Climate-induced variations in global wildfire danger from 1979 to 2013. — TEI-tagged view

W. Matt JollypersName, Mark A. CochranepersName, Patrick H. FreebornpersName, Zachary A. HoldenpersName, Timothy J. BrownpersName, Grant J. WilliamsonpersName, David M. J. S. BowmanpersName (2015date). “Climate-induced variations in global wildfire danger from 1979 to 2013..” Nature Communicationstitle·j.

Object of study — global fire weather season lengthterm·dom and its trends from 1979date to 2013date

Research question — how recent climatic changes have expanded or contracted fire weather seasonsterm·dom globally

Contribution — a simple ensemble metric of fire weather season lengthterm·dom mapped globally over 35 years

Full text — tagged

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Climate-induced variations in global wildfireterm·key danger from 1979date to 2013date.

Abstract Climate strongly influences global wildfireterm·key activity, and recent wildfireterm·key surges may signal fire weather-induced pyrogeographic shifts. Here we use three daily global climate data sets and three fire danger indicesterm·mth to develop a simple annual metric of fire weather season lengthterm·dom, and map spatio-temporal trends from 1979date to 2013date. We show that fire weather seasonsterm·dom have lengthened across 29.6 million km 2 (25.3%) of the Earth's vegetatedterm·key surface, resulting in an 18.7% increase in global mean fire weather season lengthterm·dom. We also show a doubling (108.1% increase) of global burnable area affected by long fire weather seasonsterm·dom (>1.0 σ above the historical mean) and an increased global frequency of long fire weather seasonsterm·dom across 62.4 million km 2 (53.4%) during the second half of the study period. If these fire weatherterm·key changes are coupled with ignition sources and available fuel, they could markedly impact global ecosystems, societies, economies and climate.

Wildfiresterm·key play a pivotal, dynamic role in terrestrial and atmospheric systems 1 . Global annual burned areaterm·key estimates approach 350 MHa per year 2 , and annual pyrogenic CO 2 emissions can exceed 50% of fossil fuel combustion emissions 3 4 5 . Fires play an essential ecological role in flammable ecosystems: some are managed to clear forests, promote grazing and establish plants 6 , while others are suppressed to protect human lives and property, regardless of how they ignite. Recently, there has been a surge of extremely destructive fires with corresponding social disruptions and substantial economic costs. Over the last decade, annual wildfireterm·key suppression costs on US federal lands exceeded $1.7B US dollars 7 and $1B US dollars in Canada 8 . When all components are considered, including preparedness/suppression costs and economic losses, these total costs are substantially higher. In Australia in 2005, total wildfireterm·key costs were estimated at nearly $9.4B US dollars or 1.3% of their Gross Domestic Product 9 . Therefore, the driving factors of contemporary wildfireterm·key activity changes must be understood to ensure that wildfiresterm·key are effectively managed to promote healthy ecosystems while minimizing negative socio-economic impacts.

Wildfiresterm·key occur at the intersection of dry weather, available fuel and ignition sources 10 . Weather is the most variable and largest driver of regional burned areaterm·key 11 12 13 14 . Temperature, relative humidity, precipitation and wind speed independently influence wildland fire spread rates and intensities, and the alignment of multiple weather extremes, such as the co-occurrence of hot, dry and windy conditionsterm·dom leads to the most severe firesterm·dom 15 . Global temperatures have increased by ∼0.2 °C per decade over the last three decades 16 , possibly leading to an acceleration of the global water cycle with more intense rainfall events 17 , more severe and widespread droughts 18 (despite drought frequencies appearing unchanged 19 ) and regional humidity variations 20 . Regional droughts are also tightly coupled to sea surface temperature variations 21 , and regional water availability variations can explain a significant proportion of the variations in burned areaterm·key 22 . Climatic changes are implicated in global fire variations 23 and are expected to increase fire season severity over the coming decades 24 . While several studies have examined climate-induced regional and circumboreal trends in fire danger 25 26 27 28 29 30 , a comprehensive global assessment of the interactions of recent climatic changes that lead to an expansion or contraction of fire seasons is lacking.

