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

== CCB3 Fire and climate change ==

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Raman Sukumar (India), Almut Arneth (Germany), Werner Kurz (Canada), Andrey Sirin (Russian Federation), Louis Verchot (Colombia/The United States of America)

Fires have been a natural part of Earth’s geological past and its biological evolution since at least the late Silurian, about 400 million years ago (Scott 2000 <sup>[[#fn:r430|430]]</sup> ). Presently, roughly 3% of the Earth’s land surface burns annually which affects both energy and matter exchanges between the land and atmosphere (Stanne et al. 2009 <sup>[[#fn:r431|431]]</sup> ). Climate is a major determinant of fire regimes through its control of fire weather, as well as through its interaction with vegetation productivity (fuel availability) and structure (fuel distribution and flammability) (Archibald et al. 2013 <sup>[[#fn:r432|432]]</sup> ) at the global (Krawchuk and Moritz 2011 <sup>[[#fn:r433|433]]</sup> ), regional (Pausas and Paula 2012 <sup>[[#fn:r434|434]]</sup> ) and local (Mondal and Sukumar 2016 <sup>[[#fn:r435|435]]</sup> ) landscape scales. Presently, humans are the main cause of fire ignition with lightning playing a lesser role globally (Bowman et al. 2017 <sup>[[#fn:r436|436]]</sup> ; Harris et al. 2016 <sup>[[#fn:r437|437]]</sup> ), although the latter factor has been predominantly responsible for large fires in regions such as the North American boreal forests (Veraverbeke et al. 2017 <sup>[[#fn:r438|438]]</sup> ). Humans also influence fires by actively extinguishing them, reducing spread and managing fuels.

'''Historical trends and drivers in land area burnt'''

While precipitation has been the major influence on wildfire regimes in pre-Industrial times, human activities have become the dominant drivers since then. There was less biomass burning during the 20th century than at any time during the past two millennia as inferred from charcoal sedimentary records (Doerr and Santín 2016 <sup>[[#fn:r439|439]]</sup> ), though there has been an increase in the most recent decades (Marlon et al. 2016 <sup>[[#fn:r440|440]]</sup> ). Trends in land area burnt have varied regionally (Giglio et al. 2013 <sup>[[#fn:r441|441]]</sup> ). Northern hemisphere Africa has experienced a fire decrease of 1.7 Mha yr <sup>–1</sup> (–1.4% yr <sup>–1</sup> ) since 2000, while southern hemisphere Africa saw an increase of 2.3 Mha yr <sup>–1</sup> (+1.8% yr <sup>–1</sup> ) during the same period. Southeast Asia witnessed a small increase of 0.2 Mha yr <sup>–1</sup> (+2.5% yr <sup>–1</sup> ) since 1997, while Australia experienced a sharp decrease of about 5.5 Mha yr <sup>–1</sup> (–10.7% yr <sup>–1</sup> ) during 2001–2011, followed by an upsurge in 2011 that exceeded the annual area burned in the previous 14 years. A recent analysis using the Global Fire Emissions Database v.4 (GFED4s) that includes small fires concluded that the net reduction in land area burnt globally during 1998–2015 was –24.3 ± 8.8% (–1.35 ± 0.49% yr <sup>–1</sup> ) (Andela et al. 2017 <sup>[[#fn:r442|442]]</sup> ). However, from the point of fire emissions it is important to consider the land cover types which have experienced changes in area burned; in this instance, most of the declines have come from grasslands, savannas and other non-forest land cover types (Andela et al. 2017 <sup>[[#fn:r443|443]]</sup> ). Significant increases in forest area burned (with higher fuel consumption per unit area) have been recorded in western and boreal North America (Abatzoglou and Williams 2016 <sup>[[#fn:r444|444]]</sup> ; Ansmann et al. 2018 <sup>[[#fn:r445|445]]</sup> ) and in boreal Siberia (Ponomarev et al. 2016 <sup>[[#fn:r446|446]]</sup> ) in recent times. The 2017 and 2018 fires in British Columbia, Canada, were the largest ever recorded since the 1950s with 1.2 Mha and 1.4 Mha of forest burnt, respectively (Hanes et al. 2018 <sup>[[#fn:r447|447]]</sup> ) and smoke from these fires reaching the stratosphere over central Europe (Ansmann et al. 2018 <sup>[[#fn:r448|448]]</sup> ).

Climate variability and extreme climatic events such as severe drought, especially those associated with the El Niño Southern Oscillation (ENSO), play a major role in fire upsurges, as in equatorial Asia (Huijnen et al. 2016 <sup>[[#fn:r449|449]]</sup> ). Fire emissions in tropical forests increased by 133% on average during and following six El Niño years compared to six La Niña years during 1997–2016, due to reductions in precipitation and terrestrial water storage (Chen et al. 2017 <sup>[[#fn:r450|450]]</sup> ). The expansion of agriculture and deforestation in the humid tropics has also made these regions more vulnerable to drought-driven fires (Davidson et al. 2012 <sup>[[#fn:r451|451]]</sup> ; Brando et al. 2014 <sup>[[#fn:r452|452]]</sup> ). Even when deforestation rates were overall declining, as in the Brazilian Amazon during 2003–2015, the incidence of fire increased by 36% during the drought of 2015 (Aragão et al. 2018 <sup>[[#fn:r453|453]]</sup> ).

