Climate Change Increases the Risk of WildfiresScienceBrief Review September 2020 update

Adam J. P. Smith1, Matthew W. Jones1, John T. Abatzoglou2, Josep G. Canadell3, Richard A. Betts4, 5

1 Tyndall Centre for Climate Change Research, School of Environmental Sciences, University of East Anglia (UEA), UK. 2 Management of Complex Systems, University of California, Merced, Merced, CA, USA. 3 CSIRO Oceans and Atmosphere, G.P.O. Box 1700, Canberra, ACT 2601, Australia. 4 Met Office Hadley Centre, Exeter, UK. 5 College of Life and Environmental Sciences, University of Exeter, UK.

This ScienceBrief Review is part of a collection on Critical Issues in Climate Change Science, relevant to inform the COP26 climate conference to be held in Glasgow (2021). Eds: Corinne Le Quéré, Peter Liss, Piers Forster. Time stamp: Published 25 September 2020. The evidence reviewed was published between 16 March 2013 to 21 September 2020. Search keywords used: “Climate Change” AND “Fire” (also “Fire Weather” OR “Fire Danger”).

Snap shot of the Brief on climate change and wildfiresshowing the high consensus among the 116 scientificpublications examined (81 support, 0 refute). Click hereto visit the Brief.

BackgroundHuman-induced climate change promotes theconditions on which wildfires depend, enhancing theirlikelihood and challenging suppression efforts.Although the global area burned by fires each year isdeclining, the majority of this trend is explained byconversion of natural savannahs and grasslands toagriculture in Africa (Andela et al. 2017). In contrast,the area burned by forest wildfires is increasing inmany regions, including in the western US andsoutheast Australia. Here we focus on the impacts ofclimate change on “fire weather”, which affects thelikelihood of fires occurring and the severity of fireswhen they do occur.

“Fire weather” refers to periods with a highlikelihood of fire due to a combination of hightemperatures, low humidity, low rainfall and oftenhigh winds. A number of indices are used to track fireweather based on the meteorological informationavailable, and these indices broadly quantify the risk offire based on weather conditions at any given time andplace. “Fire weather season length” refers to thenumber of days per year of fire weather. Fire weatherseason length is on the rise globally, signalling a risingrisk of wildfires in many regions (see Figure).____________________________________________1Only studies published since the release of the fifthassessment report of the Intergovernmental Panel onClimate Change (IPCC), in 2013, were examined here.

Approach. We undertook a ScienceBrief Review onthe link between climate change and wildfire risk inJanuary 2020, reviewing 57 scientific articles. 116scientific articles are now available. This updatefocusses on articles relevant to the wildfires ongoing inthe western United States, new findings relevant to thewildfires that raged southeastern Australian during the2019-2020 season, and new findings since January2020. The full Brief and all publications can beviewed here: https://sciencebrief.org/topics/climate-change-science/wildfires

Summary. New scientific publications reviewedsince January 2020 strengthen the evidence thatclimate change increases the frequency and/orseverity of fire weather – periods with a high firerisk due to a combination of high temperatures, lowhumidity, low rainfall and often high winds – inmany regions around the world. The westernUnited States is among the regions where thetrends in fire weather have been most pronouncedin the past at least 40 years. Fire activity isinfluenced by a range of other factors includingland management practices. However, landmanagement alone cannot explain recent increasesin wildfire extent and intensity in the western US orsoutheast Australia because increased fire weatheramplifies fire risk where fuels remain available.

The new analysis shows that:• Well over 100 studies published since 20131 show

clear consensus that climate change promotes theweather conditions on which wildfires depend,enhancing their likelihood.

• Natural variability is superimposed on theincreasingly warm and dry background conditionsresulting from climate change, leading to moreextreme fires and more extreme fire seasons.

• Land management can enhance or compoundclimate-driven changes in wildfire risk, eitherthrough fuel reductions or fuel accumulation asunintended by-product of fire suppression. Firesuppression efforts are made more difficult byclimate change.