Landscape-scale fire behaviour is determined by local weather conditions, but biome-level wildfireterm·key potential is more appropriately associated with fire danger indicesterm·mth, which are representative of daily synoptic weather patterns 11 . Independent fire danger indicesterm·mth have been developed and applied for different regions worldwide. All are based on daily surface weather variables that are related to the ignitability, spread rate and the control difficulty of an initiating wildland fire 31 and they are closely related to the magnitude and extent of fire activity 32 33 . Furthermore, these indices capture changes in fuel (live and dead plant material) moistures, and thus scale to fuel consumption 34 and pyrogenic emission production 5 .

Here we present an analysis of daily global fire weatherterm·key trends from 1979date to 2013date based on three sub-daily global meteorological data sets (the National Center for Environmental Predictionterm (NCEPterm) Reanalysisterm·key, NCEP-DOE Reanalysisterm·key II and the European Centre for Medium-Range Weather Forecaststerm (ECMWFterm) Interim Reanalysisterm·key) 35 36 37 between ∼0.75° and 2.0° grid cell resolution. We use these data to calculate the US Burning Index 38 , the Canadian Fire Weatherterm·key Index 39 and the Australian (or McArthur) Forest Fire Danger Index 40 . Daily fire danger indicesterm·mth (normalized to a common scale and resampled to a common resolution) were used to derive a fire weather season lengthterm·dom, defined as the number of days each year when fire danger is above half its value range, for each year in each grid cell using a technique commonly used to determine growing season length from satellite-derived vegetation indices 41 42 . Because climate studies using multi-model ensembles are generally superior to single model approaches 43 , all nine fire weather season lengthsterm·dom for each location were averaged into an ensemble mean fire weather season lengthterm·dom, hereafter referred to as ‘Fire Weather Season Lengthterm·dom' (See Supplementary Methods ). This metric was examined to identify global and regional patterns in fire weather season lengthterm·dom changes as well as changes in the frequency of, and the area affected by, long fire weather seasonsterm·dom (defined as >1.0 σ above historical mean) over the last 35 years.

Results Global fire weatherterm·key trends Ensemble mean annual maximum temperature across the vegetatedterm·key land surface increased by 0.184 °C per decade from 1979date to 2013date ( Supplementary Table 1 ). This global trend is comparable to the global land temperature trends reported by the Intergovernmental Panel on Climate Changeterm·key of 0.268, 0.315, 0.188 and 0.203 K per decade 4 . Mean annual global vegetatedterm·key surface maximum temperature was significantly correlated to the Goddard Institute of Space Studiesterm (GISSterm) land-surface temperature anomalies ( ρ =0.88, P <0.001) 16 . The amount of area witnessing unusually hot years, where maximum temperature was more than 1 s.d. from the mean, also showed a significant increase of 6.3% per decade ( Supplementary Table 1 ). Mean annual minimum relative humidity showed a weak but significant trend of −0.127% per decade but showed no significant changes in affected area. There were no significant trends in mean annual total precipitation or total precipitation affected area but we did observe a significant increase in mean annual rain-free daysterm·dom, where the mean number of dry days increased by 1.31 days per decade and the global area affected by anomalously dry years significantly increased by 1.6% per decade. Despite an absence of global trends in some key meteorological variables, there were distinct regional annual climate trends and changes in anomalous weather event affected area frequency ( Fig. 1 ).