'''GHG emissions from fires'''

Emissions from wildfires and biomass burning are a significant source of GHGs (CO <sub>2</sub> , CH <sub>4</sub> , N <sub>2</sub> O), carbon monoxide (CO), carbonaceous aerosols, and an array of other gases including non-methane volatile organic compounds (NMVOC) (Akagi et al. 2011 <sup>[[#fn:r454|454]]</sup> ; Van Der Werf et al. 2010 <sup>[[#fn:r455|455]]</sup> ). GFED4s has updated fire-related carbon emission estimates biome-wise (regionally and globally), using higher resolution input data gridded at 0.25 ̊, a new burned area dataset with small fires, improved fire emission factors (Akagi et al. 2011 <sup>[[#fn:r456|456]]</sup> ; Urbanski 2014 <sup>[[#fn:r457|457]]</sup> ) and better fire severity characterisation of boreal forests (van der Werf et al. 2017 <sup>[[#fn:r458|458]]</sup> ). The estimates for the period 1997–2016 are 2.2 GtC yr <sup>–1</sup> , being highest in the 1997 El Nino (3.0 GtC yr <sup>–1</sup> ) and lowest in 2013 (1.8 GtC yr <sup>–1</sup> ). Furthermore, fire emissions during 1997–2016 were dominated by savanna (65.3%), followed by tropical forest (15.1%), boreal forest (7.4%), temperate forest (2.3%), peatland (3.7%) and agricultural waste burning (6.3%) (van der Werf et al. 2017 <sup>[[#fn:r459|459]]</sup> ).

Fires not only transfer carbon from land to the atmosphere but also between different terrestrial pools: from live to dead biomass to soil, including partially charred biomass, charcoal and soot constituting 0.12–0.39 GtC yr <sup>–1</sup> or 0.2–0.6% of annual terrestrial NPP (Doerr and Santín 2016 <sup>[[#fn:r460|460]]</sup> ). Carbon from the atmosphere is sequestered back into regrowing vegetation at rates specific to the type of vegetation and other environmental variables (Loehman et al. 2014 <sup>[[#fn:r461|461]]</sup> ). Fire emissions are thus not necessarily a net source of carbon into the atmosphere, as post-fire recovery of vegetation can sequester a roughly equivalent amount back into biomass over a time period of one to a few years (in grasslands and agricultural lands) to decades (in forests) (Landry and Matthews 2016 <sup>[[#fn:r462|462]]</sup> ). Fires from deforestation (for land use change) and on peatlands (which store more carbon than terrestrial vegetation) obviously are a net source of carbon from the land to the atmosphere (Turetsky et al. 2014 <sup>[[#fn:r463|463]]</sup> ); these types of fires were estimated to emit 0.4 GtC yr <sup>–1</sup> in recent decades (van der Werf et al. 2017 <sup>[[#fn:r464|464]]</sup> ). Peatland fires dominated by smouldering combustion under low temperatures and high moisture conditions can burn for long periods (Turetsky et al. 2014 <sup>[[#fn:r465|465]]</sup> ).

'''Fires, land degradation/desertification and land-atmosphere exchanges'''

Flammable ecosystems are generally adapted to their specific fire regimes (Bond et al. 2005 <sup>[[#fn:r466|466]]</sup> ). A fire regime shift alters vegetation and soil properties in complex ways, both in the short- and the long-term, with consequences for carbon stock changes, albedo, fire-atmosphere-vegetation feedbacks and the ultimate biological capacity of the burnt land (Bond et al. 2004 <sup>[[#fn:r467|467]]</sup> ; Bremer and Ham 1999 <sup>[[#fn:r468|468]]</sup> ; MacDermott et al. 2016 <sup>[[#fn:r469|469]]</sup> ; Tepley et al. 2018 <sup>[[#fn:r470|470]]</sup> ; Moody et al. 2013 <sup>[[#fn:r471|471]]</sup> ; Veraverbeke et al. 2012 <sup>[[#fn:r472|472]]</sup> ) A fire-driven shift in vegetation from a forested state to an alternative stable state such as a grassland (Fletcher et al. 2014 <sup>[[#fn:r473|473]]</sup> ; Moritz 2015 <sup>[[#fn:r474|474]]</sup> ) with much less carbon stock is a distinct possibility. Fires cause soil erosion through action of wind and water (Moody et al. 2013 <sup>[[#fn:r475|475]]</sup> ) thus resulting in land degradation (Chapter 4) and eventually desertification (Chapter 3). Fires also affect carbon exchange between land and atmosphere through ozone (which retards photosynthesis) and aerosol (which slightly increases diffuse radiation) emissions. The net effect from fire on global GPP during 2002–2011 is estimated to be –0.86 ± 0.74 GtC yr <sup>–1</sup> (Yue and Unger 2018 <sup>[[#fn:r476|476]]</sup> ).