• There is an unequivocal and pervasive role ofclimate change in increasing the intensity andlength in which fire weather occurs; landmanagement is likely to have contributed too, butdoes not alone account for recent increases inwildfire extent and severity in the western US and insoutheast Australia.

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Climate change increases the risk of wildfires

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Fire weather has become more frequent andintense in western US forests. Fire weather hasbecome more frequent and intense in recent decades,lengthening by 8 days between 1979 and 2019 onaverage globally (Jolly et al., 2015; Figure 1). The USis among the regions where the frequency and severityof fire weather has shifted beyond the naturalconditions seen in the pre-industrial period (Abatzoglouet al., 2019). Goss et al. (2020) found that climatechange is increasing the likelihood of extreme autumnwildfire conditions across California with a doubling ofsuch days over the past four decades. Williams et al.(2019) also found that warming is enhancing thepotential for large wildfires in autumn, which typicallyoccur during strong, dry offshore winds and whenwinter precipitation is delayed. Khorshidia et al (2020)finds that megafire days (confluence of dry fuels/strongwinds) in southern California has increased by 3x overthis same period. Finally, Crockett et al. (2018) findsthat extreme wildfires happen more frequently duringdroughts, compounding the effects of climate trendsand climate variability. Overall, climate change isbringing hotter, drier weather to the western US andthe region is fundamentally more exposed to fire risksthan it was before humans began to alter the globalclimate. Recent trends in fire weather are projected tocontinue under a warming climate, with a 25% increasein extreme autumn fire days projected for California bythe late 21st century, even for a medium scenario offuture emissions (RCP 4.5; Goss et al., 2020).

Fire weather is driving more wildfire activity inwestern US forests. Hotter, drier weather meansforests are being primed to burn more regularly, andthis has led to an increase in fire activity in recentdecades. Williams et al. (2019) found that Californianforests experienced an eightfold increase in summerforest-fire extent during 1972–2018, and that nearly allof the increase could be explained by increased fueldryness in the context of contemporary firemanagement. In the wider western US forests,Westerling et al. (2016) found that annual burned areaincreased tenfold between 2003–2012 and 1973–1982.Increases in summer wildfire area have occurredprincipally due to increased fuel dryness under awarming climate that has been accompanied byreduced summer precipitation (Halofsky et al., 2020;Williams et al., 2019; Abatzoglou & Williams, 2016;

Human-induced warming has already led to aglobal increase in the frequency and severity of fireweather, increasing the risks of wildfire. Risingglobal temperatures and more frequent heatwavesincrease the frequency of fire weather by promoting hotand dry conditions, despite an increase in global meanprecipitation which falls more sporadically. Fire weatherseasons have significantly lengthened across 25% ofthe Earth’s vegetated surface, with detectable climatechange impact above natural variability over a similarextent (Abatzoglou et al., 2019). These impacts willbecome increasingly pervasive and intense with eachadded degree of warming. While there are regionalexceptions, the global picture is of a hotter world thatsupports longer and more extreme fire seasons (seeFigure).

Land management can ameliorate or compoundclimate-driven changes in wildfire risk. Prescribedburning during the cooler season can reduce availablefuel and therefore reduce the likelihood or intensity ofsubsequent fires. Fire suppression and increasedresidues from logging can enhance the intensity ofwildfires when they occur. Fuel reductions or themaintenance of fire breaks around population centrescan help to avoid the worst direct impacts of wildfire oncommunities, and is particularly effective aroundpopulated centres. Fire management is highly sensitiveto local climate and landscape conditions, both of whichhave changed in recent decades due to climate changeand other factors, such as population growth.

Wildfires can have broad impacts for human healthand wellbeing and for the natural environment.Impacts include deaths directly from fire and indirectlyfrom associated air pollution, loss of property,accelerated thawing of permafrost, amplification ofclimate change, and biodiversity loss.