Global fire weather season lengthterm·dom trends Even more distinct patterns and trends in annual climate anomalies emerged when the meteorological data were used to calculate fire danger indicesterm·mth and then combined into metrics characterizing the fire weather season lengthterm·dom and long fire weather season affected areasterm·def. Globally, fire weather season lengthterm·dom increased by 18.7% from 1979date to 2013date ( Fig. 2a ), with statistically significant increases observed across 25.3% (29.6 M km 2 ) of the global vegetatedterm·key area and decreases in only 10.7% (12.5 M km 2 ) ( Fig. 3a ). Long fire weather season affected areaterm·def, defined as the total global area observing fire weather seasonsterm·dom >1 s.d. from the mean, has increased by 3.1% per year from 1979date to 2013date, leading to a 108.1% increase in global long fire weather season affected areaterm·def ( Fig. 2b ). The frequency of long fire weather seasonsterm·dom increased across 53.4% of the global vegetatedterm·key area (62.4M km 2 ) as observed between 1996 and 2013date, compared with 1979date–1996, with decreased frequency only observed across 34.6% (40.4 M km 2 ) ( Fig. 3b ). Since 1979date, there have been 6 years, all in the last decade, where >20% of the global vegetatedterm·key area has been affected by long fire weather seasonsterm·dom (2005, 2007, 2009, 2010, 2012 and 2013date). Both fire weather seasonterm·dom metrics were strongly correlated with the global mean annual number of days without wetting rainfall (>0.1 mm) ( Fig. 2c ), where global mean rain-free daysterm·dom accounted for 49.7% of the variation in global fire weather season lengthterm·dom and 33.8% of the variation in global long fire weather season affected areaterm·def. Annual fire weather season lengthterm·dom anomaly maps for a subset of known severe fire years are presented in Fig. 4 and anomalies for all years are presented in Supplementary Figs 1–4 and annual ensemble-mean anomaly data are available as Supplementary Data 1 .

Fire weather season lengthterm·dom and long fire weather season affected areaterm·def increased significantly across all continents except Australia ( Table 1 ). Globally, most biomes showed significant increases in fire weather seasonterm·dom metrics with the exceptions of temperate and montane grasslands, savannas and shrublands and boreal forests/taiga and tundra ( Table 2 ). The strongest trends were observed in tropical and subtropical grasslands, savannas and shrublands. Continent × biome trends indicate that regional variations in fire weather seasonterm·dom metrics were much stronger than global trends ( Table 3 ).

Comparisons to country-wide reported burned areaterm·key Inter-annual variations in mean US fire weather season lengthterm·dom were significantly correlated with variation in annual burned areaterm·key reported by the US National Interagency Fire Center 44 over the full time series from 1979date to 2013date and also from 1992 to 2013date, when fire occurrence data quality was highest 45 ( ρ =0.679 and 0.683, respectively, P <0.001). Further, long fire weather season affected areaterm·def was also significantly correlated to the burnt area variations of the full and limited time series ( ρ =0.663 and 0.715, respectively, P <0.001) ( Table 4 and Supplementary Fig. 5 ). Inter-annual variations in mean fire weather season lengthterm·dom were also significantly correlated to inter-annual burned areaterm·key variations from 1980 to 2013date across Spain, Portugal, France, Italy, Greece and Latvia ( Table 4 ), and inter-annual variations in long fire weather season affected areaterm·def were significantly correlated with burned areaterm·key for Spain, Portugal, Italy and Latvia ( Table 4 and Supplementary Figs 6–8 ). Both mean fire weather season lengthterm·dom and long fire weather season affected areaterm·def, constrained to only boreal forests where most Canadian fires occur, were only weakly correlated to burned areaterm·key across Canadian forests ( ρ =0.3 and 0.324, respectively, P <0.1) ( Table 4 and Supplementary Fig. 5 ).

Comparisons to global land carbon uptaketerm·key Variation in fire danger metrics have been shown to be closely related to fuel consumption 34 , and fire activity is generally inversely correlated with vegetation productivity 47 . Likewise, fire weather season lengthterm·dom and long fire weather season affected areaterm·def were significantly correlated with global net land carbon flux calculated from an analysis of the global carbon budget from 1979date to 2012 (ref. 48 ) (See Supplementary Methods ) ( ρ =−0.38 and ρ =−0.48, n =33, P <0.05 and P <0.01, respectively, Supplementary Fig. 9 ). In addition, when correlations were constrained to the time period that satellite burned areaterm·key observations were available from the Moderate Resolution Imaging Spectroradiometerterm·mth (MODISterm·mth) (2001-2012), and thus where estimates of land-use change carbon emissions were more certain 2 , correlations between fire weather season lengthterm·dom, long fire season affected area and net land carbon fluxes increased substantially to ρ =−0.797 and ρ =−0.825, respectively, n =12, P <0.01). The highest correlations between the net land carbon flux and continental biome mean fire weather seasonterm·dom metrics were observed in the tropical and subtropical forests, grasslands and savannas and xeric shrublands of South America where regional fire weather season lengthterm·dom metrics accounted for between 15.7 and 29.7% of the variations in global net land carbon flux ( Table 5 ).

