Sea level and temperature extremes in a regulated Lagoon of Venice

TYPE Original Research PUBLISHED 12 January 2024 DOI 10.3389/fclim.2023.1330388 OPEN ACCESS EDITED BY Jing Yang, Beijing Normal University, China REVIEWED BY Xin Qi, Ocean University of China, China Miaoni Gao, Nanjing University of Information Science and Technology, China Tao Zhu, Chinese Academy of Sciences (CAS), China *CORRESPONDENCE Christian Ferrarin [email protected] RECEIVED 30 October 2023 ACCEPTED 28 December 2023 PUBLISHED 12 January 2024 CITATION Ferrarin C, Bonaldo D, Bergamasco A and Ghezzo M (2024) Sea level and temperature extremes in a regulated Lagoon of Venice. Front. Clim. 5:1330388. doi: 10.3389/fclim.2023.1330388 COPYRIGHT ©2024 Ferrarin, Bonaldo, Bergamasco and Ghezzo. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. Sea level and temperature extremes in a regulated Lagoon of Venice Christian Ferrarin*, Davide Bonaldo, Alessandro Bergamasco and Michol Ghezzo Institute of Marine Sciences, National Research Council, Venice, Italy Increasing sea levels and water temperatures have been detected at several coastal locations worldwide with severe consequences on the communities and ecosystems. Coastal lagoons are particularly vulnerable to such changes due to their low land elevation and limited connections with the open sea. Here the recent and future climatic changes in the Lagoon of Venice (Italy) are investigated using in-situ observations and high-resolution hydrodynamic modeling. Trend analysis was applied to observed time series of meteorological and oceanographic climate essential variables to identify significant long-term changes in mean and extreme values. The mean relative sea level rose at a rate of 4.9 mm per year in Venice due to the combined action of eustacy and subsidence while air and sea temperatures increased on average by 1.8 and 1.1◦C in 30 years, respectively. These rates, as well as climate projections, were used following a pseudo-global-warming approach to investigate the near future (up to 2050) evolution of the lagoon’s dynamics focusing on sea level and temperature extremes. The lagoon will amplify the temperature changes expected for the Adriatic Sea, especially in the shallow tidal flats where the intensity of the marine heat waves will be more than four times larger than that in the open sea. Moreover, the model allowed us to perform “what-if” scenarios to explore to which extent the flood protection MoSE barriers will modify the lagoon’s dynamics. According to the simulations, the number of floods and therefore of the MoSE closure strongly increases with sea level rise. In the most severe scenario, MoSE will have to close for more than 20% of the time in October, November, and December resulting in the reduction of water exchange with the open sea and exacerbation of marine cold spells. Some considerations on the implications of the expected changes on the lagoon’s ecology are proposed. KEYWORDS climatic trends, relative sea level rise, extreme sea levels, marine heat waves, Venice 1 Introduction The Mediterranean basin is a region of high cultural and landscape richness that has been heavily impacted by human activity and represents a focal area for climate change studies (MedECC, 2020;Chiggiato et al., 2023;Lionello et al., 2023). In this region, the long-term evolution of the physical processes is only partially explained by global trends (Sannino et al., 2022) and the intensity and trends of multiple climate-related hazards are amplified with respect to the global averages (Vautard et al., 2014;Dosio and Fischer, 2018;Lionello and Scarascia, 2018). The frequency of extreme events, including high precipitation, heat waves, and cold spells, is likely to be altered due to global warming, which might severely threaten the coastal environment and economies (Tuel and Eltahir, 2020;Ali et al., 2022). Frontiers in Climate 01 frontiersin.org Ferrarin et al. 10.3389/fclim.2023.1330388 This is most certainly true for many coastal environments, like lagoons, which have been identified as hotspots of climate change and vulnerability to environmental and anthropogenic pressures (Ferrarin et al., 2014;Newton et al., 2014). On the other hand, coastal and transitional systems are a key hub for economic productivity in the region, as well as a strategic ground in the endeavor to meet sustainable development objectives and climate change mitigation goals set at the International level. A typical example in this direction is given by the multiple ecosystem services provided by coastal wetlands and seagrass beds, which can combine ecological conservation and carbon sequestration functions with recreational implications and coastal protection instances in a nature-based solution perspective. Gattuso et al. (2018) suggest that sea level rise threatens the existence of a large fraction of the global coastal wetlands, with