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

International audience; 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 heat-induced fire-weather types that have been related to the largest wildfires in recent years. Heat-induced 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 timescales 2-5 , and, consequently, current and potential future climate-induced changes in wildfire activity might threaten ecosystems and human well-being 6. In most Euro-Mediterranean countries, wildfire activity has been declining owing to management and suppression measures undertaken since the 1980s 7. However, some recent extreme wildfire events, including those that occurred in 2016 in France 8 , 2017 in Spain and Portugal 9 and 2018 in Greece 10 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 change 11,12. 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 flammability 6,13 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 behaviour 5,14. Most of the largest wildfires occur when these conditions are met 8. 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 shrublands 15-17. 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 open