Discussion Fire weather season lengthterm·dom and long fire weather season affected areaterm·def significantly increased across all vegetatedterm·key continents except Australia. For example, significant fire weather seasonterm·dom lengthening has occurred throughout much of Africa, particularly the subtropical grasslands and savannas of the eastern half of the continent ( Fig. 3 ) and across Africa's Mediterranean forests, woodlands and scrub. Persistent fire weather season lengthterm·dom increases in ecosystems such as South Africa's Mediterranean fynbos could lead to more frequent severe burning conditions and more area burned, shortening fire return intervals and threatening these biodiversity-rich shrublands 49 . Recent climatic changes have mitigated wildland fire potential in some regions of Africa since 1979date. For example, drought during the early 1980s (ref. 50 ) lengthened fire weather seasonsterm·dom in the West African subtropics. Drought conditions have subsequently subsided, leading to a steady contraction of the fire weather season lengthsterm·dom across much of West Africa ( Fig. 3 ).

Our ensemble fire weather season lengthterm·dom metric captured important wildfireterm·key events throughout Eurasia such as the Indonesian fires of 1997–98 where peat fires, following an El Niño-induced drought, released carbon equivalent to 13–40% of the global fossil fuel emissions from only 1.4% of the global vegetatedterm·key land area ( Fig. 4 , 1997–1998) 46 and the heatwave over Western Russia in 2010 ( Fig. 4 , 2010) that led to its worst fire season in recorded history and triggered extreme air pollution in Moscow 51 . Our metric also revealed similar impacts across Eastern Canada in 2010, where high temperatures and significant water deficits led to large wildfiresterm·key ( Fig. 4 , 2010) 52 . European Mediterranean forests were also identified as being susceptible to significant changes: the inner-quartile range of fire weather season lengthterm·dom trends indicate a lengthening of 12 to 19 days, with a maximum increase of nearly a month (29 days) from 1979date to 2013date ( Table 6 ). This is consistent with a lengthening of the fire weather seasonterm·dom in Spain during 2012 ( Fig. 4 , 2012) where fires burned more area than any year in the previous decade 49 .

Over the last several decades, the US has witnessed a marked increase in large wildfireterm·key frequency and duration with the greatest increases observed in the temperate coniferous forests of the Northern Rocky Mountains 53 54 . These trends are widely attributed to shifts towards earlier snowmelt timing 54 , though fire activity in the desert Southwest has also been attributed to warming-driven increases in vapour pressure deficit 55 and variability in the timing of spring precipitation 56 . Our results extend these findings by demonstrating that areas with the most significant change in fire weather season lengthterm·dom occur where not only temperature but also changes in humidity, length of rain-free intervals and wind speeds are most pronounced. In 2012, for example, longer-than-normal fire weather seasonsterm·dom across an unprecedented 47.4% of the vegetatedterm·key area of the US ( Fig. 4 , 2012) culminated in a near-record setting ∼3.8 MHa of burned areaterm·key. In addition, our results show significant fire weather seasonterm·dom lengthening throughout much of the Eastern US Coastal Plains. Over the last decade, this region has witnessed a marked uptick in wildfiresterm·key and a group of large fires in Okefenokee National Wildlife Refuge, the Osceola National Forest and adjacent lands burned ∼243 KHa in 2007 (ref. 57 ), one of those fires ranked as the twelfth largest fire in the US history.

