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.

Increasing drought conditions under global warming are expected to alter the frequency and distribution of large and high-intensity wildfires. However, our understanding of the impact of increasing drought on extreme wildfires events remains incomplete. Here, we analyzed the weather conditions associated with the extreme wildfires events that occurred in Mediterranean France during the exceptionally dry summers of 2003 and 2016. We identified that these fires were related to two distinct shifts in the fire weather space towards fire weather conditions that had not been explored before and resulting from specific interactions between different types of drought and different fire weather types. In 2016, a long-lasting “press drought” intensified wind-driven fires. In 2003, a “hot drought” combining a heat wave with a press drought intensified heat-induced fires. Our findings highlight that increasing drought conditions projected by climate change scenarios might affect the dryness of fuel compartments and lead to a higher frequency of extremes wildfires events.

Background: ‘Megafire’ is an emerging concept commonly used to describe fires that are extreme in terms of size, behaviour, and/or impacts, but the term’s meaning remains ambiguous.Approach: We sought to resolve ambiguity surrounding the meaning of ‘megafire’ by conducting a structured review of the use and definition of the term in several languages in the peer‐reviewed scientific literature. We collated definitions and descriptions of megafire and identified criteria frequently invoked to define megafire. We recorded the size and location of megafires and mapped them to reveal global variation in the size of fires described as megafires.Results: We identified 109 studies that define the term ‘megafire’ or identify a megafire, with the term first appearing in the peer‐reviewed literature in 2005. Seventy‐one (~65%) of these studies attempted to describe or define the term. There was considerable variability in the criteria used to define megafire, although definitions of megafire based on fire size were most common. Megafire size thresholds varied geographically from > 100–100,000 ha, with fires > 10,000 ha the most common size threshold (41%, 18/44 studies). Definitions of megafire were most common from studies led by authors from North America (52%, 37/71). We recorded 137 instances from 84 studies where fires were reported as megafires, the vast majority (94%, 129/137) of which exceed 10,000 ha in size. Megafires occurred in a range of biomes, but were most frequently described in forested biomes (112/137, 82%), and usually described single ignition fires (59% 81/137).Conclusion: As Earth’s climate and ecosystems change, it is important that scientists can communicate trends in the occurrence of larger and more extreme fires with clarity. To overcome ambiguity, we suggest a definition of megafire as fires > 10,000 ha arising from single or multiple related ignition events. We introduce two additional terms – gigafire (> 100,000 ha) and terafire (> 1,000,000 ha) – for fires of an even larger scale than megafires.

Understanding how the interactions between anthropogenic and biophysical factors control fire regimes is increasingly becoming a major concern in a context of climate, economic and social changes. On a short time scale, fire activity is mainly driven by the variations in weather conditions. But while the assessment of this fire‐weather relationship is an essential step towards fire hazard estimations, reconstructions or projections, still little is known about the impact of human practices on this relationship. In this study, we examined the recent fire history in southern France where a new fire policy, introduced during the 1980s, suddenly brought new fire suppression and prevention practices. We aimed at assessing the impact of these changes on fire activity and on the relationships between fire and weather, usually assumed to be constant over time. To do so, we used a statistical framework based on spatially explicit daily fire occurrence data, the corresponding weather variables and the associated fuel moisture derived from a process‐based model. Our results showed that the introduction of the new fire policy resulted in a sharp decrease in fire activity but also impacted the daily fire‐weather relationship in two main ways. On the one hand, fewer wildfires ignited for similar weather conditions. On the other hand, the probability of a fire to spread over significant surfaces shifted from a fuel‐dryness driven system to a system driven by the concomitance of fuel dryness and strong winds. These observations suggest that mid‐term (decadal) social factors can affect the short‐term (seasonal to daily) relationship between weather conditions and fire activity. Thus, the interactions between human and climate factors should be taken into account when reconstructing or projecting fire activity and including the impact of fire policies on the fire‐weather relationships in fire models would be an important step towards more realistic fire regimes simulations.