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

==== 2.4.4.2 Observed Changes in Wildfire ====

<div id="h3-25-siblings" class="h3-siblings"></div>

<div id="2.4.4.2.1" class="h4-container"></div>

<span id="detection-and-attribution-of-observed-changes-in-wildfire"></span>
===== 2.4.4.2.1 Detection and attribution of observed changes in wildfire =====

<div id="h4-17-siblings" class="h4-siblings"></div>

Wildfire is a natural and essential component of many forest and other terrestrial ecosystems. Excessive wildfire, however, can kill people, cause respiratory disease, destroy houses, emit carbon dioxide and damage ecosystem integrity (see Sections 2.4.4.2 and 2.4.4.4). Anthropogenic climate change increases wildfire by exacerbating its three principal driving factors: heat, fuel and ignition ( [[#Moritz--2012|Moritz et al., 2012]] ; [[#Jolly--2015|Jolly et al., 2015]] ). Non-climatic factors also contribute to wildfires—in tropical areas, fires are set intentionally to clear forest for agricultural fields and livestock pastures ( [[#Bowman--2020|Bowman et al., 2020]] ). Urban areas and roads create ignition hazards. Governments in many temperate-zone countries implement policies to suppress fires, even natural ones, producing unnatural accumulations of fuel in the form of coarse woody debris and high densities of small trees ( [[#Ruffault--2015|Ruffault and Mouillot, 2015]] ; [[#Hessburg--2016|Hessburg et al., 2016]] ; [[#Andela--2017|Andela et al., 2017]] ; [[#Balch--2017|Balch et al., 2017]] ; [[#Lasslop--2017|Lasslop and Kloster, 2017]] ; [[#Aragao--2018|Aragao et al., 2018]] ; [[#Kelley--2019|Kelley et al., 2019]] ). Globally, 4.2 million km 2 of land per year burned on average from 2002 to 2016 ( [[#Giglio--2018|Giglio et al., 2018]] ), with the highest fire frequencies in the Amazon rainforest, deciduous forests and savannas in Africa and deciduous forests in northern Australia ( [[#Earl--2018|Earl and Simmonds, 2018]] ; [[#Andela--2019|Andela et al., 2019]] ).

Since the AR5 and the IPCC Special Report on Land, published research has detected increases in the area burned by wildfire, analysed relative contributions of climate and non-climate factors and attributed burned area increases above natural (recent historical) levels to anthropogenic climate change in one part of the world, western North America ( ''robust evidence'' , ''high agreement)'' ( [[#Abatzoglou--2016|Abatzoglou and Williams, 2016]] ; [[#Partain--2016|Partain et al., 2016]] ; [[#Kirchmeier-Young--2019|Kirchmeier-Young et al., 2019]] ; [[#Mansuy--2019|Mansuy et al., 2019]] ; [[#Bowman--2020|Bowman et al., 2020]] ). Across the western USA, increases in vegetation aridity due to higher temperatures from anthropogenic climate change doubled burned area from 1984 to 2015 over what would have burned due to non-climate factors including unnatural fuel accumulation from fire suppression, with the burned area attributed to climate change accounting for 49% (32–76%, 95% confidence interval) of cumulative burned area ( [[#Abatzoglou--2016|Abatzoglou and Williams, 2016]] ). Anthropogenic climate change doubled the severity of a southwest North American drought from 2000 to 2020 that has reduced soil moisture to its lowest levels since the 1500s ( [[#Williams--2020|Williams et al., 2020]] ), driving half of the increase in burned area ( [[#Abatzoglou--2016|Abatzoglou and Williams, 2016]] ; [[#Holden--2018|Holden et al., 2018]] ; [[#Williams--2019|Williams et al., 2019]] ). In British Columbia, Canada, the increased maximum temperatures due to anthropogenic climate change increased burned area in 2017 to its highest extent in the 1950–2017 record, seven to eleven times the area that would have burned without climate change ( [[#Kirchmeier-Young--2019|Kirchmeier-Young et al., 2019]] ). In Alaska, USA, the high maximum temperatures and extremely low relative humidity due to anthropogenic climate change accounted for 33–60% of the probability of wildfire in 2015, when the area burned was the second highest in the 1940–2015 record ( [[#Partain--2016|Partain et al., 2016]] ). In protected areas of Canada and the USA, climate factors (temperature, precipitation, relative humidity and evapotranspiration) accounted for 60% of burned area from local human and natural ignitions from 1984 to 2014, outweighing local human factors (population density, roads and built area) ( [[#Mansuy--2019|Mansuy et al., 2019]] ).

