pasture and crops, and the felling of trees for the wood used as building material and fuel (5). To prevent the lagoon filling up, the main water courses were diverted outside the lagoon from the XV to the XVII century; for example, the rivers Brenta, Marzenego, Bacchiglione and Piave. These works, in addition to reducing silting, had three main consequences on the lagoon (1):

1. the salinity progressively increased, as a result of the reduced freshwater load; in fact at present, most of the lagoon has salinity values of the same order of magnitude as those measured in the Adriatic Sea (30-33‰) except in the area near the inner coast;
2. the nutrient loads, and particularly that of phosphorus, decreased as a consequence of the reduced input from the terra firma and
3. the reduced silting no longer compensated for the subsidence rate, so that the mean depth of the lagoon and the erosion increased and the lagoon bottom became more flattened.

The lagoon of Venice - Subsidence and sea level rise in the future

For the future, controls on groundwater extraction should prevent a return to the large subsidence rates that occurred between 1930 and 1970. However, natural processes, notably consolidation, and anthropogenic activities are observed to continue. The sum of natural processes of subsidence, including sediment compaction, tectonics and GIA (Glacial Isostatic Adjustment), is estimated to be about 1.0 mm/year (30), with a constant rate on a centennial timescale. All natural and anthropogenic contributions have the potential to increase relative sea level rise in Venice, exacerbating the negative impact of local absolute sea level rise, with a contribution on the order of 10 cm per decade at the sub-regional scale and potentially much larger locally (27).

The likely range of relative sea level rise in Venice by 2100 ranges between 32 and 62 cm for a low-end scenario of climate change (RCP2.6) and between 58 and 110 cm for a high-end scenario of climate change (RCP8.5), respectively. A plausible but unlikely high-end scenario linked to strong ice-sheet melting yields about 180 cm of relative sea-level rise in Venice by 2100 (27). 

Damage by salt

Damage is caused by salts in the bricks or stone dissolving and then recrystallizing. The situation has been made worse by the dredging of deep-water channels for shipping, allowing more sea water to enter the lagoon and increasing the salinity of the water (20). 

The lagoon of Venice - History of flood protection

To protect the lagoon from the violence of the sea, the efficiency of the natural barriers (sandbars) was improved with various works. For example, after 1282 the burning or felling of trees growing on the littoral was forbidden, as was the removal of sand from the same zone after 1334. By the middle of the XIV century, the littoral zone of the sandbars had been protected with chalk boulders set in the areas most exposed to waves and currents, and the dunes were consolidated and protected against the winds with tamarisk (Tamarix gallica) plantations. Because the sandbars were not adequately protected by the chalk boulders, hurdles and piles, in 1740 at Malamocco and Pellestrina construction work started on the murazzi, which are walls made of Istria rocks cemented together to form an adequate barrier against the sea (1).

The Venetian Republic fell in 1797. After the annexation of Venice to the Italian Kingdom (1866) a new model was planned to stimulate the economy of the city. This plan involved extending Venice to the terra firma keeping separate its different rôles: the ancient city as a residential, touristic and cultural centre, and the terra firma (Marghera, Mestre) as an industrial port (1).

The necessity for protecting Venice and its lagoon from the high water became clear after an evaluation of the damage caused by the dramatic stormy sea of November 4, 1966, when the water level of the lagoon reached about 2 meters above the reference value. As a consequence of this damage to people, buildings and monuments, special laws were passed to save Venice and some short term measures have been taken. Because it is probable that in the near future the sea level, compared to that of the land, will continue to rise, as a combined effect of subsidence and eustatism, the prediction is that Venice and the other urban centers on the lagoon will be flooded more often (1).

Man-induced changes over the past century have increased the amounts of water exchanged with the sea through the inlets (due to their reconfiguration and construction of jetties) and reduced the lagoon’s resistance to incoming tides as a result of loss of extensive areas of saltmarsh and the excavation of the principal navigation channels. Consequently currents are stronger and erosion rates are high (2).

To protect the lagoon from the violence of the sea, the murazzi, seriously damaged by a storm surge on November 4, 1966, were restored at the beginning of the 90s, new sea-shores were built up and those eroded were enlarged (1).

To reduce the risk of subsidence in the lagoon area, methane extraction and the drilling of new artesian wells has been forbidden, and the quantity of water taken from the pre-existing wells reduced (1).

In 2003 the project MOSE was started: to build a storm surge barrier that could close the lagoon of Venice during high flood levels in the Adriatic Sea.

For the present sea level situation, the annual closure frequency of the gates of this barrier would be about 12 times with a total closure of 42 hours. Consequently, the total duration of the lagoon’s isolation from the sea would be too short to involve any damage to the lagoon ecosystem. Comparing the advantages and disadvantages of each proposal, the group of international experts concluded that Venice could be protected against exceptional episodes of high water both by mobile gates located at the three inlets of the lagoon and by raising the low-lying parts of the urban centers to 100 centimeters above the reference level (1).

The meteorological component (wind and atmospheric pressure (6)) is the main factor of the frequent flooding of Venice.

