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with a camera obscura, were a precious tool that preserved the memory of the ... Canaletto painted photographic views on site, and also made duplicates in his.

SIXTY-CM SUBMERSION OF VENICE DISCOVERED THANKS TO CANALETTO’S PAINTINGS DARIO CAMUFFO and GIOVANNI STURARO National Research Council, Institute of Atmospheric Sciences and Climate, Corso Stati Uniti 4, 35127 Padova, Italy E-mail: [email protected]

Abstract. Relative sea level (RSL) rise is a crucial issue for the safeguard of Venice and its historical buildings. The phenomenon over the last three centuries has been investigated by using a proxy of mean sea level: the height of the algae front on palaces. This indicator was accurately drawn by Canaletto and his pupils in their ‘photographic’ paintings made with an optical camera obscura. The positions of the fronts in the 18th century and the present were compared. The RSL rise is due to a combination of natural and anthropogenic factors, both local and global, which affected the land subsidence. An analysis was performed to establish the long-term trend and distinguish between natural and local man contributions. A prudent scenario for the future would suggest a rate between 1.9 ± 0.4 mm yr−1 and 2.3 ± 0.4 mm yr−1 .

1. Introduction Venice risks being submerged as a consequence of two problems: land subsidence and sea level rise due to global warming (Wigley and Raper, 1992; IPCC, 2001). The overall result of land subsidence and sea level rise is usually referred as ‘relative sea level (RSL) rise’, which represents the point of view of the city that is progressively submerging and apparently sees the sea level rising. Ammerman et al. (1999) and Ammerman and McClennen (2000), on the grounds of archaeological evidence, better documented from AD 200 to 1400, propose an average RSL change of 1.3 mm yr−1 , with a lower rate in the early period and a greater one in the modern period. Light still needs to be cast on the dark period between archaelogical and instrumental data. Regular tide-gauge monitoring was started in 1872 and shows an RSL rise of about 30 cm. The frequency of storm surges flooding the city has increased, as has the level reached by waters (Camuffo, 1993; Enzi and Camuffo, 1995). In the last three decades, the rise seems to have come to a halt. In view of city safeguard, the question is whether the sinking of the land has definitively come to a halt after the regulation of underground water extraction or if it is simply a question of a temporary negative fluctuation of the sea level. The answer can only be found from the knowledge of the natural trend of the RSL change over a time period which should be as long as possible. To this aim, the ‘photographic’ paintings by Canaletto and pupils were studied to understand how Climatic Change 58: 333–343, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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the RSL has changed over the last three centuries by looking at the shift in the level of the algae belt on buildings.

2. The Photographic Paintings by Canaletto The Venetian painter Antonio Canal, nicknamed Canaletto (1697–1768) and his pupil Bernardo Bellotto (1722–1780) made accurate reproductions of Venetian buildings using a camera obscura on the site. The camera obscura operates like a modern camera. The light beam penetrates through a lens, is reflected by a mirror or a prism, and is projected onto a glass surface, where a sheet of paper, or canvas, was placed. Then the painter drew in all the contours, obtaining a ‘photographic’ painting. Paintings were used to rebuild, as close as possible to the original, some historical buildings in Dresden and Warsaw, which were completely destroyed in World War II. The author of these views was Bernardo Bellotto who often used the nickname of his more famous uncle ‘Canaletto’. These paintings, also made with a camera obscura, were a precious tool that preserved the memory of the architectonic features of fabrics, but were used only from a qualitative point of view, not from a quantitative one. In this paper we show that Bellotto’s paintings are reliable and, in the particular case of Venice, they can be used as proxy data for a quantitative evaluation of the RSL rise. In a number of paintings, the brown-green front left by algae is visible on the buildings facing the canals. This front is a precious biological indicator of the average high-tide level (HTL) made by the laminaria alga (Camuffo, 2001). In the past, the upper level of the algae belt, called Commune Marino (CM), was in some cases also engraved on buildings with the initials ‘CM’, but this information is unreliable for the earliest times because the early labels were damaged, or lost, or displaced during building restorations. The algae belt is determined by the frequent tidal wetting that dampens bricks and stones: the cumulative effect of the frequent submersions, as well as the water kept in the pores, constitutes the habitat favorable for seaweed. Spot or unusual events, like rainfall or exceptional tides are vanished by the long term dryness and do not alter this level. In Venice, a comparison between the algae belt (CM) and the tide gauge observations showed that the CM is 31 cm above the mean sea level (MSL) and coincides with the HTL (Rusconi, 1983). Canaletto painted photographic views on site, and also made duplicates in his workshop. As copies are suspected to be less reliable, in this study, the earliest version of each view was preferred. The CM level was measured on the paintings and compared with the present-day situation in Venice with site inspections. A quantitative evaluation of CM displacement was possible by making reference to certain architectural features (e.g., steps, stone decorations) that constituted a useful frame within which to take and compare measurements (Figures 1–4). The cases

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Figure 1. Reading Venice submersion from paintings. (left) B. Bellotto, S. Giovanni e Paolo (1741), detail. The two arrows give the level of the algae belt in 1741 (lower) and today (upper) as derived from on-site observations. The painting shows that there were two front steps above the green belt. The displacement is 77 ± 10 cm. (right) The same door today. The picture was taken during low tide and the top step of the old front stairs is just visible (green arrow). The door was walled up with bricks in the first 70 cm above the front step to avoid water penetration.

