Editing IPCC:AR6/WGII/Cross-Chapter-Paper-4 (section)

=== CCP4.1.3 Observed and Projected Climate Change ===

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The Mediterranean Basin is located in a transition zone between mid-latitude and subtropical atmospheric circulation regimes, with large topographic gradients. The analysis of observed climate changes and their impacts is strongly affected by the imbalance of observations between northern and southern countries, where available time series have often not allowed past climate evolution to be reconstructed over a sufficiently long-time scale ( [[#Cramer--2018|Cramer et al., 2018]] ).

Since the 1980s, Mediterranean atmospheric warming has exceeded global average rates ( ''high confidence'' ) (WGI AR6 Chapter 11, Seneviratne et al., 2021; [[#Lionello--2018|Lionello and Scarascia, 2018]] ; [[#Cherif--2020|Cherif et al., 2020]] ). Future annual and summer warming rates are projected to be 20% and 50% larger than the global annual average, respectively. Summer warming is projected to be particularly strong in the north (Figure CCP4.2, WGI AR6; Chapter 11, Seneviratne et al., 2021; [[#Mariotti--2015|Mariotti et al., 2015]] ; [[#Lionello--2018|Lionello and Scarascia, 2018]] ). Temperature extremes and heat waves have increased in intensity, number, and length during recent decades, particularly in summer, and are projected to continue increasing ( ''high confidence'' ) (WGI AR6 Chapter 11, Seneviratne et al., 2021; [[#Zittis--2016|Zittis et al., 2016]] ; [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ; [[#Cherif--2020|Cherif et al., 2020]] ).

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[[File:9398d7b61edeea62675553a4bec9511e IPCC_AR6_WGII_Figure_CCP4_002.png]]

'''Figure CCP4.2 |''' '''Changes in climate impact drivers with respect to the 1995–2014 period for 1.''' '''5°C (left column) and 3°C (right column) global warming:''' mean summer (June to August) temperature (°C, a, b), number of days with maximum temperature above 40°C (days, c, d), total precipitation during the cold (October to March) season (%, e, f) and 1-day maximum precipitation (mm, g, h). Values based on CMIP6 global projections and SSP5-8.5. Sea level rise concerns the long term (2081–2100) and SSP1-2.6 for (i) and SSP3-7.0 for (j) (source: Annex I: Atlas).

Sea surface temperatures have increased in recent decades ( ''high confidence'' ), with regional variation between +0.29°C and +0.44°C per decade ( [[#Darmaraki--2019a|Darmaraki et al., 2019a]] ), and stronger trends in the eastern basin ( [[#Iona--2018|Iona et al., 2018]] ; [[#Pastor--2019|Pastor et al., 2019]] ), involving the whole upper mixed layer ( [[#Rivetti--2017|Rivetti et al., 2017]] ). Towards the end of the 21st century, ocean warming in the range 0.8°C–3.8°C is projected near the surface ( ''high confidence'' ), 0.8°C–3.0°C at intermediate depth and 0.15°C–0.18°C in deeper waters ( [[#Darmaraki--2019b|Darmaraki et al., 2019b]] ; [[#Soto-Navarro--2020|Soto-Navarro et al., 2020]] ). The duration and intensity of marine heat waves have increased ( ''high confidence'' ) ( [[#Darmaraki--2019a|Darmaraki et al., 2019a]] ) and both parameters are projected to continue increasing in the future ( [[#Galli--2017|Galli et al., 2017]] ). Under Representative Concentration Pathway (RCP) 8.5, at least one long-lasting marine heat wave is projected for every year by 2100, up to 3 months longer and about four times more intense than present-day events (WGI AR6 Chapter 9, Fox-Kemper et al., 2021; [[#Darmaraki--2019b|Darmaraki et al., 2019b]] ). Salinity is projected to increase, with anomalies from +0.48 to +0.89 psu by the end of the century ( ''medium confidence'' ) (WGI AR6 Chapter 9, Fox-Kemper et al., 2021; [[#Adloff--2015|Adloff et al., 2015]] ).

Observed trends in annual precipitation are significant only in some areas and some periods, and they are stationary over the long term throughout the region ( ''medium confidence'' ) (WGI AR6 Chapter 11, Seneviratne et al., 2021; Figure CCP4.3; [[#Harris--2014|Harris et al., 2014]] ; [[#Lionello--2018|Lionello and Scarascia, 2018]] ; [[#Vicente-Serrano--2020|Vicente-Serrano et al., 2020]] ). Precipitation is projected to decrease ( ''high confidence'' for global warming levels above 2°C) (Figure CCP4.2) by approximately 4% per 1°C global warming, for all seasons in the central and southern basin, and mostly in summer in the north ( [[#Mariotti--2015|Mariotti et al., 2015]] ; [[#Hertig--2017|Hertig and Tramblay, 2017]] ; [[#Lionello--2018|Lionello and Scarascia, 2018]] ). Precipitation extremes have increased in some northern areas ( ''medium confidence'' ), and are projected to increase in the north ( ''high confidence'' for global warming levels above 2°C), potentially accompanied by an increase in of flash floods ( [[#Llasat--2016|Llasat et al., 2016]] ), with no change in the south ( ''low confidence'' ) (WGI AR6 ATLAS, Gutiérrez et al. 2021; Figures CCP4.2; CCP4.3; [[#Tramblay--2018|Tramblay and Somot, 2018]] ; [[#Lionello--2020|Lionello and Scarascia, 2020]] ). These trends enhance the gradient between northern (already characterised by more intense events) and southern areas (where extreme precipitation events are comparatively milder) ( [[#Giorgi--2014|Giorgi et al., 2014]] ; [[#Jacob--2014|Jacob et al., 2014]] ; [[#Vautard--2014|Vautard et al., 2014]] ; [[#Lionello--2020|Lionello and Scarascia, 2020]] ).