In summary, field evidence shows that anthropogenic climate change has increased the area burned by wildfire above natural levels across western North America in the period 1984–2017, at GMST increases of 0.6°C–0.9°C, increasing burned area up to 11 times in one extreme year and doubling it (over natural levels) in a 32-year period ( ''high confidence'' ).

<div id="2.4.4.2.2" class="h4-container"></div>

<span id="observed-changes-in-wildfire-globally"></span>
===== 2.4.4.2.2 Observed changes in wildfire globally =====

<div id="h4-18-siblings" class="h4-siblings"></div>

Regarding global terrestrial area as a whole, wildfire trends vary depending on the time period of analysis. From 1900 to 2000, global average fire frequency, based on field data, increased 0.4% but the change was not statistically significant ( [[#Gonzalez--2010|Gonzalez et al., 2010]] ). Fire frequency increased on one-third of global land, mainly from burning for agricultural clearing in Africa, Asia and South America, slightly less than the area of fire frequency decrease, mainly from fire suppression across Australia, North America and Russia ( [[#Gonzalez--2010|Gonzalez et al., 2010]] ). Analyses of the Global Fire Emissions Database document shows that, from 1996 to 2015, global burned area decreased at a rate of −0.7% yr -1 ( [[#Forkel--2019|Forkel et al., 2019]] ) but the change was not statistically significant ( [[#Giglio--2013|Giglio et al., 2013]] ). From 1998 to 2015, global burned area decreased at a rate of −1.4 ± 0.5% yr -1 ( [[#Andela--2017|Andela et al., 2017]] ). The area of fire increases was one-third of the area of decreases, due to reduced vegetation cover from agricultural expansion and intensification ( [[#Andela--2017|Andela et al., 2017]] ) and from increased precipitation ( [[#Forkel--2019|Forkel et al., 2019]] ). Furthermore, much of the decreasing trend derives from two years: 1998 with a high burned area and 2013 with low burned area ( [[#Forkel--2019|Forkel et al., 2019]] ). Wildfire does not show a clear long-term trend for the world as a whole because of increases and decreases in different regions ( ''medium evidence'' , ''medium agreement'' ).

Where the global average burned area has decreased in the past two decades, higher correlations of rates of change in burning to human population density, cropland area and livestock density than to precipitation indicate that agricultural expansion and intensification were the main causes ( [[#Andela--2017|Andela et al., 2017]] ). The global decrease of fire frequency from 2000 to 2010 is correlated with increasing human population density ( [[#Knorr--2014|Knorr et al., 2014]] ). The fire-reducing effect of reduced vegetation cover following expansion of agriculture and livestock herding can counteract the fire-increasing effect of the increased heat and drying associated with climate change ( [[#Lasslop--2017|Lasslop and Kloster, 2017]] ; [[#Arora--2018|Arora and Melton, 2018]] ; [[#Forkel--2019|Forkel et al., 2019]] ). The reduced burning needed after the initial clearing for agricultural expansion drives much of the decline in fires in the Tropics ( [[#Andela--2017|Andela et al., 2017]] ; [[#Earl--2018|Earl and Simmonds, 2018]] ; [[#Forkel--2019|Forkel et al., 2019]] ). The human influence on fire ignition can be seen through the decrease documented on holy days (Sundays and Fridays) and traditional religious days of rest ( [[#Earl--2015|Earl et al., 2015]] ). Overall, human land use exerts an influence on wildfire trends for global terrestrial area as a whole that can be stronger than climate change ( ''medium confidence'' ).