more than 50% loss by the end of the present century in response to a relatively optimistic 50-cm rise. More generally, it has been pointed out that extreme thermal stressors such as marine heat waves can have notably strong effects on shallow environments, particularly intertidal shores or tidal flats, where organisms and communities typically live close to their tolerance limits (Domínguez et al., 2021). The relatively small scale of the processes that characterize coastal and transitional systems, in particular in the occurrence of extreme events (Schlegel et al., 2017), alongside with the responsiveness to local forcing (Ozer et al., 2022), and the relevance of these environments call for a high-resolution characterization of climate-change impacts over a multi-decadal time scale. The understanding of the dynamics leading to extreme sea levels and temperatures and of the future evolution of their intensity and frequency is thus of paramount importance for a realistic assessment of present and future risks. Moreover, the description and comprehension of the interconnections between sea level and sea temperature extremes represent a major concern in coastal environments. Their role in the medium-term reshaping of the local habitats features in transitional waters and consequently the extension of available ecological niches is still little studied, although it deserves extreme attention, especially in the presence of the undoubted climate drift underway. This study focuses on the Lagoon of Venice, which according to Ferrarin et al. (2014), is— as well as other coastal environments in the Mediterranean Sea— highly responsive to climate change and can act as a sentinel system for the observation of climate change. Due to its unique historical and cultural value, a flood protection plan (called MoSE, from the Italian acronym for Experimental Electromechanical Module) is operating to protect the City of Venice from flooding. The MoSE is formed by a series of mobile barriers located at the inlets which rise up to emerge and block the flow of the incoming sea level signal in the lagoon during high-tide events. Even if climate change, sea level rise and coastal flooding in Venice have been deeply investigated by several authors (see the review papers of Lionello et al., 2021;Zanchettin et al., 2021), the changes in the dynamics of the lagoon in the future—when the exchange with the open sea will increasingly be regulated by the MoSE barriers—are still not investigated in detail. Nonetheless, it has already been pointed out that the operation of the MoSE system in a rising sea scenario will bring along a number of implications for the efficiency of marine traffic (Umgiesser and Matticchio, 2006), the morphodynamic processes of shoals, and saltmarshes (Tognin et al., 2022), and more broadly the long-term physical and socialecological evolution of Venice, its lagoon and the neighboring coastal plain (Tagliapietra and Umgiesser, 2023). The aims of the research reported here are (i) characterizing the recent (1991– 2020) climate in the Northern Adriatic Sea and the Lagoon of Venice from observations (Section 3), (ii) investigating future (up to 2050) climatic changes in the Lagoon of Venice and their effects particularly on extreme sea levels and water temperatures (Section 4), (iii) exploring to which extent the MoSE barriers will modify the lagoon’s hydrodynamics and thermohaline regime (Section 5). Some considerations on the impact of climate change and MoSE on the ecological dynamics in the lagoon are finally provided (Section 6). 2 Methods 2.1 The Lagoon of Venice As shown in Figure 1, Venice is located at the center of a coastal shallow lagoon connected to the Northern Adriatic Sea via three inlets (Lido, Malamocco, and Chioggia) which ensure an active exchange of water and substances with the open sea (Umgiesser et al., 2004). The Lagoon of Venice is characterized by a complex network of tidal channels (maximum depth around 15 m), shallow water tidal flats with an average depth on the order of 1 m and salt marshes (Madricardo et al., 2017). The hydrodynamics in the Venice Lagoon is mainly driven by the tide (about 1 m excursion during spring tides) and the wind. The tide propagates into the lagoon along the deep, narrow channels onto the tidal flats and tidal marshes (Umgiesser et al., 2004). The sea level oscillations in the Northern Adriatic propagate into the lagoon through the three inlets and then follow the main tidal channels. These oscillations, once they have entered the lagoon, propagate nearly without damping to the City of Venice, where the sea levels are comparable to the ones close to the inlets, with a typical 1-h delay (Figure 2). Other more remote areas of the lagoon show a higher phase shift of up to 3 h with respect to the inlets and an attenuation of the sea level (Ferrarin et al., 2015). Due to the high tidal