Editing IPCC:AR6/WGI/Chapter-5 (section)

==== 5.4.3.2 Fire and Other Disturbances ====

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The SRCCL assessed that climate change is playing an increasing role in determining wildfire regimes alongside human activity ( ''medium confidence'' ), with future climate variability expected to enhance the recurrence and severity of wildfires in many biomes, such as tropical rainforests ( ''high confidence'' ). Projections of increased fire weather in a warmer climate are widespread ( [[IPCC:Wg1:Chapter:Chapter-12#12.3.2.8|Section 12.3.2.8]] ) and may drive increased fire frequency and severity in several regions, including Arctic and boreal ecosystems ( [[#Gauthier--2015|Gauthier et al., 2015]] ; X.J. [[#Walker--2019|]] [[#Walker--2019|Walker et al., 2019]] ), Mediterranean-type ecosystems ( [[#Turco--2014|Turco et al., 2014]] ; [[#Jin--2015|Jin et al., 2015]] ), degraded tropical forests ( [[#Aragão--2018|Aragão et al., 2018]] ), and tropical forest-savanna transition zones ( [[#Lehmann--2014|Lehmann et al., 2014]] ).

Wildfire is included in some CMIP6 ESMs (Table 5.4) and is thus only partially represented in estimates of carbon–climate feedbacks from these models. The CMIP5 ESMs that include fire project an 8–58% increase of fire carbon emissions under future scenarios, with higher emissions under higher warming scenarios; the ensemble spread is driven by differing factors such as population density, fire management, and other land-use processes ( [[#Kloster--2017|Kloster and Lasslop, 2017]] ). Fire dynamics in CMIP6 models, as evaluated in land-only configurations of CMIP6-generation land surface models, also show large variations but better agreement with observations ( [[#Teckentrup--2019|Teckentrup et al., 2019]] ; [[#Hantson--2020|Hantson et al., 2020]] ; [[#Lasslop--2020|Lasslop et al., 2020]] ).

Climate change also drives changes to vegetation composition and ecosystem carbon storage through other disturbances such as forest dieback that lead to biome shifts in tropical forests ( [[#Cox--2004|Cox et al., 2004]] ; [[#Jones--2009|Jones et al., 2009]] ; [[#Brando--2014|Brando et al., 2014]] ; [[#Le%20Page--2017|Le Page et al., 2017]] ; [[#Zemp--2017|Zemp et al., 2017]] ), and temperate and boreal regions ( [[#Joos--2001|Joos et al., 2001]] ; [[#Lucht--2006|Lucht et al., 2006]] ; [[#Scheffer--2012|Scheffer et al., 2012]] ; [[#Lasslop--2016|Lasslop et al., 2016]] ). The AR5 assessed that large-scale loss of tropical forests due to climate change is ''unlikely'' (WGI, Section 6.4.9). Newer ecosystem modelling approaches that include a greater degree of ecosystem heterogeneity and diversity show a reduced sensitivity of such forest dieback-type changes ( [[#Levine--2016|Levine et al., 2016]] ; [[#Sakschewski--2016|Sakschewski et al., 2016]] ), supporting the AR5 assessment ( [[#5.4.9|Section 5.4.9]] ). Beyond such biome shifts, observations of tropical forests also show that increasing tree mortality rates within tropical forests may reduce carbon turnover times and storage ( [[#Brienen--2015|Brienen et al., 2015]] ), that increased tree mortality rates in tropical forests and elsewhere are expected with increased temperatures and vapour pressure deficit (Cross-Chapter Box 5.1; [[#Allen--2015|Allen et al., 2015]] ; [[#McDowell--2018|McDowell et al., 2018]] ; [[#Grossiord--2020|Grossiord et al., 2020]] ), and that these processes are not well represented in ESMs ( [[#Powell--2013|Powell et al., 2013]] ; [[#Fisher--2018|Fisher et al., 2018]] ). An ensemble of land models that includes ecological processes such as forest demography shows that changes to mortality may be a more important driver of carbon dynamics than changes to productivity ( [[#Friend--2014|Friend et al., 2014]] ).

Overall, climate change will force widespread increases in fire weather throughout the world ( [[IPCC:Wg1:Chapter:Chapter-12#12.3.2.8|Section 12.3.2.8]] ). Because of incomplete inclusion of fire in ESMs, a separate compilation of fire-driven carbon–climate feedback estimates is shown in Figure 5.29, based on results from [[#Eliseev--2014a|Eliseev et al. (2014a)]] and [[#Harrison--2018|Harrison et al. (2018)]] . There is ''low agreement'' in magnitude and ''medium agreement'' in sign which leads to an assessment of ''medium confidence'' that fire represents a positive carbon–climate feedback, but ''very low confidence'' in the magnitude of that feedback. Other disturbances such as tree mortality will increase across several ecosystems ( ''medium agreement'' ) with decreased vegetation carbon ( ''medium confidence'' ). However, the lack of model agreement and key process representation in ESMs leads to a ''low confidence'' assessment in the projected magnitude of this feedback.

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