Climate Change and Wildfire in theWestern US

The western US is exposed to greater wildfire risksthan it was before humans began to alter the globalclimate. The legacy of fire suppression andexclusion of indigenous land stewardship haveincreased vegetation density in the region therebycontributing to recent wildfires.

Change in the length of the fire weather season between 1979 – 2019 as seen in meteorological data (figure produced by M. Jones and J. Abatzoglou following Jolly et al. (2015); data from Vitolo et al., 2020, using the ERA5 dataset).

implementation of widespread fuel treatments reducethe proportion of high–severity fires (Liang et al., 2018)under future climates in the Sierra Nevada. Theinfluence of land–management practices on fire activityis ecosystem–dependent; for example, fire suppressionpractices may have little bearing on changes in burnedarea in sub-alpine ecosystems with climate change(Hansen et al., 2019). However, Goss et al. (2020)concluded that “the broad geographic extent ofincreased burned area in California and the westernUS suggests that demographic and forest managementfactors alone are insufficient to explain the magnitudeof the observed increase in wildfire extent over the pasthalf–century”.

Climate Contribution to the 2019–2020Australian Bushfires

Extreme heat and drought in Australia during thefire season of 2019–2020 (and prior years ofdrought) led to an unprecedented fire season in theforests of the southeast of the country. Theweather conditions were partly a result of human–driven climate change. Long term accumulation offuels as a result of fire suppression and loggingpractices likely contributed to the severity andextent of fire activity.

The scale of the 2019–2020 bushfires wasunprecedented. A globally unprecedented 21% of theAustralian temperate and broadleaf mixed (TBLM)forest biome burned during the 2019-2020 bushfires.This is believed to be due to record low levels ofmoisture in the leaf litter layer, which propagate TBLMfires, following a period of record-breaking heat andextended drought, coupled with windy conditions inearly September 2019 (Boer et al., 2020). While thisperiod of elevated fire weather is consistent withpredictions of climate change, formal attribution studiesfor the 2019–2020 Australian wildfires are still inprogress.

Fuel management through prescribed burns andimproved logging practice cannot fully mitigateincreased wildfire risk due to climate change.Recent analysis found 36% of the 2019–2020 burnedarea in Victoria had been burned twice or more in theprevious 25 years, (Lindenmayer & Taylor, 2020)suggesting limited effectiveness of prescribed burns tomitigate wildfires in these areas. Lindenmayer et al.(2020) identified that, in addition to climate change,logging residues from poor forestry practices in Victoriacontributed to the intensity of the 2019–2020megafires. However, Di Virgilio et al. (2020)demonstrate that in southeastern Australia, climatechange has reduced the window for safe use ofprescribed burns to manage forest fuel loads, limitingfuture effectiveness of this tool. Extended fire–seasonlength in other regions would mean this also applieselsewhere. To prevent future extreme or catastrophicfires in forests, using fuel management alone, Clarke etal. (2020) suggest that landscape–scale changes invegetation type would be required, not merelymanagement of existing vegetation.

Holden et al., 2018), in the context of modern landmanagement and fire suppression practices.

Demographic factors alone cannot account for themagnitude of the observed increase in wildfires inthe western US, but increased population leads togreater impacts. In addition to climate change, theincreased population at the wildland-urban interface(WUI) is contributing to greater impacts of wildfires,with more people, property and infrastructure at risk(Syphard et al. 2019, Hanberry, 2020; Radeloff et al.,2020). Strader (2018) found that there was a >100–foldincrease in homes built in high fire risk zones during1940–2010, while 97% of US wildfires in the WUI resultfrom human ignition (Mietkiewicz et al., 2020).Observations reveal weaker climate–fire relationshipswhere human populations are higher, due to bothignitions, land fragmentation, and suppression(Syphard et al., 2017). While studies from elsewhere inthe world have shown that a rising rural populationleads to higher human ignitions, especially along newhighways (Oliveira et al., 2017), studies in Californiahave not revealed such an effect where the number ofignitions has declined slightly over the past fourdecades (Keeley and Syphard, 2018).