<div id="2.4.4.2.3" class="h4-container"></div>

<span id="observed-changes-in-wildfire-in-individual-regions-with-complex-attribution"></span>
===== 2.4.4.2.3 Observed changes in wildfire in individual regions with complex attribution =====

<div id="h4-19-siblings" class="h4-siblings"></div>

While burned area has increased in parts of Asia, Australia, Europe and South America, published research has not yet attributed the increases to anthropogenic climate change ( ''medium evidence'' , ''high agreement'' ).

In the Amazon, deforestation for agricultural expansion and the degradation of forests adjacent to deforested areas cause wildfire in moist humid tropical forests not adapted to fire ( ''robust evidence'' , ''high agreement'' ) ( [[#Fonseca--2017|Fonseca et al., 2017]] ; [[#van%20Marle--2017|van Marle et al., 2017]] ; [[#da%20Silva--2018|da]] [[#Silva--2018|Silva et al., 2018]] ; [[#da%20Silva--2021|da Silva et al., 2021]] ; [[#dos%20Reis--2021|dos Reis et al., 2021]] ; [[#Libonati--2021|Libonati et al., 2021]] ). Roads facilitate deforestation, fragmenting the rainforest and increasing the dryness and flammability of vegetation ( [[#Alencar--2015|Alencar et al., 2015]] ). Extreme droughts that occur during warm phases of the ENSO and the Atlantic Multi-Decadal Oscillation combine with the degradation of vegetation to cause extreme fire events ( ''robust evidence'' , ''high agreement'' ) ( [[#Fonseca--2017|Fonseca et al., 2017]] ; [[#Aragao--2018|Aragao et al., 2018]] ; [[#da%20Silva--2018|da]] [[#Silva--2018|Silva et al., 2018]] ; [[#Burton--2020|Burton et al., 2020]] ; [[#dos%20Reis--2021|dos Reis et al., 2021]] ; [[#Libonati--2021|Libonati et al., 2021]] ). In the State of Roraima, Brazil, distance to roads and infrastructure that enable deforestation and ENSO were the factors most explaining fire occurrence in the extreme 2015–2016 fire season ( [[#Fonseca--2017|Fonseca et al., 2017]] ). From 1973 to 2014, burned area increased in the Amazon, coinciding with increased deforestation ( [[#van%20Marle--2017|van Marle et al., 2017]] ). In the State of Acre, Brazil, burned area increased 36-fold from 1984 to 2016, with 43% burned in agricultural and livestock settlement areas ( [[#da%20Silva--2018|da]] [[#Silva--2018|Silva et al., 2018]] ). In the extreme fire year 2019, 85% of the area burned in the Amazon occurred in areas deforested in 2018 ( [[#Cardil--2020|Cardil et al., 2020]] ). Even though relatively higher moisture in 2019 led to burning below the 2002–2019 average across most of South America, burning in areas of recent deforestation in the Amazon were above the 2002–2019 average, indicating that deforestation, not meteorological conditions, triggered the 2019 fires ( [[#Kelley--2021|Kelley et al., 2021]] ; [[#Libonati--2021|Libonati et al., 2021]] ). Furthermore, from 1981 to 2018, deforestation in the Amazon reduced moisture inputs to the lower atmosphere, increasing drought and fire in a self-reinforcing feedback ( [[#Xu--2020|Xu et al., 2020]] ). In the Amazon, deforestation exerts an influence on wildfire that can be stronger than climate change ( ''robust evidence'' , ''high agreement'' ).