Figure 2. A view of Canal Cannaregio painted by Canaletto in 1735 (right), and a detail of the same building today (left). The algae shift is 71 ± 10 cm. Some minor restorations are visible.

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Figure 3. A view of Palace Giustinian-Lolin painted by Bellotto in 1735 (left), and a detail of the main entrance today (right). The algae shift is 66 ± 10 cm. The main staircase is now submersed and a new wooden wharf was necessary to enter.

Figure 4. A view of Palace Flangini painted by Bellotto in 1741 (left), and a detail of the main entrance today (right). The algae shift is 71 ± 12 cm. The main staircase is now submersed and covered with algae.

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Table I Canaletto’s (C) and Bellotto’s (B) paintings with clearly visible algae belt in buildings, which remained unaltered since the 18th century Name of painting

Date

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

1727 1727 1730/31 1732 1735 1735 (?) 1735 1740 1741 1741 1758

Punta Dogana (C) Grand Canal: the Rialto Bridge from the North (C) The Grand Canal from Balbi Palace to Rialto (C) The Grand Canal from the Campo San Vio (C) Entrance to Cannaregio with S. Geremia Church (C) The Grand Canal at S. Maria della Carit`a, looking S. Vio (C) The Grand Canal from Grimani Palace to Foscari Palace (C) The Grand Canal near S. Stae Church (B) Campo S. Giovanni and Paolo (B) The Grand Canal from Flangini Palace to Vendramin Calergi Palace (B) The Grand Canal from S. Sofia Church to the Rialto Bridge (C)

where building restoration has altered these architectonic features were discarded. Although Canaletto painted some 200 views of Venice, most of them cannot be utilized due to the absence of a view on a canal or because there is no clear indication of the algae belt, or because the old buildings have since been demolished or transformed. After an extensive search in catalogues and collections, we found only 16 potentially useful views, either in the original, or in a good reproduction. After inspection in Venice of the present-day conditions, the number of paintings was reduced to 11, i.e., the only ones which reproduce buildings which have remained unaltered since the 18th century (Table I). Computer image processing of the paintings was used for a more precise identification of details; however, it was not influential for the quantitative evaluation of algae displacement.

3. Data Analysis and Discussions An analysis of the paintings showed that the CM has risen on average by CMobs = 69 ± 11 cm since the first half of the 18th century (Table II). The largest errors occurred when the green line was superimposed on a local frame and did not allow a fine resolution (e.g., uncertainty over the height of a step or a brick row). In this case, the uncertainty was resolved by the actual height of the step or row. In other circumstances, where the position of the algae was measured as a distance from an architectonic reference, e.g., a floor or a window, the uncertainty derived from the errors made in measuring these distances both on the site and in the paintings. The estimated errors are reported in Table II.

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Table II Relative sea level (RSL) change measured in terms of algae belt displacement CMobs , after the Canaletto’s and Bellotto’s paintings. Corrections are made for changes in wave regime (motor boats) and lagoon hydrodynamics (excavation of deep channels favouring the penetration of sea water), which contribute to raise the algae belt, but not the average RSL. Painting numbers as in Table I Site

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Punta Dogana Fontego Tedeschi Balbi Palace Ca’ Grande Palace Emo Palace – Cannaregio Giustinian-Lolin Palace Grimani Palace S. Stae Church S. Giovanni e Paolo Flangini Palace S. Sofia Church

Date of painting

Observed algae displacement – CMobs (cm)

Corrected values CMcor (cm)

1727 1727 1730/31 1732 1735 1735 (?) 1735 1740 1741 1741 1758

69 ± 16 68 ± 8 67 ± 6 70 ± 9 71 ± 10 66 ± 10 66 ± 10 77 ± 18 77 ± 10 71 ± 12 65 ± 7

61 ± 17 60 ± 9 59 ± 7 62 ± 10 63 ± 11 58 ± 11 58 ± 11 69 ± 19 69 ± 11 62 ± 13 57 ± 8