The lagoon of Venice - Pollution

The development of industry and the use of fertilizers and biocides in agriculture are the main causes of air and water pollution in the lagoon. Domestic sewage, rich in algal nutrients (e.g., phosphorus and nitrogen compounds), must be added to these pollutant loads. Even now the untreated sewage produced by the inhabitants of Venice and its numerous tourists (about twelve million annual presences) is directly discharged into the city’s 180 canals (called rii) and then dispersed in the lagoon by the tidal cycle (1).

In spite of the high concentration of nutrients in the lagoon water, before the 70s there did not seem to be any dramatic effects of eutrophication (8). During the 80s the bloom of Ulva rigida, Agard Had a dramatic effect on the lagoon ecosystem and represented an important nuisance to man and his activities (e.g. problems for navigation, bad smell, sulphydric acid emission, mass mortality of fish). Ulva was partly harvested each year during this period; for example, in 1987, 10,200 tons of the alga were removed from the lagoon (7). Many scientists believed that the most important cause of the Ulva bloom was probably the increased load of nutrients, and particularly nitrogen compounds, reaching the lagoon.

Apart from the nutrient load, there are other, no less important, causes such as the works which have modified the hydrological and morphological characteristics of the lagoon (e.g., deepening of inlets, digging of canals), and subsidence and eustatism (7). At the same time as the Ulva bloom appeared, there was a great increase in numbers of the midge Chironomus salinarius. … During the emergence of this insect, the enormous number of adults led to the Liberty Bridge becoming so slippery as to cause several car accidents. … During the late summer, in the same areas, when the decomposition of Ulva attained its maximum, the oxygen consumption was so high that anoxic conditions were produced, with the consequent decline of the benthic populations and the elimination of the most sensitive species. … it is very difficult to explain why at the beginning of the 90s the species progressively declined … It is interesting to note that up to now Ulva has not been replaced by other algal blooms of equivalent importance, and the decrease of nutrient concentrations in the water, the effect of effluent treatment at the border of the lagoon, is not so marked as to justify the decline of this opportunistic species.

During recent years, as a result of both the decline of industry and the implementation of stricter legislation aimed at reducing air and water pollution, the concentrations of nutrient and toxic substances in the environment have decreased (1).

Under the Venetian Republic the city canals were periodically dredged to prevent the excessive accumulation of sediments and activate the circulation of tidal currents, with a consequent cleaning of the canals. In the last few years, after about fifty years with no intervention, the dredging of the canals of Venice has started again(1).

The lagoon of Venice - Increase in storm surge frequency

A key problem for Venice is the increasing frequency of storm surges with the sea flooding the city (locally named acqua alta) that has reached an unsustainable level. After careful examination of both proxy documentary and instrumental data, it has been possible to reconstruct the extreme weather conditions and natural hazards, which have taken place in the last two millennia, with particular reference to surges (17). The results showed that the frequency of surges has dramatically increased since the 1960s, reaching about two flooding events per year, the greatest value since 792 AD.

The increase in the frequency of flooding events with the future rise of sea level has been estimated for the historic centers of Venice and Chioggia (1):

Tidal level (cm)*

Venice

Chioggia

Chioggia

Mean frequency Maximum frequency Mean frequency +80 39 times/year 60 times/year (1979)   +100 7 times/year 19 times/year (1979) 15 times/year +120 once/year 4 times/year (1992) 3 times/year +140 1 in 6 years once/year (1979)

1 in 3 years

+195 1 in 180 years   1 in 85 years

* with respect to Punta della Salute hydrometric zero.

The lagoon of Venice - Economic consequences of climate change

Significant efforts have been made to estimate the costs of sea level rise and temperature increases in the Adriatic Sea, in particular, in the city of Venice.). The impacts of climate change on coastal tourism, focusing on the historical centre of Venice, for instance, have been estimated at a loss of 34.9 – 42.9 million Euros in 2030 (24). Similar estimates have been made for clams’ aquaculture, focusing on the most important area of the Venice lagoon in terms of productivity (a loss of 10.4 – 16.5 million Euros in 2030), and sea level rise, quantifying the economic costs of floods for economic activities in the city of Venice (a loss of 7.6 – 9.5 million Euros in 2030) (9).

The lagoon of Venice - Future flood frequency

Will tidal flooding events in Venice increase in frequency and intensity during the 21st century? This has been debated recently. It was concluded from research that the frequency of extreme tides in Venice might largely remain unaltered in the 21st century; the impact of sea level rise would be compensated by a 30% decrease of the frequency of extreme storm surge events by the end of the 21st century and a decrease of extreme tides under sea level rise (18). Others, however, oppose these conclusions by stating that projected sea level rise has been underestimated; they conclude that the projected reduction in the frequency of storm surges will be likely overwhelmed by sea level rise, resulting in an increase in the number and intensity of flooding in Venice (12).

Model results on high resolution sea level pressure and wind data now and at the end of the 21st century present no statistical evidence of changes of storm surge statistics in the future climate scenario (13). This confirms previous studies (15) and shows that likely the main hazard to the north Adriatic flat coast and to Venice is posed by future sea level rise and not by an increasing number of storms (13).