energy and relatively shallow water, the water masses in the lagoon are generally vertically well-mixed (Pivato et al., 2018). Stratification of water masses occurs only in the inner lagoon where the tidal energy is attenuated. Winter months, particularly November, are more frequently characterized by meteo-marine conditions favorable to the onset of severe storm surges in the Northern Adriatic Sea, mostly driven by southeasterly winds (Scirocco), determining to flood in Venice (Lionello et al., 2021). The frequency of flooding events in Venice increased constantly over the last century (Ferrarin et al., 2022) and the flood risk is expected to rise in future due to the combined action of sea level rise and subsidence (Schlumberger et al., 2022). Indeed, since October 2020, the MoSE (from the Italian acronym for Experimental Electromechanical Module) mobile barriers at the three inlets have started to be in a pre-operational phase closing the lagoon during severe events to protect Venice from flooding. An example of the effect of the MoSE barriers in blocking the incoming tide and limiting the sea level in Venice is presented in Figure 2 Frontiers in Climate 02 frontiersin.org Ferrarin et al. 10.3389/fclim.2023.1330388 FIGURE 1 Unstructured model grid and bathymetry of the Lagoon of Venice with the dots marking the monitoring stations (in green the ARPAV meteorological stations). The pink triangle marks the analysis point in the shallow tidal flat (STF). The magenta bars indicate the MoSE barriers at the inlets. for the high tide events of 2, 4, 5, 6, and 9 December of the same year. On December 8th, due to an erroneous interpretation of the forecast, the MoSE was not activated, resulting in flooding in the City of Venice. 2.2 Observational dataset Among the different observed parameters, we decided to focus our investigation on a few Essential Climate Variable (ECV, defined as the variables that critically contribute to the characterization of the climate; https://gcos.wmo.int/en/essential-climate-variables) for which long-term timeseries are available in the area of interest. We selected three variables characterizing the surface ocean physics (sea level, sea temperature, and salinity) and three variables describing the surface atmosphere (air temperature, precipitation, and wind speed). 2.2.1 In-situ observations The Northern Adriatic Sea and the Lagoon of Venice are wellmonitored by several meteorological and marine stations (Ferrarin et al., 2020;Pérez Gómez et al., 2022). We considered the 1991– 2020 period which represents an observational record of 30 years, generally recognized as adequate to assess climatic changes and trends (Mudelsee, 2019). Among the different observational sites, we considered those for which long-term hourly or daily time series were available: •The Acqua Alta oceanographic tower—AAOT in the shelf sea (15 km Offshore the coast on a 15 m depth) monitoring several meteorological (air temperature, wind speed and direction, precipitation, relative humidity and solar radiation) and oceanographic (sea level, sea temperature, salinity, current velocity) variables. •Five meteorological stations along the Venetian littoral belonging to the Regional Agency for Environmental Protection and Prevention of the Veneto (ARPAV) measuring air temperature and precipitation (Cavallino Treporti, Mira, Chioggia Sant’Anna, Codevigo, Eraclea). •The meteo-marine station of Palazzo Cavalli in Venice (CAV) monitoring air and sea temperatures from 2000 to 2020. •The tide gauge of Punta della Salute in Venice (PDS). All sea level values reported in this manuscript refer to the Punta della Frontiers in Climate 03 frontiersin.org Ferrarin et al. 10.3389/fclim.2023.1330388 FIGURE 2 Observed sea level outside (AAOT) and inside (PDS) the lagoon during the first decade of December 2020. The horizontal green line identifies the threshold level for the activation of the MoSE system. Salute official datum (called ZMPS; corresponding to the mean sea level of the 1885–1909 period). •The marine station of Trieste (TRI), located about 110 km from Venice. Even if this station is quite far from the study site, it has a long record of sea level and sea temperature (Raicich and Colucci, 2019) which can be used as a benchmark for describing the Northern Adriatic Sea conditions. Indeed, Trieste is not affected by significant vertical land motions and therefore it has been widely used as a reference for estimating subsidence in Venice (Zanchettin et al., 2021). The monitoring stations considered in this study are indicated with dots in Figure 1. 