Editing IPCC:AR6/WGII/Chapter-2 (section)

==== 2.5.2.4 Risk to Mediterranean-Type Ecosystems (MTEs) ====

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The regions containing MTEs all show ''high confidence'' in projected increases in the intensity and frequency of hot extremes and decreases in the intensity and frequency of cold extremes, and ''medium confidence'' in increasing ecological drought due to increased evapotranspiration (in all regions) and reduced rainfall (excluding California, USA, where model agreement is low) (see WGI Chapter 11). Projections also show a ''robust'' increase in the intensity and frequency of heavy precipitation in the event of ≥2°C warming for MTEs in South Africa, the Mediterranean Basin and California, USA, but are less clear for Australia and Chile ( [[#Seneviratne--2021|Seneviratne et al., 2021]] ).

MTEs are characterised by the distinctive seasonal timing of precipitation and temperature, and the disruption of this regime is likely to be critical for their maintenance. Unfortunately, projections of changes in rainfall seasonality have received less attention and are far more uncertain than many other aspects of climate change ( [[#Pascale--2016|Pascale et al., 2016]] ; [[#Breinl--2020|Breinl et al., 2020]] ), thus limiting our ability to predict the ecological consequences of climate change in MTEs. Responses to experimental manipulation of rainfall seasonality show the potential for shifts in plant functional composition and diversity loss, but results vary with soil type ( [[#van%20Blerk--2021|van Blerk et al., 2021]] ).

Unfortunately, global- and regional-scale dynamic vegetation models show a poor performance for large areas of MTEs, because they do not characterise shrub and Crassulacean acid metabolism (CAM)-photosynthetic plant functional types well ( [[#Moncrieff--2015|Moncrieff et al., 2015]] ). Furthermore, the grain of these models is too coarse for quantifying impacts to many vegetation formations which are patchy or of limited extent (e.g., small stands of trees). There is ''high confidence'' that observations of high mortality in trees and other growth forms, reduced reproductive and recruitment success, range shifts, community shifts towards more thermophilic species and type conversions are set to continue, due to either direct climate impacts through drought and other extreme weather events or to their interaction with factors like fire and pathogens (Sections 2.4.3.5, 2.4.3.6; 2.4.3.7; 2.4.4.2; 2.4.4.3; 2.5.2.5, 2.5.2.6, 2.5.2.7, 2.5.4).

Fire is a key driver across most MTEs due to summer-dry conditions. Climate projections for the MTEs translate into high confidence that periods of low fuel moisture will become more severe and prolonged, and that episodes of extreme fire weather will become more frequent and severe (see ( [[#Douville--2021|Douville et al., 2021]] ; [[#Seneviratne--2021|Seneviratne et al., 2021]] )). This will lead to the birth of novel fire regimes in MTEs, characterised by an increase in the probability of greater burned area and extreme wildfire events (e.g., megafires), with associated loss of human life and property, long-term impacts on ecosystems and acceleration of the possible loss of resilience and capacity to recover ( [[#Abatzoglou--2016|Abatzoglou and Williams, 2016]] ; [[#González--2018|González et al., 2018]] ; [[#Boer--2020|Boer et al., 2020]] ; [[#Moreira--2020|Moreira et al., 2020]] ; [[#Nolan--2020|Nolan et al., 2020]] ; [[#Duane--2021|Duane et al., 2021]] ; [[#Gallagher--2021|Gallagher et al., 2021]] ).

Fire is virtually certain to have additional impacts through compound events (see Section 11.8 in ( [[#Seneviratne--2021|Seneviratne et al., 2021]] )). Extreme post-fire weather is extremely likely to continue to impact diversity ( [[#Slingsby--2017|Slingsby et al., 2017]] ), retard vegetation regrowth ( [[#Slingsby--2020a|Slingsby et al., 2020a]] ) and accelerate vegetation shifts ( [[#Batllori--2019|Batllori et al., 2019]] ). Any increases in the intensity and frequency of heavy precipitation are highly likely to compromise soil stability in recently burnt areas ( [[#Morán-Ordóñez--2020|Morán-Ordóñez et al., 2020]] ). The impacts of fire often depend on interactions with non-climatic factors such as habitat fragmentation ( [[#Slingsby--2020b|Slingsby et al., 2020b]] ) and management ( [[#Steel--2015|Steel et al., 2015]] ) or the spread of flammable exotic plantation forestry and invasive species ( [[#Kraaij--2018|Kraaij et al., 2018]] ; [[#McWethy--2018|McWethy et al., 2018]] ). Managing these factors provides opportunities for adaptation and mitigation ( [[#Moreira--2020|Moreira et al., 2020]] ). (See sections 2.4.4.2 and 2.5.3.2).

Human adaptation and mitigation responses to climate change may create additional threats to MTEs. MTEs have dry summers by definition, posing a challenge for the year-round supply of water to growing human populations and agriculture. With recent major droughts in all MTEs ( [[#2.4.3.6|Section 2.4.3.6]] ), there is increasing reliance on groundwater for the bulk of the water supply ( [[#Kaiser--2018|Kaiser and Macleod, 2018]] ). The majority of groundwater systems have exceeded or are rapidly approaching their environmental flow limits ( [[#de%20Graaf--2019|de Graaf et al., 2019]] ), threatening human populations and ecosystems that depend on these systems for their persistence through unfavourable climatic conditions ( [[#McLaughlin--2017|McLaughlin et al., 2017]] ). Similarly, much of the MTEs are open shrublands and grasslands and proposed extensive tree-planting to sequester atmospheric CO 2 could result in a loss of biodiversity and threaten water security ( [[#Doblas-Miranda--2017|Doblas-Miranda et al., 2017]] ; [[#Bond--2019|Bond et al., 2019]] ).

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