Editing IPCC:AR6/WGII/Chapter-13 (section)

===== 13.2.1.2.3 Water Temperature and Quality =====

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Water temperatures in rivers and lakes have increased over the past century by ~1–3°C in major European rivers (CBS, 2014; [[#EEA--2017a|EEA, 2017a]] ; [[#Woolway--2017|Woolway et al., 2017]] ). Warming is accelerating for all European river basins ( [[#Wanders--2019|Wanders et al., 2019]] ) increasing by 0.8°C in response to 1.5°C GWL and 1.2°C for 3°C GWL relative to 1971–2000 ( [[#van%20Vliet--2016a|van Vliet et al., 2016a]] ) aggravated by declines in summer river flow.

(Ground)water extractions or drainage have caused saltwater intrusions ( [[#Rasmussen--2013|Rasmussen et al., 2013]] ; [[#Ketabchi--2016|Ketabchi et al., 2016]] ). During summer, seawater will also penetrate estuaries further upstream in response to reduced river flow and SLR, resulting in more frequent closure of water inlets in the downstream part of the rivers in a period when water is most needed ( ''high agreement, low evidence'' ) (e.g., [[#Haasnoot--2020b|Haasnoot et al., 2020b]] ).

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'''Box 13.1 | Venice and Its Lagoon'''
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Venice and its lagoon are a UNESCO World Heritage Site. This socio-ecological system is the result of millennia of interactions between people and the natural environment. It is exposed to climatic and non-climatic hazards: more frequent floods, warming, pollution, invasive species, reduction of salt marshes, hydrodynamic and bathymetric changes, and waves generated by cruise ships and boat traffic.

The elevation of the average city pedestrian level and of its inner historic area are, respectively, 105 and 55 cm above the present relative mean sea level (RMSL). Consequently, even small surges and compound events cause floods when they coincide with high tide ( [[#Lionello--2021a|Lionello et al., 2021a]] ). During the 20th century, RMSL rose at about 2.5 mm yr –1 due to SLR and land subsidence ( [[#Zanchettin--2021|Zanchettin et al., 2021]] ). The frequency of floods affecting the city has increased from once per decade in the first half of the 20th century to 40 times per decade in the period 2010–2019 (Figure Box 13.1.1a).

In 1973, the Italian government established a legal framework for safeguarding Venice and its lagoon. Construction of the flood protection system started in 2003 and was used for the first time in October 2020 ( [[#Lionello--2021b|Lionello et al., 2021b]] ). This system of mobile barriers (MoSE) closes the lagoon inlets to avoid floods when needed, while under normal conditions they lay on the seabed, thus allowing ship traffic and the exchange between the lagoon and the sea ( [[#Molinaroli--2019|Molinaroli et al., 2019]] ). To prevent flooding of the central monument area, additional measures have been proposed including inlets, expansion of salt marshes and pumping seawater into deep brackish aquifers to raise the city’s level ( [[#Umgiesser--1999|Umgiesser, 1999]] ; [[#Umgiesser--2004|Umgiesser, 2004]] ; [[#Teatini--2011|Teatini et al., 2011]] ).

Without adaptation, potential economic damages between 7 and 17 billion EUR have been estimated for the next 50 years ( [[#Caporin--2016|Caporin and Fontini, 2016]] ). Additionally, the ecosystem is vulnerable to warming ( [[#Solidoro--2010|Solidoro et al., 2010]] ) and SLR (Day Jr et al., 1999; [[#Marani--2007|Marani et al., 2007]] ). The duration of the closure of the lagoon inlets is expected to increase from 2 to 3 weeks yr –1 for RMSL rises of 30 cm, to 2 months yr –1 for 50 cm and 6 months yr –1 for 75 cm (Figure Box 13.1.1b; [[#Umgiesser--2020|Umgiesser, 2020]] ; [[#Lionello--2021b|Lionello et al., 2021b]] ), resulting in disconnection from the sea for most of the time for RMSL rise exceeding 75 cm. Frequent closures of the inlets would prevent ship traffic and in/outflow of water. For Venice, adaptation pathways considering the full range of plausible RMSL (Figure Box 13.1.1c) levels are not available, indicating a long-term adaptation gap. As planning and implementation of adaptation of this extent can take several decades ( [[#Haasnoot--2020b|Haasnoot et al., 2020b]] ; Cross-Chapter Box SLR in Chapter 3), this increases the risk that the city will not be prepared in case of rapid SLR.

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'''Figure Box 13.1.1 |''' '''Venice sea level rise (SLR) and coastal flooding: (a)''' ''evolution of relative and mean sea level in Venice and decadal frequency of floods above the safeguard level in the city centre (Frederikse et al.'' '','' 2020; [[#Lionello--2021a|Lionello et al., 2021a]] ; [[#Lionello--2021b|Lionello et al., 2021b]] ; [[#Zanchettin--2021|Zanchettin et al., 2021]] ); '''(b)''' projected relative SLR at the Venetian coast ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ); “very likely ” corresponds to 5–95th percentile range, “likely ” to 17–83rd percentile range; '''(c)''' timing when critical relative sea level thresholds will be reached depending on scenarios and confidence level ( [[#Lionello--2012|Lionello, 2012]] ; [[#Umgiesser--2020|Umgiesser, 2020]] ; [[#Lionello--2021a|Lionello et al., 2021a]] ), the upper limit of the medium confidence range under SSP5–8.5 represents a low-likelihood, high-impact storyline, low confidence processes include ice sheet instability; '''(d)''' Landsat view of Venice and its lagoon with the three inlets connecting it to the Adriatic Sea.

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