2.2.2 Satellite observations Among the ocean variables monitored by satellite, we considered: •Sea level anomaly (SLA): 1993–2020 daily mean estimates of sea level anomaly based on satellite altimetry measurements and delivered at 0.125◦by the Copernicus Climate Change Service (C3S, 2018). Sea level anomaly is defined as the height of water over the mean sea surface in a given time and region computed with respect to a 20-year mean reference period (1993–2012); •Sea surface temperature (SST): 1993–2020 daily (nighttime) optimally interpolated 0.05◦grid resolution SST maps over the Mediterranean Sea (Pisano et al., 2020) and delivered by the Copernicus Marine Service (https://doi.org/10.48670/ moi-00269); •Sea surface salinity (SSS): 2010–2020 daily dataset at 0.0625◦ grid resolution covering the entire Mediterranean Sea (Sammartino et al., 2022). The time series data have been extracted from the gridded datasets in proximity to the Acqua Alta oceanographic station (as the 3 ×3 pixel box average), which is close to the open sea boundary of the numerical simulations. 2.3 Climate model downscaling Coastal climate downscaling is performed here using the pseudo-global-warming (PGW) approach (Brogli et al., 2023) which consists in imposing changes in the climate system on a control climate simulation (usually representing current conditions) by modifying the atmospheric forcing and initial and boundary conditions. This approach has been preferred with respect to the classical dynamical downscaling technique—where outputs from a large-scale climate model are directly used to force limited-area models (Drenkard et al., 2021)—because no detailed regional climate simulations representing correctly the past distribution of the main meteorological and ocean variables are actually available for the Northern Adriatic Sea. The pseudo-global-warming approach, by applying climate change deltas to the simulation forcings, assumes the preservation of the historical statistical distribution of the specific variable with the advantage of representing correctly both the mean and extreme values (which is often misrepresented in global and regional climate models, Mishra et al., 2023) and the disadvantage that potential changes in the intra-annual, interannual, and future variability might be missed (Brogli et al., 2023). Moreover, PWG has the advantage of performing short numerical simulations, thus limiting Frontiers in Climate 04 frontiersin.org Ferrarin et al. 10.3389/fclim.2023.1330388 the computational and storage costs. The PWG approach has been used by Ferrarin et al. (2014) for simulating climate changes in several Mediterranean coastal environments. The numerical experiments consisted of simulating the circulation in the Lagoon of Venice using the open-source System of HydrodYnamic Finite Element Modules (SHYFEM, Arpaia et al., 2023) (https://github.com/SHYFEM-model/shyfem). In a 3D approach, the model solves the shallow water equations in their formulations with levels and transports using a finite element numerical method and a semi-implicit time stepping. The model has already been applied in the Mediterranean Sea and several coastal environments (Umgiesser et al., 2022, and references therein). The hydrodynamic numerical computation is performed on a spatial domain that represents the Venice Lagoon and its adjacent shore (Figure 1). The use of elements of variable sizes, typical of finite element methods, is fully exploited to suit the complicated geometry of the basin, the sharp spatial gradients of the topographic features, and the complex bathymetry. The water column is discretised into 17 vertical levels with progressively increasing thickness, varying from 1 m for the topmost 10 to 7 m for the deepest layer of the outer shelf. The model bathymetry was obtained from the data collected in 2002 by Magistrato alle Acque di Venezia—merged with later surveys—and the high-resolution bathymetry acquired in the main channels of the lagoon in 2014 (Madricardo et al., 2017). Climate anomalies are computed as differences with respect to a control situation. The year 2020 is considered for the control simulation using the observations (hourly sea level, current velocity, temperature, salinity, wind speed, wind direction, air temperature, mean sea level pressure, solar radiation, relative humidity, and cloud cover) acquired at the Acqua Alta oceanographic tower as boundary and forcing conditions. This period has been chosen because the year 2020 is at the end of the 30-year climate period considered for assessing the recent climate evolution, and also because in October 2020 the MoSE barrier system started operating for the protection of the historical city of Venice from flooding (https://www.MoSEvenezia.eu/MoSE/). The numerical simulation of the MoSE closure was modeled by increasing bottom shear stress and viscosity in the inlet areas. The future reference time horizon is 2050, thus considering a 30-year projection of the actual situation. The climate change deltas used to perturb the control situation were both extrapolated from the observed past trends over the 1991–2020 period (where statistically significant) and derived from climate