In Australia, burned area increased significantly between the periods 1950–2002 and 2003–2020 in the southeast state of Victoria, with the area burned in the 2019–2020 bushfires being the highest on record ( [[#Lindenmayer--2020|Lindenmayer and Taylor, 2020]] ). In addition to the deaths of dozens of people and the destruction of thousands of houses, the 2019–2020 bushfires burned almost half of the area protected for conservation in Victoria, two-thirds of the forests allocated for timber harvesting ( [[#Lindenmayer--2020|Lindenmayer and Taylor, 2020]] ), wildlife and extensive areas of habitat for threatened plant and animal species ( [[#Geary--2021|Geary et al., 2021]] ). Generally, past timber harvesting did not lead to more severe fire canopy damage ( [[#Bowman--2021b|Bowman et al., 2021b]] ). Across southeastern Australia, the fraction of vegetated area that burned increased significantly in eight of the 32 bioregions from 1975 to 2009, but decreased significantly in three bioregions ( [[#Bradstock--2014|Bradstock et al., 2014]] ). Increases in four bioregions were correlated to increasing temperature and decreasing precipitation. Decreases in burned area occurred despite increased temperature and decreased precipitation. Analyses of climate across Australia from 1950 to 2017 ( [[#Dowdy--2018|Dowdy, 2018]] ; [[#Harris--2019|Harris and Lucas, 2019]] ) and during periods with extensive fires in 2017 in eastern Australia ( [[#Hope--2019|Hope et al., 2019]] ), in 2018 in northeastern Australia ( [[#Lewis--2020|Lewis et al., 2020]] ), and in period 2019–2020 in southeastern Australia ( [[#Abram--2021|Abram et al., 2021]] ; [[#van%20Oldenborgh--2021|van Oldenborgh et al., 2021]] ) indicate that temperature and drought extremes due to the ENSO, Southern Annular Mode and other natural inter-decadal cycles drive inter-annual variability of fire weather. While the effects of inter-decadal climate cycles on fire are superimposed on long-term climate change, the relative importance of anthropogenic climate change in explaining changes in burned area in Australia remains unquantified ( ''medium evidence'' , ''high agreement'' ).

In Africa, the rate of change of burned area on the continent as a whole ranged from a non-statistically significant −0.45% yr -1 in the period 2002–2016 ( [[#Zubkova--2019|Zubkova et al., 2019]] ) to a significant −1.9% yr -1 in the period 2001–2016 ( [[#Wei--2020|Wei et al., 2020]] ). These decreases coincided with areas of agricultural expansion or areas where drought reduced fuel loads ( [[#Zubkova--2019|Zubkova et al., 2019]] ; [[#Wei--2020|Wei et al., 2020]] ). It is possible, however, that the 500-m spatial resolution of Modis remote-sensing fire data underestimates the area burned in Africa by half, by missing small fires ( [[#Ramo--2021|Ramo et al., 2021]] ). In the Serengeti-Mara savanna of east Africa, burned area showed no significant change from 2001 to 2014, although an increase in domestic livestock would tend to reduce the grass cover that fuels savanna fires ( [[#Probert--2019|Probert et al., 2019]] ).

In Mediterranean Europe, the area burned in the region as a whole decreased from 1985 to 2011 ( [[#Turco--2016|Turco et al., 2016]] ), although the burned area for Spain did not show a significant long-term increase from 1968 to 2010 ( [[#Moreno--2014|Moreno et al., 2014]] ) whereas that for Portugal in 2017 was the highest in the period 1980–2017 ( [[#Turco--2019|Turco et al., 2019]] ). Increased summer maximum temperature and decreased soil moisture explained most of the burned area observed, suggesting a contribution of climate change, but fire suppression, fire prevention, agricultural abandonment and reforestation as well as the reduction in forest area exerted even stronger influences on burned area than the climate across Mediterranean Europe ( ''robust evidence'' , ''high agreement'' ) ( [[#Moreno--2014|Moreno et al., 2014]] ; [[#Turco--2017|Turco et al., 2017]] ; [[#Viedma--2018|Viedma et al., 2018]] ; [[#Turco--2019|Turco et al., 2019]] ).