One question was whether Canaletto and Bellotto were accurate in drawing the exact position of the algae front. The answer is given by the scatter of all the data: the better their accuracy, the smaller the scatter. The scattering of the data is limited to 12 cm, part of this being explained by the uncertainty in reading certain details and the individual response of building foundations which are based on poles planted in mud. This confirms the reliability of the paintings and the consistency of the method. Another question concerns the distortion between the horizontal and vertical axes or other optical imperfections of the camera, which might influence the results. However, only the vertical axis was influential in the measurements and the reference was made to a local frame, e.g., a step. The absolute value of the distortion over such a small distance is negligible; e.g., even supposing a 1% distortion over a typical step of 18 cm, the error is 0.18 cm, which is two orders of magnitude less than the estimated total uncertainty. The observed shift, i.e., CMobs = 69 ± 11 cm, is primarily determined by RSL change and, secondarily, by wave height change. Waves generated by motor boats have a typical height of some 10 cm that is about twice the value of those generated by an 18th-century row-boat, as estimated from wave observations in the Grand Canal under differing conditions, motor traffic being either allowed or

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Figure 5. Relative sea level (RSL) at Venice from tide gauges (continuous grey line, period 1872–2000) and from Canaletto’s and Bellotto’s paintings (white dots with error bars, period 1727–1758). RSL from paintings was estimated from the difference in level of the algae belt as it was in the paintings and as it is today.

forbidden (Canestrelli and Cossutta, 2001). This is equivalent to an apparent 5-cm CM rise. Another factor is the amplitude of the tidal wave. After the excavation of two deep channels, the ingress of sea water into the Lagoon was facilitated, slightly amplifying the tidal wave in Venice. Analysis of tide gauge observations demonstrated that this dynamic effect contributes to the yearly average tidal amplitude raising the CM by another 3 cm. The combination of both the above factors gives an RSL contribution equal to RSLwave = 8 ± 1 cm. The MSL in Canaletto’s day (MSL1700s) is obtained as MSL1700s = MSL2000 − CMcor , where MSL2000 is the average MSL for the year 2000 and CMcor = CMobs − RSLwave = 61 ± 12 cm. The reference to the year 2000 is only virtual, useful to draw the data (Figure 5), not to estimate RSL changes that are calculated from CMcor . The paintings show that in the period 1727–2000 the bulk RSL rise (or submersion rate, SR) was 2.3 ± 0.4 mm yr−1 . This trend is close to that computed for the instrumental period 1872–2000, which was 2.4 ± 0.1 mm yr−1 (Figure 5). In this period, the apparent RSL rise is determined by both natural and anthropogenic factors. The latter are both global, i.e., global warming and sea level rise, and local, e.g., caused by underground water pumping, canal excavations, hydraulic works in the lagoon. The impact of the local factors was dominant especially in the period 1930–1970.

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4. The Land Subsidence and the Sea Level Rise Venice submersion is explained in terms of three main causes. The first is the natural subsidence due to tectonic movements in the deep layers (Colombo, 1972; Bondesan et al., 2001). This is worsened by the compaction of clay and peat sediments, although over the past centuries attempts have been made to reduce lagoon sediments and compaction by diverting rivers. During the Holocene, the tectonic subsidence in this area (Bondesan et al., 2001) was estimated to be 1.25 mm yr−1 . The second cause is the land subsidence induced by man. The main factor was extensive pumping of ground water from the upper aquifers for industrial purposes. In the period 1930–1970, highly affected by water pumping, subsidence was estimated to be 10–12 cm in comparison with sea level on other sites (Teatini et al., 1995; Carbognin and Taroni, 1996; Pirazzoli and Tomasin, 1999). Pumping was stopped in 1970, followed by an apparent immediate benefit and a small rebound of 1–2 cm. Fortunately, this occurred before too much irreversible damage had taken place. Another, less relevant local anthropogenic factor was past building construction. Especially in the 17th century, many magnificent Baroque palaces were built, which increased static pressure and sinking in marshy ground. The third cause of submersion is the sea-level change (eustatism) in the Mediterranean. This sea is poorly connected to the ocean through the Gibraltar Strait and is affected by local evaporation, reduced rainfall, inverse barometer response and change in deep water salinity and temperature (Teatini et al., 1995; Piervitali et al., 1998; Tsimplis and Baker, 2000). The climatic features determined a variable sea level, with a negative trend after 1960 (Tsimplis and Baker, 2000). Eustatism in the Mediterranean in the last century has been estimated to be 1.1–1.5 mm yr−1 . Both the instrumental and painting estimates of the RSL rate of change up to the present include the local anthropogenic effect and are therefore higher than the natural contribution. In principle, the natural rate can be obtained from the tide gauge data in the short period before the extensive water pumping (1930). This means assuming the value 1.9 ± 0.4 mm yr−1 for the natural submersion rate SR. On the other hand, the longer the series, the less relevant the influence of fluctuations. In order to identify the trend rate, the SR is represented in Figure 6 with the uncertainty (standard error corrected for the degrees of freedom). The SR was calculated over a mobile period starting in 1872 and ending with the running year, indicated in the abscissa. The SR had a generally rising trend until 1920, when it reached 2.4 ± 0.4 mm yr−1 . Then, the SR continued to vary because of natural and/or local and/or global anthropogenic factors. Generally, it increased during the period of intense groundwater pumping (1930–1970) and the present-day value is 2.4 ± 0.1 mm yr−1 . Briefly, within the instrumental period, the actual SR can be assumed to lie between 1.6 and 2.6 mm yr−1 . The natural contribution is illustrated in the first part of Figure 6, but is affected by greater uncertainty. From Canaletto’s and Bellotto’s paintings it is possible to know what happened since the beginning of the 18th century. If CMcor values (Table II) are corrected

