Increased likelihood of heat-induced large wildfires in the Mediterranean Basin

www.nature.com/scientificreports OPEN Increased likelihood of heat‑induced large wildfires in the Mediterranean Basin Julien Ruffault1,9*, Thomas Curt2, Vincent Moron3, Ricardo M. Trigo4,10, Florent Mouillot5, Nikos Koutsias6, François Pimont7, Nicolas Martin‑StPaul7, Renaud Barbero2, Jean‑Luc Dupuy7, Ana Russo4 & Chiraz Belhadj‑Khedher8 Wildfire activity is expected to increase across the Mediterranean Basin because of climate change. However, the effects of future climate change on the combinations of atmospheric conditions that promote wildfire activity remain largely unknown. Using a fire-weather based classification of wildfires, we show that future climate scenarios point to an increase in the frequency of two heatinduced fire-weather types that have been related to the largest wildfires in recent years. Heatinduced fire-weather types are characterized by compound dry and warm conditions occurring during summer heatwaves, either under moderate (heatwave type) or intense (hot drought type) drought. The frequency of heat-induced fire-weather is projected to increase by 14% by the end of the century (2071–2100) under the RCP4.5 scenario, and by 30% under the RCP8.5, suggesting that the frequency and extent of large wildfires will increase throughout the Mediterranean Basin. Wildfire is a complex phenomenon that occurs when three conditions are met: available fuel, an ignition source (due to lightning or human activities) and weather conditions conducive to fires (fire weather)1. Climate and weather are important drivers of wildfire activity across a range of timescales2–5, and, consequently, current and potential future climate-induced changes in wildfire activity might threaten ecosystems and human well-being6. In most Euro-Mediterranean countries, wildfire activity has been declining owing to management and suppression measures undertaken since the 1980s7. However, some recent extreme wildfire events, including those that occurred in 2016 in F rance8, 2017 in Spain and P ortugal9 and 2018 in G reece10 have highlighted the limits of wildfire suppression capabilities under exceptional fire-weather conditions. Furthermore, studies show that wildfire activity is expected to increase across the Mediterranean Basin due to climate change11,12. However, how the combinations of climate and weather conditions that promote the largest wildfires will respond to climate change remain largely unknown. Climate and weather are both drivers of wildfire activity. Soil moisture deficit over days to months increases fuel aridity and flammability6,13 while a number of synoptic weather conditions associated to different combinations of short-term and instantaneous meteorological fields (precipitation, temperature, relative humidity and wind speed) influence wildfire behavior5,14. Most of the largest wildfires occur when these conditions are met8. For instance, the combination of extreme drought with extreme wind or heatwaves have both been identified as crucial factors in the occurrence of crown wildfires in Mediterranean forests and s hrublands15–17. In this study, we focus on the frequency of current and future weather and climate conditions associated with wildfires in four countries (France, Portugal, Greece, Tunisia) of the Mediterranean basin covering most of its diverse biogeographic and climatic conditions (Supplementary Figs. S1–S3). Building on the insights gained 1 Institut Méditerranéen de Biodiversité et d’Ecologie marine et continentale (IMBE), Aix Marseille Université, CNRS, IRD, Avignon Université, Avignon, France. 2INRAE, Aix Marseille Univ, RECOVER, Aix‑en‑Provence, France. 3Aix Marseille University, CNRS, IRD, INRAE, Coll. de France, CEREGE, Aix‑en‑Provence, France. 4Instituto Dom Luiz (IDL), Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749‑016 Lisbon, Portugal. 5CEFE, UMR 5175, Univ. Montpellier, CNRS, EPHE, IRD, univ. Paul Valery Montpellier 3, 1919 route de Mende, 34293 Montpellier Cedex 5, France. 6Department of Environmental Engineering, University of Patras, G. Seferi 2, 30100 Agrinio, Greece. 7INRAE, Ecologie des Forêts Méditerranéennes (UR 629), Avignon, France. 8Geomatics and Geosystems, University of Manouba, Manouba, Tunisia. 9Present address: INRAE, Ecologie des Forêts Méditerranéennes (UR 629), Avignon, France. 10Departamento de Meteorologia, Instituto de Geociências, Universidade Federal do Rio de Janeiro, Rio de Janeiro 21941‑916, Brazil. *email: Scientific Reports | (2020) 10:13790 | https://doi.org/10.1038/s41598-020-70069-z 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1.  Fire-weather types (FWTs) in the wildfire niche shown in terms of the first two orthogonal combinations of weather and climate variables determined by principal component analysis (PCA). Wildfire records (wildfires > 30 ha) were extracted for the period 1985–2015 from four countries covering most of the biogeographical conditions found in the Mediterranean Basin (France, Greece, Portugal and Tunisia). (a) Local daily values of the weather and climate variables associated with each wildfire (colored dots) and centroids (colored squares) for each FWT. (b) The wildfire niche is the atmospheric space that contains all the wildfires in the dataset, the All Voxels space covers all days and grid cells (including non-fire voxels) in the summer fire season. The Unfavourable weather conditions correspond to voxels in which wildfires are unlikely to occur as conditions are moister and cooler than those observed in the wildfire niche. from a series of previous s tudies8,17–19, we decompose “fire-weather” into robust and distinct combinations of local-scale short-term weather (i.e. daily mean temperature, relative humidity and wind speed) and long-term climate (i.e. monthly to seasonal drought) conditions called fire-weather types (FWTs). First, to identify FWTs, we classify the local-scale weather and climate conditions associated with more than 17,000 records of wildfires (wildfires larger than 30 ha) using an objective clustering. Next, we evaluate whether the largest wildfires (from 80 up to 2,150 ha fire size thresholds) occur preferentially under specific FWTs. Then, we extrapolate this wildfirebased classification to the whole Mediterranean Basin to assess the potential current and future changes to FWT frequency over the twenty-first century. Finally, we discuss the potential consequences in terms of future wildfire activity across the Mediterranean Basin. Results Identification and characterization of fire weather types (FWTs). The set of weather and climate conditions under which wildfires occur (the “wildfire niche”; Fig. 1a) was described by five variables that characterize different levels of fuel aridity (from weekly to monthly scale) and the synchronous, short-term weather conditions (i.e. daily scale) that control the occurrence and spread of wildfires. The variables include the daily mean temperature, wind speed, relative (...truncated)