projections according to three Representative Concentration Pathway scenarios (RCPs 2.6, 4.5, and 8.5). Future climate change deltas for the three RCPs scenarios were computed as the multi-model ensemble mean of EURO-CORDEX (https://www.euro-cordex. net/) and Med-CORDEX (https://www.medcordex.eu/) model experiments for the meteorological and oceanographic climate variables, respectively. The EURO-CORDEX (taken here as the mean of an ensemble of the HadGEM2-ES_RACMO22E, MPI-ESM-LR_REMO2009, EC-EARTH_CCLM4-8-17, ECEARTH_RACMO22E, and EC-EARTH_RCA4 models) and Med-CORDEX (CNMR-CM5_CNRM-RCSM4) model experiments were selected according to the availability of data and scenarios for the study area and following the study of expected climate change in the nearby Friuli Venezia Giulia region, where Trieste is located (ARPA-FVG, 2018). Future projections of the mean sea level for the three RCPs scenarios were taken from the IPCC 6th Assessment Report (Fox-Kemper et al., 2021) and made available via the NASA Sea Level Projection Tool (https:// sealevel.nasa.gov/ipcc-ar6-sea-level-projection-tool). The climate perturbations were applied to air temperature, wind speed, sea level, salinity, and sea temperature. To summarize, for analyzing the climate change effects on lagoon hydrodynamics, we performed five pairs of 13month numerical experiments with SHYFEM applied over the Lagoon of Venice with different initial and forcing conditions as summarized below: •CTRL-2020: control simulation for 2020 using observations as forcings; •TREND-2050: climate change simulation extrapolating the past observed 30-year perturbations to 2050; •RCP2.6-2050: climate change simulation using RCP2.6 scenario perturbations for 2050; •RCP4.5-2050: climate change simulation using RCP4.5 scenario perturbations for 2050; •RCP8.5-2050: climate change simulation using RCP8.5 scenario perturbations for 2050. All above-mentioned simulations were performed also in “what-if” scenarios considering the closure of the MoSE barriers. The SHYFEM numerical model allows the fluxes at the inlets to be reduced for simulating the closing and subsequent opening of the mobile barriers (Ferrarin et al., 2013;Umgiesser, 2020). According to the tide gauge observations registered since October 2020, the MoSE is raised when a sea level is forecasted to surpass the 1.1 m flooding threshold. However, to save Venice from flooding, the MoSE barriers have to be closed in advance, generally when the sea level in Venice is between 60 and 80 cm (depending on the meteorological situation) and opened in the subsequent descending tide (Figure 2). In the case of consecutive flooding events, the MoSE is kept closed continuously for several hours or even days. The closing of the barriers takes 30 min, and in all three inlets the barriers are generally closed at the same moment. These managing procedures were considered in projecting the future operation of MoSE in the 2050 climate change scenarios. A similar MoSE operation procedure was adopted in the modeling study by Umgiesser (2020). The initial condition in the numerical simulations is always the calm state. This is certainly no problem for the current velocity and the water level since these quantities approach a dynamic state very fast (less than a day) in such a dynamic tidal and wind-driven environment. The time to reach a steady state distribution is larger for the water temperature and salinity. Therefore, salinity and water temperature values measured in the lagoon in December 2019 (and modified by the climate change deltas in the climatic simulations) have been chosen as initial conditions and we considered a spinup time of 1 month (December 2019). Such a period is larger than the average residence time of the lagoon (Ferrarin et al., 2013) and therefore is enough for the variables to approach a dynamic steady state. Frontiers in Climate 05 frontiersin.org Ferrarin et al. 10.3389/fclim.2023.1330388 The application of the SHYFEM model to the Lagoon of Venice has been validated in previous work reproducing correctly tidal propagation, storm surge, water flows at the lagoons’ inlets, and water temperature, and salinity variability (Ferrarin et al., 2021, and references therein). The control experiment (CTRL-2020) has been validated by comparing the model results with sea level (SL) and sea temperature (ST) observations acquired at PDS. The model results compare reasonably well with the measurements: the root mean square error is 7 cm for SL and 1.0◦C for ST; the correlation coefficient is about 0.98 for both variables; the difference between the averages of simulated and observed values (BIAS) is −2 cm for SL and 0.7◦C for ST. 2.4 Data analysis Monthly averaged time series data were processed with the Python pyMannKendall analysis tool (https://pypi.org/project/ pymannkendall/,Hussain and Mahmud, 2019) for detecting consistently increasing or