'''Fires under future climate change'''

Temperature increase and precipitation decline would be the major driver of fire regimes under future climates as evapotranspiration increases and soil moisture decreases (Pechony and Shindell 2010 <sup>[[#fn:r477|477]]</sup> ; Aldersley et al. 2011 <sup>[[#fn:r478|478]]</sup> ; Abatzoglou and Williams 2016 <sup>[[#fn:r479|479]]</sup> ; Fernandes et al. 2017 <sup>[[#fn:r480|480]]</sup> ). The risk of wildfires in future could be expected to change, increasing significantly in North America, South America, central Asia, southern Europe, southern Africa and Australia (Liu et al. 2010 <sup>[[#fn:r481|481]]</sup> ). There is emerging evidence that recent regional surges in wildland fires are being driven by changing weather extremes, thereby signalling geographical shifts in fire proneness (Jolly et al. 2015 <sup>[[#fn:r482|482]]</sup> ). Fire weather season has already lengthened by 18.7% globally between 1979 and 2013, with statistically significant increases across 25.3% but decreases only across 10.7% of Earth’s land surface covered with vegetation. Even sharper changes have been observed during the second half of this period (Jolly et al. 2015 <sup>[[#fn:r483|483]]</sup> ). Correspondingly, the global area experiencing long fire weather seasons (defined as experiencing a fire weather season greater than one standard deviation (SD) from the mean global value) has increased by 3.1% per annum or 108.1% during 1979–2013. Fire frequencies under 2050 conditions are projected to increase by approximately 27% globally, relative to the 2000 levels, with changes in future fire meteorology playing the most important role in enhancing global wildfires, followed by land cover changes, lightning activities and land use, while changes in population density exhibit the opposite effects (Huang et al. 2014 <sup>[[#fn:r484|484]]</sup> ).

However, climate is only one driver of a complex set of environmental, ecological and human factors in influencing fire regimes (Bowman et al. 2011 <sup>[[#fn:r485|485]]</sup> ). While these factors lead to complex projections of future burnt area and fire emissions (Knorr et al. 2016a <sup>[[#fn:r486|486]]</sup> , b <sup>[[#fn:r487|487]]</sup> ), human exposure to wildland fires could still increase due to population expansion into areas already under high risk of fires (Knorr et al. 2016a <sup>[[#fn:r488|488]]</sup> , b <sup>[[#fn:r489|489]]</sup> ). There are still major challenges in projecting future fire regimes and how climate, vegetation and socio/economic factors will interact (Hantson et al. 2016 <sup>[[#fn:r490|490]]</sup> ; Harris et al. 2016 <sup>[[#fn:r491|491]]</sup> ). There is also need for integrating various fire management strategies, such as fuel-reduction treatments in natural and planted forests, with other environmental and societal considerations to achieve the goals of carbon emissions reductions, maintain water quality, biodiversity conservation and human safety (Moritz et al. 2014 <sup>[[#fn:r492|492]]</sup> ; Gharun et al. 2017 <sup>[[#fn:r493|493]]</sup> ).

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'''Cross-Chapter Box 3, Figure 1'''

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'''The probability of low-fire regions becoming fire prone (positive values), or of fire-prone areas changing to a low-fire state (negative values) between 1971–2000 and 2071–2100 based on eight-Earth system model (ESM) ensembles, two Shared Socio-economic Pathways (SSPs) and two Representative Concentration Pathways (RCPs). Light grey: areas where at least one ensemble simulation predicts a positive […]'''
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[[File:477cef9af00c89e76edade38abff4a21 CCB3-Figure1-1024x770.jpg]]

The probability of low-fire regions becoming fire prone (positive values), or of fire-prone areas changing to a low-fire state (negative values) between 1971–2000 and 2071–2100 based on eight-Earth system model (ESM) ensembles, two Shared Socio-economic Pathways (SSPs) and two Representative Concentration Pathways (RCPs). Light grey: areas where at least one ensemble simulation predicts a positive and one a negative change (lack of agreement). Dark grey: area with &gt;50% past or future cropland. Fire-prone areas are defined as having a fire frequency of &gt;0.01 yr-1, (a) RCP4.5 emissions with SSP3 demographics, and (b) RCP8.5 emissions with SSP5 demographics (Knorr et al. 2016a).

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In summary, climate change is playing an increasing role in determining wildfire regimes alongside human activity ( ''medium confidence'' ), with future climate variability expected to enhance the risk and severity of wildfires in many biomes, such as tropical rainforests ( ''high confidence'' ). Fire weather seasons have lengthened globally between 1979 and 2013 ( ''low confidence'' ). Global land area burned has declined in recent decades, mainly due to less burning in grasslands and savannas ( ''high confidence'' ). While drought remains the dominant driver of fire emissions, there has recently been increased fire activity in some tropical and temperate regions during normal to wetter-than-average years due to warmer temperatures that increase vegetation flammability ( ''medium confidence'' ). The boreal zone is also experiencing larger and more frequent fires, and this may increase under a warmer climate ( ''medium confidence'' ).

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