In the Arctic tundra and boreal forest, where wildfire has naturally been infrequent, burned area showed statistically significant increases of ~50% yr -1 across Siberia, Russia, from 1996 to 2015 ( [[#Ponomarev--2016|Ponomarev et al., 2016]] ) and 2% yr -1 across Canada from 1959 to 2015 ( [[#Hanes--2019|Hanes et al., 2019]] ). Wildfire burned ~6% of the area of four extensive Arctic permafrost regions in Alaska, USA, eastern Canada and Siberia from 1999 to 2014 ( [[#Nitze--2018|Nitze et al., 2018]] ). In boreal forest in the Northwest Territories, Canada and Alaska, USA, the area burned by wildfire increased at a statistically significant rate of 6.8% yr -1 in the period 1975–2015, ( [[#Veraverbeke--2017|Veraverbeke et al., 2017]] ), with smouldering below-ground fires that lasted through the winter covering ~1% of burned area in the period 2002–2016 ( [[#Scholten--2021|Scholten et al., 2021]] ). While burned area was correlated with temperature and reduced precipitation in Siberia ( [[#Ponomarev--2016|Ponomarev et al., 2016]] ; [[#Masrur--2018|Masrur et al., 2018]] ) and correlated with lightning, temperature and precipitation in the Northwest Territories and Alaska ( [[#Veraverbeke--2017|Veraverbeke et al., 2017]] ), no attribution analyses have examined relative influences of climate and non-climate factors.

In Indonesia, deforestation and draining of peat swamp forests dries out the peat, providing substantial fuel for fires ( [[#Page--2016|Page and Hooijer, 2016]] ). Extreme fire years in Indonesia, including 1997, 2006 and 2015, coincided with extreme heat and aridity during the warm phase of the ENSO ( [[#Field--2016|Field et al., 2016]] ). Fire-resistant forest in 2019 covered only 3% of peatlands and 4.5% of non-peatlands on Sumatra and Kalimantan ( [[#Nikonovas--2020|Nikonovas et al., 2020]] ).

In Chile, the area burned in the summer of 2016–2017 was 14 times the mean for the period 1985–2016 and the highest on record ( [[#Bowman--2019|Bowman et al., 2019]] ). While this extreme fire year coincided with the highest daily mean maximum temperature in the period 1979–2017 ( [[#Bowman--2019|Bowman et al., 2019]] ) in central Chile (the area of highest fire activity), burned area from 1976 to 2013 showed the highest correlation with the precipitation cycles of the ENSO and the temperature cycles of the Antarctic Oscillation ( [[#Urrutia-Jalabert--2018|Urrutia-Jalabert et al., 2018]] ).

Overall, burned area has increased in the Amazon, Arctic, Australia and parts of Africa and Asia, consistent with, but not formally attributed to anthropogenic climate change ( ''medium evidence'' , ''high agreement'' ). Deforestation, peat draining, agricultural expansion or abandonment, fire suppression and inter-decadal cycles such as the ENSO exert a stronger influence than climate change on wildfire trends in numerous regions outside of North America ( ''high confidence'' ).

<div id="2.4.4.2.4" class="h4-container"></div>

<span id="observed-changes-in-fire-seasons-globally"></span>
===== 2.4.4.2.4 Observed changes in fire seasons globally =====