decreasing linear trends (monotonic trends). The Mann-Kendall Trend (MKT) test is a non-parametric test, which means it works for all distributions (i.e., data doesn’t have to meet the assumption of normality), but data should have no serial correlation (Mann, 1945). In this study, the seasonal MKT was used to remove the effect of seasonality in the data. The trend is here considered statistically significant when the p-value of the MKT is less than 0.05. Sea level extremes are commonly defined in Venice as the high tide event (locally called acqua alta) exceeding the flooding threshold of 1.1 m above ZMPS (Lionello et al., 2021). Such a flooding threshold is also used in the sea level forecasts for the activation of the MoSE system. Marine heat waves (MHW) occur when the sea temperatures are abnormally warm for the time of the year relative to historical temperatures, with that extreme warmth persisting for a prolonged period (five consecutive days according to Hobday et al., 2016). A single day of sea temperature above the threshold value is defined as a marine heat spike (MHS). The day-of-the-year threshold is computed as being the daily 90th percentile of the sea temperature distribution over a long (ideally 30-year) historical baseline period (Hobday et al., 2016). In the MHW analysis, we used the marineHeatWaves python module (https://github.com/ecjoliver/ marineHeatWaves) which implements the MHW definition of Hobday et al. (2016). The daily climatological mean and threshold time series are smoothed using a 30-day moving window. Contrary to MHW, Marine Cold Spells (MCS; Schlegel et al., 2021), happen when the sea temperatures are unusually low for the time of year compared to climatological values for a prolonged time (5 days). As a threshold for the identification of MCSs, we used the 10th percentile of the daily climatological sea temperature distribution. 3 Recent climatic changes in the Northern Adriatic Sea In this study, we analyzed in-situ and satellite observations acquired in the period 1991–2020 to assess recent climatic changes in the Northern Adriatic Sea and the Lagoon of Venice. The characterization of the recent climate and its changes in time are presented and discussed separately for meteorological and ocean variables, with special attention on the second group. Linear climatic trends for all considered variables are reported in Table 1. 3.1 Meteorological variables The analysis of the air temperature datasets revealed an increasing trend of 0.58◦C/10y over the Veneto Coast with values up to 0.9◦C/10y in the City of Venice (calculated over the 2003– 2020 period). This means that in the study area, the air temperature rose by at least 1.8◦C in the last three decades, therefore already overpassing the global limit of 1.5◦C above pre-industrial levels defined by the Paris Agreement (https://unfccc.int/process-andmeetings/the-paris-agreement). Temperature-derived indicators reveal an increasing trend in heat waves of 3 days per decade (not statistically significant), tropical nights (days when the temperature does not fall under 20◦C during the night time) of about 4 days per decade (statistically significant) and frost-free days (days with a minimum daily temperature above 0◦C) of 8 days per decade (statistically significant) (Figure 3). These results confirm previous climatic studies highlighting the Northern Adriatic Sea, and generally the Mediterranean, as a hotspot for climate change (MedECC, 2020). The long-term wind speed timeseries recorded at AAOT show a slight decreasing trend of 0.01 m s−1, in accordance with the detected decrease of storm waves in the Adriatic Sea (Pomaro et al., 2018). Decreasing trends in wind intensity and wave height over the Adriatic Sea have also been found in several climate studies (e.g., Bonaldo et al., 2017;Benetazzo et al., 2022). The precipitation dataset used in this study does not reveal any significant linear trend (Table 1), in partial contrast with the findings by Philandras et al. (2011), which identified the Adriatic region as one of the areas in which the long-term trends in precipitation decrease have been the most intense in the last century. Nevertheless, it is worth noting that the region enclosed between the Northern Adriatic Sea and the Alpine Ridge exhibits strong spatial gradients in terms of precipitation rates, recent past trends (Christidis and Stott, 2022), and future projections (Zittis et al., 2021), suggesting that global and regional-scale modeling and coarse-resolution gridded observational products could fail in matching the measured precipitation trend in the Veneto coastal region. End-of-century RCP8.5 regional projections by Rajczak et al. (2013) show for the Adriatic Sea and its hydrographic basin decreasing precipitation frequency throughout the whole year (except in the north in winter), wetter autumn/winters (in the north) and dryer springs/summers, increasing intensity particularly in autumn and winter. 