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Figure 6. RSL rate of change for a mobile periods starting in 1872 and ending in the running year, indicated in the abscissa. Thin curves give the uncertainty intervals as standard errors corrected for the degrees of freedom. Before 1930 data are not affected by local anthropogenic land subsidence, although fluctuations are evident.

also for local anthropogenic effects 1930–1970, i.e., decreased by 11 ± 1 cm, the estimate of the natural SR is 1.9 ± 0.5 mm yr−1 .

5. Conclusions and Open Issues Canaletto’s and Bellotto’s paintings enable us to extend the estimate of the Venice submersion back in time for almost three centuries. From these paintings, the estimate of the natural SR is 1.9 ± 0.5 mm yr−1 in agreement with instrumental data 1872–1930 on a different time scale. One still open issue is the apparent RSL stasis after 1970, which might be either a short-term fluctuation from the main trend (e.g., temporary compensation of various effects), or a real change in trend (i.e., a long-term change in forcing factors). Looking at the tide gauge data (1872–2001), we can see that the trend was generally rising, but with many short and some medium term fluctuations, temporarily determined by a combination of weather factors. Periods with temporary sea level stasis can be found in 1879–1894, 1899–1908, 1910–1950, 1937–1951 and 1970–today. Although the last period is among the longest ones, it benefits from two fortunate events: a 2–3 cm aquifer rebound, and an increase in atmospheric pressure (1 hPa) that has characterised the last four decades, equivalent to 1cm sea level decrease. The bulk effect of these two factors is equivalent to counteract 15–20 year natural RSL rise. From this point of view, the last thirty years, affected by this positive combination, is too short a period to give a definite answer. However, Canaletto

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and Bellotto made it possible to know the time-history of RSL over the last three centuries: it was always characterised by a well-marked and almost constant rising trend. No evidence has been found to justify a change in trend. The hypothesis of a fluctuation, supported by other repeated events, seems the more plausible. Although the extraction of underground water has stopped, an enhanced land subsidence cannot be excluded in the future, maybe due to a climate change, causing a reduction in precipitation and aquifer level. For this reason, in planning mobile gates to safeguard the city from surges, it may be advisable to consider a prudential rate between 1.9±0.4 mm yr−1 (i.e., the past ‘natural’ rate) and 2.3±0.4 mm yr−1 (i.e., the past overall rate) as deduced from the paintings. A positive expectation may be a possible reduction of the frequency of the southerly wind (the Sirocco), which pushes water towards Venice, as a consequence of the expected climate changes. However, in the last 40 years, although the Sirocco gales have decreased by some 50% (Piervitali and Colacino, 1998), the frequency of the flooding tides has increased by 100% (Camuffo, 1993; Camuffo et al., 2002). The non-linearity between sea level, frequency of adverse meteorological conditions and flooding tides, make surge frequency unsustainable. In the short term, mobile gates may face this challenge, although with some uncertainty (Pirazzoli, 2002), and not before the ten years required for works. In the long term, a better solution is needed, e.g. to raise the city by pumping sea water deeply into the subsoil. However, this method requires further research in order to ascertain the feasibility, possible risks and limits. The Venetian palaces were originally protected against groundwater rise by a belt of non-permeable Istria stone, but now the protective belt has sunk with the city and the flooding waters reach brick and plaster which are destroyed by salt crystallisation cycles. Whatever the causes of RSL rise, the problem remains, Venice has lost 60 cm of its historical buildings to the sea and the walls are at risk of rapid decay. This is a very high price to pay and this situation must not be worsened if we wish to leave Venice to future generations. On the grounds of this consideration, safeguarding measures, taken in view of the most pessimistic scenarios, are justified.

Acknowledgements The idea arose under researches funded by the European Commission, Directorate Research (Environment and Climate Programme) and was developed with funding by CORILA, Venice. Special thanks are due to Dr. Luigi Alberotanza, Director of CNR-ISDGM, Venice for having facilitated this research.

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