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

== CCP4.1 Climate Change in the Mediterranean Basin ==

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=== CCP4.1.1 The Mediterranean Sea, Land and People ===

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The Mediterranean Basin, known for its exceptional environmental and socio-cultural richness, comprises the semi-enclosed Mediterranean Sea and the countries and regions bordering it, [[#footnote-000|3]] which belong to Europe, Asia and Africa (Figure CCP4.1). The region has a unique historical and environmental identity ( [[#Abulafia--2011|Abulafia, 2011]] ), despite undeniable variations in the environment, socioeconomic conditions and cultural traditions. The countries in the Mediterranean Basin hosted approximately 542 million people in 2020, a number which is expected to increase to 657 million by 2050 and 694 million by 2100. In 1950, only 23.7% of the Mediterranean population lived in countries of the south, this number increased to 41.2% in 2000, 46.3% in 2020, and is projected to reach 55.5% in 2050 and 64.6% in 2100 ( [[#UN%20DESA--2019|UN DESA, 2019]] ).

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'''Figure CCP4.1 |''' '''The Mediterranean region: Topography and bathymetry''' '''(colour bar in metres), main urban areas (population in thousands for 2020 from''' [http://www.naturalearthdata.com '''w''' '''ww.''' '''naturalearthda''' '''ta.com'''] '''), container ports (millions of TEU [twenty-foot container equivalent units] in 2017, from International Association of Ports and Harbours) and borders of the Mediterranean region used in WGI AR6 [[IPCC:Wg2:Chapter:Chapter-10|Chapter 10]] (Doblas-Reyes et al., 2021).'''

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=== CCP4.1.2 Main Findings from Previous Assessments ===

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All previous assessments of climate change for the Mediterranean Basin and its sub-regions indicate ongoing warming of the atmosphere and the sea, as well as projected warming and changes in rainfall ( [[#Stocker--2013|Stocker et al., 2013]] ; [[#Cherif--2020|Cherif et al., 2020]] ). The projected increase in climate hazards, in combination with high regional vulnerability and exposure make it a prominent ‘climate change hotspot’ ( [[#Giorgi--2006|Giorgi, 2006]] ), with a large number of vulnerable natural systems and socioeconomic sectors ( [[#Field--2014|Field et al., 2014]] ; [[#MedECC--2020|MedECC, 2020]] ). In addition to high temperatures, the main risk factor identified is drought, generally expected to increase in the region, significant already at global warming of only 1.5°C, reaching, for higher warming levels, intensities unprecedented during the past 10 ka ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ). In Southern Europe and North Africa, groundwater recharge and soil water content will consequently decline, especially during summer ( [[#Kovats--2014|Kovats et al., 2014]] ; [[#Niang--2014|Niang et al., 2014]] ).

With the changing climate, marine ecosystems have already undergone changes in structure, including the spread of tropical species from the Atlantic Ocean and the Red Sea ( ''high confidence'' ) and mass mortality in at least 25 invertebrate species, threatening, along with ocean acidification, marine ecosystems, including seagrass meadows ( [[#Hoegh-Guldberg--2014|Hoegh-Guldberg et al., 2014]] ; [[#Nurse--2014|Nurse et al., 2014]] ; [[#Pörtner--2014|Pörtner et al., 2014]] ; [[#Wong--2014|Wong et al., 2014]] ). Endemic marine species are at higher risk of extinction due to limited possibilities for migrating northward ( [[#Kovats--2014|Kovats et al., 2014]] ; [[#Poloczanska--2014|Poloczanska et al., 2014]] ; [[#Balzan--2020|Balzan et al., 2020]] ). Southern and eastern Mediterranean coastal systems with narrow dune belts and often rapid urbanisation are vulnerable to both warming and sea level rise ( [[#Seneviratne--2012|Seneviratne et al., 2012]] ; [[#Wong--2014|Wong et al., 2014]] ; [[#Balzan--2020|Balzan et al., 2020]] ).

Most Mediterranean land ecosystems are impacted negatively by drier conditions, causing the ranges of many endemic species to shrink, and the health and growth rates of trees to decline ( [[#Kovats--2014|Kovats et al., 2014]] ; [[#Niang--2014|Niang et al., 2014]] ; [[#Nurse--2014|Nurse et al., 2014]] ; [[#Settele--2014|Settele et al., 2014]] ). Climate change is expected to increase wildfire risk in the region ( [[#Kovats--2014|Kovats et al., 2014]] ), although earlier estimates of burnt area have been reduced in the most recent assessments to approx. 40–100%, considering that prevention and mitigation actions have successfully reduced this risk so far ( [[#Balzan--2020|Balzan et al., 2020]] ). Wetlands and mountain summits are hotspots for biodiversity loss and extinctions ( ''medium confidence'' ) ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ; [[#Nurse--2014|Nurse et al., 2014]] ; [[#IPBES--2018a|IPBES, 2018a]] ; [[#IPBES--2018b|IPBES, 2018b]] ; [[#Balzan--2020|Balzan et al., 2020]] ). Along with unsustainable land use practices, climate change is projected to increase soil erosion in semiarid areas ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ).