3.2 Ocean variables According to Zanchettin et al. (2021), the relative mean sea level (RMSL) in Venice has risen by more than 30 cm since 1870 with an average linear trend of 2.5 mm y−1. A higher rate Frontiers in Climate 06 frontiersin.org Ferrarin et al. 10.3389/fclim.2023.1330388 TABLE 1 Linear climatic trends from observations for the 1991–2020 period (if not differently specified) at the 95% significant level. Variable/station Venice (PDS/CAV) Shelf sea (AAOT) N. Adriatic (TRI) Veneto coast (ARPAV) Satellite (AAOT) Air temperature (◦C 10y−1) 0.90 ±0.01b0.69 ±0.01b0.58 ±0.01dWind speed (m s−110y−1) - −0.1 ±0.00 - - - Precipitation (mm y−1) - NSTbNSTdRelative sea level (mm y−1) 4.9 ±0.3 4.8 ±0.3 3.4 ±0.3 - 3.0 ±0.0a Sea temperature (◦C 10y−1) 0.48 ±0.01b0.44 ±0.02b0.48 ±0.01 - 0.45 ±0.01 Salinity (pss 10y−1) - NSTb- - 0.08 ±0.01c a1993–2020; b2003–2020; c2010–2020; d1992–2020. NST stands for non-significant trend. FIGURE 3 Mean decadal values of heat waves, tropical nights, and frost-free days in the Veneto coast. of sea-level rise has been observed in more recent years (1991– 2020) with values reaching 4.9 mm y−1(Table 1 and black line in Figure 4). A similar trend value has been found for the AAOT monitoring station (4.8 mm y−1). However, it is well-known these two locations are strongly influenced by subsidence, whose rate is estimated by the comparison with the sea level trend registered in Trieste (3.4 mm y−1) which is considered unaffected by vertical land movements (Zanchettin et al., 2021, and references therein). The comparative assessment of the two tide gauges (PDS and TRI) estimates the mean subsidence in Venice of 1.5 mm y−1over the last three decades. The analysis of the satellite data recorded in the shelf sea reported an increasingly significant trend of 3.0 mm y−1, a value similar to what was observed in Trieste and within the range estimated by Mohamed et al. (2019) (3.0 ±0.5 mm y−1over the 1993–2017 period) and Meli et al. (2023) (2.6 ±0.8 mm y−1over the 1993–2019 period) for the entire Adriatic Sea. As reported by Lionello et al. (2021) and Baldan et al. (2023), the frequency of floods in Venice has increased over the decades (Figure 4). Ferrarin et al. (2022) found that this is mostly due to the rise of the RMSL and partially to changes in the long-term forcings associated with planetary atmospheric waves and seasonal to inter-annual oscillations. It has to be noted that in the Lagoon of Venice, Caruso et al. (2010) found a statistically significant difference between yearly mean and maximum sea level trends and suggested a possible explanation for the different contributions of the lagoon morphology to energy dissipation. The analysis of the sea temperature revealed an increasing long-term trend in all monitoring stations with values ranging from 0.44 to 0.48◦C per decade (Table 1). The rise of the mean sea temperature determined a general increase (not statistically significant) in the number of marine heatwaves and a decrease in marine cold spells (Figure 5). The catalogs of MHW and MCS events in Venice are proposed in Supplementary Tables 1,2, respectively. We detected a total of 49 MHW events; the strongest occurred in February/March 2007 and lasted for 35 days with a cumulative intensity (computed as the integral of the intensity over the duration of the event) of 36◦C·day (Figure 5). The year 2007 was characterized by anomalously high temperatures during the first half of the year (with six marine heatwaves) and unusually low sea temperatures from September to December (with four marine cold spells). The year 2020, considered here as a control period, shows a sea temperature variability in line with the longterm climatology with only a short MHW event in December and no MCSs. Frontiers in Climate 07 frontiersin.org Ferrarin et al. 10.3389/fclim.2023.1330388 FIGURE 4 Number of events per year exceeding the 1.1 m flooding threshold. The blue bar indicates the number of MoSE closures in 2020. The time evolution of the relative mean sea level is shown as a black dashed line. Within the Mediterranean basin, the Northern Adriatic Sea has been identified as a hotspot for marine heatwave intensity (Dayan et al., 2023), particularly in spring and summer, with the number of total yearly MHW days increased from a few units to around one hundred per year (Juza et al., 2022;Martínez et al., 2023). Schlegel et al. (2017) have shown that the occurrence of marine heatwaves and cold spells in coastal environments can only partially be explained by mesoscale ocean conditions while being strongly controlled by local and sub-mesoscale processes. Salinity recorded at AAOT in the period 2003–2020 does not show any significant trend. On the contrary, the sea surface salinities recorded by satellite manifested an increasing trend of 0.08 pss from 2010 to 2020, highlighting the Northern Adriatic Sea salinification in the last decade (Sammartino et al., 2022). Camatti et al. (2023) detected an increasing trend in salinity in the inner part of the Venice Lagoon and not near the inlet. This is explained by the fact that the relative sea level rise (RSLR) produces an increase in the lagoon volume that, even considering no significant reduction in the freshwater runoff, induces the long-term salinization of the lagoon. 