Land management practices are contributingfactors, but cannot alone explain the magnitude ofthe observed increase in wildfires extent in thewestern US forests in recent decades. Prescribedburning is one mechanism for removing the mostflammable excess fuels from the forest floor. However,negative perceptions towards the practice and a closingwindow of opportunity to conduct the burns safely eachyear mean that these prescribed burns are conductedless regularly than desired in some regions (Miller etal., 2020). Limited fuel management through hazardreduction fires has been suggested by some (Kolden,2019; Miller et al., 2020; Moreira et al., 2020) as acontributing factor to the increasing scale and impact ofwildfires in the western US (and other regions withMediterranean–type climate). A long history of firesuppression has also contributed to elevated wildfirerisk because fuel stocks are out of sync with theirnatural dynamics. Increased forest residues fromreduced frequency of low–intensity fires enhance thelikelihood of fires being highly impactful when they dooccur (Moreira et al., 2020). Likewise, the

Joanne Francis – Unsplash.com

future, climate change signal emerges from naturalvariability in most of France after 2060. Positivetemperature, negative humidity and zonal windanomalies in the Iberian Peninsula correlate with largefires (Vieira et al., 2020) that are projected to increasein the number of extreme days and normalised burnedarea by 2071–2100, under both a medium (RCP 4.5)and a high (RCP 8.5) emissions scenario (Calheiros etal., 2021). For the Mediterranean region as a whole,fire weather is projected to increase 14–30% by thelate 21st century, depending on future emissionsscenarios. In Central Asia, fire weather is projected toincrease by 63–146% and burned area by 3–13% by2071–2099, depending on future emissions scenarios(Zong et al., 2020).

The full Brief and all references to publications canbe viewed and explored here:

https://sciencebrief.org/topics/climate-change-science/wildfires/explorerExtreme weather and Pyroconvection are projected

to increase wildfire risk under future climatechange in southeastern Australia. Researchpublished before the 2019-2020 bushfires revealed thatsurface fire weather conditions and atmosphericinstability can interact during major fires, as heat andmoisture are released (Dowdy & Pepler, 2018; DiVirgilio et al., 2019; Dowdy et al., 2019). Known aspyroconvective circulation, temperature and humiditychanges above the fire and can develop thunderstormsand pyrocumulonimbus clouds, resulting in catastrophicwildfires that are even more dangerous, because windspeed and direction changes erratically (Dowdy &Pepler, 2018). Reanalysis data (1979–2016) forsoutheast Australia, show pyroconvection riskincreased over time, especially during spring andsummer, due to worsening surface fire weatherconditions (Dowdy & Pepler, 2018). Future projections(2060–2079) suggest that climate change maycontinue to increase the risk of pyroconvection insoutheastern Australia, compared to 1990–2009. Thisincrease is notable in November (spring) and to alesser extent during December (summer), though atrend was not clear for all regions of the country (DiVirgilio et al., 2019; Dowdy et al., 2019).

New Evidence from Elsewhere in theWorld

Scientific evidence that climate change is causingan increase in the frequency and extent of fireweather, contributing to extreme wildfires aroundthe world, continues to mount.

In the Mediterranean region, anthropogenic climatechange signal has been detected separately fromnatural variability in southern France and wasresponsible for 50% of the increase in fire weather(Barbero et al., 2020). Fargeon et al. (2020) report thatunder a high emissions scenario (RCP 8.5) for the

Suggested citation:Smith, A.J.P., Jones, M.W., Abatzoglou, J.T., Canadell,J.G. & Betts, R.A. ScienceBrief Review: Climatechange increases the risk of wildfires, September 2020.In: Critical Issues in Climate Change Science, editedby: C. Le Quéré, P. Liss, P. Forster.https://doi.org/10.5281/zenodo.4570195

Acknowledgements.This ScienceBrief Review was supported by theEuropean Commission via projects CRESCENDO, 4C,and VERIFY(grants no. 641816, 776810, 821003).

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