Fire weather season lengthterm·dom and long fire weather season affected areaterm·def were only weakly correlated to Canadian boreal forest burned areaterm·key ( Table 4 ). This may be caused by averaging across the large zonal and meridional climatic gradients across the country. Future work should examine relationships between provincial fire activity and fire weather seasonterm·dom metrics. In addition, our fire weather season lengthterm·dom metric captures variations in the number of days each year that fires are likely to burn, but it does not account for inter-annual variations in fire season severity. Further work should consider both a lengthening fire season and an increase in within-season fire weatherterm·key severity as causal mechanisms of burned areaterm·key variations.

South America's tropical and subtropical forests, grasslands and savannas have experienced tremendous fire weather season lengthterm·dom changes, with a median increase of 33 days over the last 35 years ( Fig. 3a and Table 6 ). Our metric captured a rare drought in the Amazon in 2005 that prompted long fire weather seasonsterm·dom, leading to a dramatic increase in basin-wide fire activity 58 ( Fig. 4 , 2005). Longer fire seasons prolong conducive burning conditions, potentially expanding areas susceptible to escaped deforestation fires 59 and subsequently strengthening feedbacks in regional climate-fire dynamics 60 .

In contrast to all other continents in our analysis, Australia showed no significant trends in biome-level fire season length, but we identified regional increases in the frequency of anomalously long fire weather seasonsterm·dom, especially from 1996 to 2013date ( Fig. 3b ). Our analysis also captures climatic droughts that contributed to the Ash Wednesday fires in Victoria (1983) ( Fig. 4 , 1982–1983), the Canberra bushfires (2003) ( Supplementary Fig. 3 ), the Black Saturday fires (2009) ( Supplementary Fig. 4 ) and the widespread heatwave across Australia in the summer of 2012–2013date that promoted long fire weather seasonsterm·dom and intense bushfire activity across most of the Eastern half of the country ( Fig. 4 , 2013date). Australian fires seasons are dominated by periods of benign weather followed by years with extreme fire weatherterm·key conditions. Such high inter-annual variability reflects climate modes, such as the El Niño Southern Oscillationterm·dom (ENSOterm·dom) and the Indian Ocean Dipole, which can heavily influence inter-annual rainfall variationsterm·dom and subsequently affect wildfireterm·key potential throughout Australia 61 62 .

Fire weather season lengthterm·dom imperfectly scales with actual fire activity because fires may not be ignited, there may be no available fuel, or they may be suppressed by humans. Nonetheless, our global fire weather season lengthterm·dom metrics were significantly correlated to global net land carbon flux. These correlations were negative, suggesting that when average fire weather seasonsterm·dom are longer-than-normal or when long seasons impacted more global burnable area, net global terrestrial carbon uptaketerm·dom is reduced. Generally, low correlations between fire weather season lengthterm·dom and global land carbon uptaketerm·key are to be expected because wildfiresterm·key represent a small proportion of the total land carbon flux. However, if our fire season metrics were combined with other metrics of global land carbon uptaketerm·key that have been produced by others 63 64 , they may improve our ability to assess the cumulative impacts of climatic changes on terrestrial carbon fluxes. Correlations between global net land carbon flux and continental-scale, biome mean fire weather season lengthterm·dom metrics were highest across South American tropical and subtropical forests, savannas and grasslands and xeric shrublands ( Table 5 ), highlighting that the strongest coupling between fire weatherterm·key and global carbon emissions is occurring in an area of intense land-use pressure.