4 Future climatic changes in the Lagoon of Venice Four scenario simulations were performed to assess the future changes in the lagoon characteristics. The climate change deltas used in the numerical experiments are reported in Table 2. The table reports also the deltas uncertainties as derived from the analysis of the observed climatic trends and from the standard deviation of the multi-model ensembles. The results of the climate experiments were used to investigate anomalies with respect to the 2020 control simulation. Even if we are aware that changes in the climate do not follow a linear evolution, the reported values allow the comparison of the observed past climate changes with future projections. In this context, the observed-based scenario—which replicates the actual trend of the site-specific climatic conditions—provides a benchmark for the global and regional model projections. From a first assessment, we can observe that the rate of change of the relative sea level in the recent past (last 30 years) is slightly lower than what is expected for the future (next 30 years) by all climate scenarios. On the other side, a much more intense climate warming was recently experienced in the study site (1.8 and 1.4◦C for the air and sea temperatures, respectively) than foreseen by the climate models in the different RCPs (1.1 and 1.0◦C for the air and sea temperatures, respectively, in the most extreme scenario). The number of extreme sea levels and consequent flooding of Venice will increase with sea level rise (Table 3). With a rise of the mean sea level of 0.22 m—as predicted in the RCP8.5 scenario for 2050—the number of acqua alta events will triplicate (54 in total) with 9 exceptional floods (considered when the peak sea level reaches 1.4 m and causing the flooding of about 60% of the city of Venice). The flooding events will remain concentrated in the winter period even if some events will occur in spring and summer. It has to be noted that our simulations do not consider changes in the storm surge component of the sea level boundary conditions and therefore the evolution of the extreme sea levels within the lagoon is mainly due to the rise of the RMSL. Climate studies generally agree in predicting a slight weakening of severe weather conditions in the Adriatic Sea (Lionello et al., 2017;Bonaldo et al., 2020;Benetazzo et al., 2022) with the consequent attenuation of the storm surge (Makris et al., 2023). However, our simulations consider changes in the tide and storm propagation within the lagoon due to future sea level rise—as a consequence of modifications in the dissipative processes—which could impact the extreme sea levels more than Frontiers in Climate 08 frontiersin.org Ferrarin et al. 10.3389/fclim.2023.1330388 FIGURE 5 Daily sea temperature time series recorded in the Venice Lagoon at PDS (black line). The dashed red, the dot-dashed green and the dashed blue lines indicate the daily 90th percentile threshold defining MHW, the mean daily climatology and the daily 10th percentile threshold defining MCS, respectively. Light red and light blue shadings display the detected MHW and MCS events, respectively. Frontiers in Climate 09 frontiersin.org Ferrarin et al. 10.3389/fclim.2023.1330388 Conceptualization, Writing—original draft, Writing—review & editing. AB: Writing—review & editing. MG: Data curation, Software, Writing—original draft, Writing—review & editing. Funding The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This work was partially supported by the Interreg project AdriaClim (Climate change information, monitoring and management tools for adaptation strategies in Adriatic coastal areas; Project ID 10252001) funded by the European Union under the V A Interreg Italy-Croatia CBC programme. Acknowledgments The authors wish to thank the Italian Institute for Environmental Protection and Research (ISPRA), the Tide Forecast and Early Warning Center of the city of Venice (CPSM), the Regional Agencies for Environmental Protection and Prevention of the Veneto (ARPAV), and Friuli Venezia-Giulia (ARPA FVG) for providing meteorological and oceanographic data. We thank Dr. Michela Sammartino for providing SSS data over the Northern Adriatic Sea. We thank Dr. Georg Umgiesser for critically reviewing the manuscript. Conflict of interest The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision. 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