Widespread increase of evaporative demand and some decrease of precipitation explain the drying of the Mediterranean region during recent decades ( ''high confidence'' ) (WGI AR6 Chapter 11, Seneviratne et al., 2021; Figure CCP4.3) ( [[#Spinoni--2015|Spinoni et al., 2015]] ; [[#Gudmundsson--2016|Gudmundsson and Seneviratne, 2016]] ; [[#Spinoni--2017|Spinoni et al., 2017]] ; [[#Stagge--2017|Stagge et al., 2017]] ; [[#Caloiero--2018|Caloiero et al., 2018]] ). Droughts are projected to become more severe, more frequent and longer under moderate emission scenarios, and strongly enhanced under severe emission scenarios ( ''high confidence'' ) (WGI AR6 Chapter 11, Seneviratne et al. 2021; [[#Hertig--2017|Hertig and Tramblay, 2017]] ; [[#Lehner--2017|Lehner et al., 2017]] ; [[#Ruosteenoja--2018|Ruosteenoja et al., 2018]] ; [[#Spinoni--2018b|Spinoni et al., 2018b]] ; [[#Grillakis--2019|Grillakis, 2019]] ; [[#Lionello--2020|Lionello and Scarascia, 2020]] ).

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'''Figure CCP4.3 |''' '''Observed and projected (at global warming levels of 1''' '''.''' '''5°C and 3°C) direction of change of climate drivers and confidence levels for Mediterranean land sub-regions.'''

No trends in mid-latitude cyclones crossing the Mediterranean Basin have been detected for recent decades ( [[#Lionello--2016|Lionello et al., 2016]] ). For Mediterranean hurricanes (‘medicanes’), no observed trends are known because of insufficient monitoring. In the future, mid-latitude cyclones and medicanes are projected to decrease in frequency, but medicane intensity will ''likely'' increase ( [[#Cavicchia--2014|Cavicchia et al., 2014]] ; [[#Nissen--2014|Nissen et al., 2014]] ; [[#Romera--2017|Romera et al., 2017]] ).

Mediterranean waters have acidified since the pre-industrial period, more rapidly than the global ocean, due to faster ventilation times ( ''high confidence'' ) ( [[#Palmiéri--2015|Palmiéri et al., 2015]] ). Acidification is projected to continue ( ''virtually certain'' ) (WGI AR6 Chapter 11, Seneviratne et al., 2021), with a pH decrease of up to -0.46 in a high emission scenario ( [[#Goyet--2016|Goyet et al., 2016]] ).

Mediterranean mean sea level has risen by 1.4±0.2 mm yr −1 during the 20th century ( [[#Wöppelmann--2012|Wöppelmann and Marcos, 2012]] ) and accelerated to 2.4±0.5 mm yr −1 for 1993 to 2012 ( [[#Bonaduce--2016|Bonaduce et al., 2016]] ) and 3.4 mm yr −1 for 1990 to 2009 in the northwest ( ''medium confidence'' ) ( [[#Calvo--2011|Calvo et al., 2011]] ). The accelerating trend is robust, although different methods and time horizons yield slightly different rates of change ( [[#Meyssignac--2011|Meyssignac et al., 2011]] ; [[#Cazenave--2018|Cazenave et al., 2018]] ; [[#von%20Schuckmann--2020|von Schuckmann et al., 2020]] ). For 2150, sea level is ''likely'' to reach 0.52 m [0.32–0.81] for SSP1-1.9, to 1.22 [0.91–1.78] for SSP5-8.5 relative to 1996–2014 ( ''medium confidence'' ) (WGI AR6 Chapter 9, Fox-Kemper et al., 2021; Figure FAQ CCP4.2; SMCCP4.4), with uncertain variation between sub-basins ( [[#Slangen--2017|Slangen et al., 2017]] ). Melting processes in Greenland and Antarctica could result in even higher levels ( ''low confidence'' , WGI AR6 Chapter 9, Fox-Kemper et al., 2021; Cross-Chapter Box SLR in Chapter 3).

The Mediterranean Basin includes within small distances a large variety of climatic conditions that are ''likely'' to shift northwards with global warming. Consequently, ecoregions will be exposed to potentially unsuitable conditions: more arid climate for the Mediterranean forests of North Africa, more subtropical climate and temperate climate for the mountain forests of the Balkans and of the Alps, respectively, and Mediterranean climate for the temperate forests of North Anatolia (Figure CCP4.4; [[#Lelieveld--2012|Lelieveld et al., 2012]] ; [[#Simpson--2014|Simpson et al., 2014]] ).

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'''Figure CCP4.4 |''' '''Climate and natural land ecosystems in the Mediterranean Basin, based on Köppen-Geiger climate types, for the baseline climate (a, 1985–2014) and the future climate (b, 2076–2100, A1FI scenario (corresponding to global warming of approximately 4°C), based on ( [[#Rubel--2010|Rubel and Kottek, 2010]] ), with the three terrestrial biodiversity hot spots that are present in the region (see WG2 Cross-Chapter Paper 1: Biodiversity Hotspots).'''

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