Global fire regimes are the combined results of available fuel, ignition sources and conducive fire weatherterm·key 10 . As such, leveraging well-established fire danger indicesterm·mth to explore changes in global wildfireterm·key weather only capture part of the potential variations in global pyrogeographyterm·key. Regionally, our documented fire weatherterm·key changes may not appreciably alter fire regimes if fires are not ignited or if there is no fuel. However, we observed an overall lengthening of the number of days each year that wildfiresterm·key may burn across more than a quarter of the Earth's vegetatedterm·key surface and these fire weatherterm·key changes could manifest themselves as a positive feedback to global atmospheric carbon accumulations if all the requirements for wildfiresterm·key are present. In addition, this study may improve our ability to explore the complex drivers of global fire activity by isolating the climate-induced variations in fire potential from changes in either fuel availability or human and nature-caused ignitions, which lead to the realized burned areaterm·key and subsequent fire emissions. In summary, we have shown that combined surface weather changes over the last three and a half decades have promoted global wildfireterm·key weather season lengthening. If these trends continue, increased wildfireterm·key potential may have pronounced global socio-economic, ecological and climate system impacts.

Methods Meteorological data Three global reanalysisterm·key projects provided gridded, sub-daily surface meteorological data from 1979date to 2013date. Two data sets at ∼210 km spatial resolution were obtained from NCEPterm, including the Reanalysisterm·key I 36 and the DOE Reanalysisterm·key II data sets 37 . Six-hourly data fields for 2 m maximum temperature, minimum temperature, specific humidity, surface pressure, precipitation rate, water equivalent of actual snow depth and 10 m U and V wind components were summarized to daily data ( Supplementary Table 2 ). We assumed that diurnal variations in actual vapour pressure are small and thus daily mean actual vapour pressure was calculated from the NCEPterm data using mean daily specific humidity and surface pressure 65 , and saturation vapour pressure was calculated from daily maximum and minimum temperature to calculate daily maximum and minimum relative humidity. In addition, we used the ECMWFterm ERA Interim Reanalysisterm·key 35 . This data set is similar to NCEPterm's but at a higher spatial and temporal resolution (∼78 km resolution). We extracted 3-hourly 2 m air temperature, dewpoint temperature, surface total precipitation, and 10 m U and V wind components using the ECMWFterm GRIdded Binary Application Programming Interface (GRIB-API) and used them to derive daily maximum and minimum temperature, maximum and minimum relative humidity, maximum wind and total daily precipitation amount and daily precipitation duration ( Supplementary Table 3 ). Daily maximum and minimum relative humidity were calculated using mean daily dewpoint temperature and minimum and maximum daily 2 m air temperature, respectively.

Surface meteorological data analysis Trends in ensemble mean annual values were analysed for five climate variables that are important to wildland fire potential: mean maximum daily temperature, mean minimum daily relative humidity, total precipitation, rain-free daysterm·dom and mean maximum 10 m wind speed. Annual means or totals for each pixel were averaged across all three reanalysisterm·key data sets to produce an ensemble mean value. Two types of analysis were performed on each ensemble climate variable. First, we examined areas that showed long-term trends in mean annual weather conditions. All trends were evaluated using Mann–Kendallterm·key trend tests, following a four-step process to reduce the effects of serial autocorrelation on significance tests 66 (see Supplemental Methods). Second, we examined the change in frequency of occurrence of unusually hot, dry or windy conditions by comparing the number of years that maximum temperature, rain-free daysterm·dom or wind speed was >1 s.d. above the mean or when minimum relative humidity was <1 s.d. from the mean in 1996–2013date, as compared with the number of similar events observed in 1979date–1996. One year, 1996, overlaps each period to provide 18 years in each period. Further, all data sets were masked using the vegetatedterm·key (burnable) land area defined by a global landcover data set developed from AVHRR satellite data 67 . Mean trends and event frequency changes were then displayed spatially ( Fig. 1 ) and summarized to average global values ( Supplementary Table 1 ).

Additional information How to cite this article: JollypersName, W. M. et al. Climate-induced variations in global wildfireterm·key danger from 1979date to 2013date. Nat. Commun. 6:7537 doi: 10.1038/ncomms8537 (2015).

Supplementary Material Supplementary Figures 1-11, Supplementary Table 1-4, Supplementary Methods and Supplementary References Annual Ensemble Fire Weather Season Lengthterm·dom Standardized-Anomalies from 1979-2013 rasters in GeoTIFF format.