<div id="h4-20-siblings" class="h4-siblings"></div>

The IPCC AR6 WGI assessed fire weather ( [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ), while this chapter assesses the impacts of changes in fire weather: burned area and fire frequency. The global increases in temperature from anthropogenic climate change have increased aridity and drought, lengthening the fire weather season (the annual period with a heat and aridity index greater than half of its annual range) on one-quarter of global vegetated area and increasing the average fire season length by one-fifth from 1979 to 2013 ( [[#Jolly--2015|Jolly et al., 2015]] ). Climate change has contributed to increases in the fire weather season or the probability of fire weather conditions in the Amazon ( [[#Jolly--2015|Jolly et al., 2015]] ), Australia ( [[#Dowdy--2018|Dowdy, 2018]] ; [[#Abram--2021|Abram et al., 2021]] ; [[#van%20Oldenborgh--2021|van Oldenborgh et al., 2021]] ), Canada ( [[#Hanes--2019|Hanes et al., 2019]] ), central Asia ( [[#Jolly--2015|Jolly et al., 2015]] ), East Africa ( [[#Jolly--2015|Jolly et al., 2015]] ) and North America ( [[#Jain--2017|Jain et al., 2017]] ; [[#Williams--2019|Williams et al., 2019]] ; [[#Goss--2020|Goss et al., 2020]] ). In forest areas, the burned area correlates with fuel aridity, a function of temperature; in non-forest areas, the burned area correlates with high precipitation in the previous year, which can produce high grass fuel loads ( [[#Abatzoglou--2018|Abatzoglou et al., 2018]] ). Fire use in agriculture and raising livestock or other factors have generated a second fire season on approximately one-quarter of global land where fire is present, despite sub-optimal fire weather in the second fire season ( [[#Benali--2017|Benali et al., 2017]] ). In summary, anthropogenic climate change, through a 0.9°C surface temperature increase since the pre-industrial period, has lengthened or increased the frequency of periods with heat and aridity that favour wildfire on up to one-quarter of vegetated area since 1979 ( ''robust evidence, high agreement'' ).

<div id="2.4.4.2.5" class="h4-container"></div>

<span id="observed-changes-in-post-fire-vegetation"></span>
===== 2.4.4.2.5 Observed changes in post-fire vegetation =====

<div id="h4-21-siblings" class="h4-siblings"></div>

Globally, fire has contributed to biome shifts ( [[#2.4.3.2|Section 2.4.3.2]] ) and tree mortality (Sections 2.4.4.2, 2.4.4.3) attributed to anthropogenic climate change. Research since the AR5 has also found vegetation changes from wildfire due to climate change. Through increased temperature and aridity, anthropogenic climate change has driven post-fire changes in plant regeneration and species composition in South Africa ( [[#Slingsby--2017|Slingsby et al., 2017]] ), and tree regeneration in the western USA ( [[#Davis--2019b|Davis et al., 2019b]] ). In the fynbos vegetation of the Cape Floristic Region, South Africa, post-fire heat and drought and the legacy effects of exotic plant species reduced the regeneration of native plant species, decreasing species richness by 12% from 1966 to 2010 and shifting the average temperature tolerance of species communities upward by 0.5°C ( [[#Slingsby--2017|Slingsby et al., 2017]] ). In burned areas across the western USA, the increasing heat and aridity of anthropogenic climate change from 1979 to 2015 pushed low-elevation ponderosa pine ( ''Pinus ponderosa'' ) and Douglas fir ( ''Pseudotsuga menziesii'' ) forests across critical thresholds of heat and aridity that reduced the post-fire tree regeneration by half ( [[#Davis--2019b|Davis et al., 2019b]] ). In the southwestern USA, where anthropogenic climate change has caused drought ( [[#Williams--2019|Williams et al., 2019]] ) and increased wildfire ( [[#Abatzoglou--2016|Abatzoglou and Williams, 2016]] ), high-severity fires have converted some forest patches to shrublands ( [[#Barton--2018|Barton and Poulos, 2018]] ). Field evidence shows that anthropogenic climate change and wildfire, together, altered vegetation species composition in the southwestern USA and Cape floristic region, South Africa, reducing post-fire natural regeneration and species richness of tree and other plant species, between 1966 and 2015, at GMST increases of 0.3°C–0.9°C ( ''medium evidence'' , ''high agreement'' ).

<div id="2.4.4.3" class="h3-container"></div>

<span id="observed-changes-in-tree-mortality"></span>