The increasing water scarcity was found to be a significant threat to agriculture ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ; [[#Kovats--2014|Kovats et al., 2014]] ; [[#Niang--2014|Niang et al., 2014]] ; [[#Mrabet--2020|Mrabet et al., 2020]] ). Associated with increased extreme temperatures, the Mediterranean is expected to become less attractive for tourism ( [[#Kovats--2014|Kovats et al., 2014]] ; [[#Nurse--2014|Nurse et al., 2014]] ; [[#Wong--2014|Wong et al., 2014]] ; [[#Dos%20Santos--2020|Dos Santos et al., 2020]] ). Several critical risks for human health increase due to climate change, including heat waves and vector-borne diseases ( [[#Kovats--2014|Kovats et al., 2014]] ; [[#Nurse--2014|Nurse et al., 2014]] ; [[#Linares--2020|Linares et al., 2020]] ). Adaptation options have been identified for many risks (buildings, water management, coastal protection, etc.) ( [[#Murray--2012|Murray et al., 2012]] ; [[#Revi--2014|Revi et al., 2014]] ; [[#Wong--2014|Wong et al., 2014]] ). There are synergies between adaptation and mitigation, for example, renewable energies or nature-based solutions focused on the conservation and restoration of ecosystems ( [[#Nurse--2014|Nurse et al., 2014]] ; [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ; [[#Vafeidis--2020|Vafeidis et al., 2020]] ).

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=== 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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'''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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=== CCP4.1.4 Detection and Attribution of Climate Change Impacts ===

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New evidence published since Working Group II Assessment Report 5 (WGII AR5) confirms that climate change is increasingly affecting many systems and sectors in the Mediterranean region ( ''high confidence'' ) (Figure CCP4.5; Chapters 9, 13 and 16). There is ''high confidence'' that climate change has worsened heat waves and droughts (CCP4.1.3; [[#Lionello--2014|Lionello et al., 2014]] ; [[#Caloiero--2018|Caloiero et al., 2018]] ; [[#Mathbout--2018|Mathbout et al., 2018]] ; [[#Spinoni--2019|Spinoni et al., 2019]] ), and ''medium to high confidence'' that heat waves are impacting marine ( [[#Rivetti--2014|Rivetti et al., 2014]] ; [[#Tsikliras--2014|Tsikliras and Stergiou, 2014]] ; [[#Stergiou--2016|Stergiou et al., 2016]] ; [[#Corrales--2017|Corrales et al., 2017]] ), freshwater and terrestrial ecosystems ( [[#Peñuelas--2018|Peñuelas et al., 2018]] ; [[#Bartsch--2020|Bartsch et al., 2020]] ; [[#Carosi--2021|Carosi et al., 2021]] ), as well as agriculture ( [[#El-Maayar--2013|El-Maayar and Lange, 2013]] ; [[#Ortas--2013|Ortas and Lal, 2013]] ; [[#Ponti--2014|Ponti et al., 2014]] ; [[#Garcia-Mozo--2015|Garcia-Mozo et al., 2015]] ; [[#Moore--2015|Moore and Lobell, 2015]] ; [[#Oteros--2015|Oteros et al., 2015]] ; [[#Di%20Lena--2018|Di Lena et al., 2018]] ) and fisheries ( [[#Fortibuoni--2015|Fortibuoni et al., 2015]] ; [[#Givan--2018|Givan et al., 2018]] ; [[#IPBES--2018a|IPBES, 2018a]] ). Heat waves have also increased thermal discomfort, especially in urban areas (WGI AR6 Chapter 10, Doblas-Reyes et al., 2021; WGI AR6 Chapter 12, Ranasinghe et al., 2021; [[#Zinzi--2017|Zinzi and Carnielo, 2017]] ). Despite increasing wildfire hazard, forest fires are generally decreasing in the European part of the basin, due to more efficient risk management ( ''medium confidence'' ) ( [[#Turco--2016|Turco et al., 2016]] ; 2017). Mixed trends of increasing and decreasing flash and river floods across the Mediterranean are reported, but there is ''low confidence'' in their attribution to climate change ( [[#Mediero--2014|Mediero et al., 2014]] ; [[#Baahmed--2015|Baahmed et al., 2015]] ; [[#Gaume--2016|Gaume et al., 2016]] ; [[#Paprotny--2018|Paprotny et al., 2018]] ; [[#Blöschl--2019|Blöschl et al., 2019]] ; [[#Vicente-Serrano--2019|Vicente-Serrano et al., 2019]] ).

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'''Figure CCP4.5 |''' '''Attribution of observed impacts of climate change in the Mediterranean region (see SMCCP4.''' '''1 for supporting references).'''

Flooding, erosion and salinisation are significant observed impacts in coastal regions, especially where subsidence is significant, such as in the region of Thessaloniki in Greece or the eastern Nile Delta in Egypt ( [[#Raucoules--2008|Raucoules et al., 2008]] ; [[#Frihy--2010|Frihy et al., 2010]] ), with only ''low confidence'' in attribution to climate change so far (Section SMCCP4.1). Coastal urbanisation and engineering protection are expanding in the Mediterranean, resulting in substantial impacts on coastal biodiversity ( [[#Masria--2015|Masria et al., 2015]] ; [[#Carranza--2020|Carranza et al., 2020]] ).

The attribution of impacts displays little variability across sub-regions, but confidence in attribution to climate change is higher in the north, due to the larger number of observations and studies in Europe. While land use and fisheries are still major non-climatic drivers of changing hazards and biodiversity losses ( [[#Aguilera--2015|Aguilera et al., 2015]] ; [[#Turco--2016|Turco et al., 2016]] ; [[#IPBES--2018a|IPBES, 2018a]] ; 2018b; [[#Tramblay--2019|Tramblay et al., 2019]] ; [[#Vicente-Serrano--2019|Vicente-Serrano et al., 2019]] ), impacts of climate change are now being observed in all parts of the Mediterranean region ( ''high confidence'' ).

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