When catastrophic wildfires capture global headlines, such as the unprecedented blazes sweeping through southern Europe this summer or the devastating 2019–20 “Black Summer” in Australia, they are almost always preceded by severe drought. Dry vegetation and parched landscapes are necessary ingredients for infernos. But not all droughts are created equal – the way a drought evolves can drastically alter how a fire behaves once it ignites. As global temperatures rise, a new and dangerous phenomenon is becoming increasingly common: the “flash drought”. Unlike standard droughts, which take place over months or years, flash droughts are rapidly occurring dry spells, driven by intense heat and unusually low humidity. They suck moisture out of the landscape at an alarming rate. While scientists know standard droughts increase fire risks, a critical question has remained unanswered: do flash droughts further increase this risk, and what happens when these two drought types collide? Our latest research mapped two decades of global satellite data to answer this question. We found when landscapes transition from a prolonged standard drought into a rapid flash drought – or when the two overlap – they create a “hyper-flammable” state. This acts as a massive fire accelerant; they spread faster, last longer and grow significantly larger than other wildfires.How hyper-flammability worksTo understand why this happens, we have to look at how different droughts affect the environment. The major difference is the speed at which they occur.A standard drought acts like a slow, relentless oven. Over months, it gradually dries out vegetation and severely reduces soil moisture, leaving a dry environment primed to burn.A flash drought, on the other hand, is like hitting the landscape with a high-powered hairdryer. The atmospheric dryness rapidly increases, characterised by extreme heat and low humidity, and quickly depletes remaining soil moisture. When a standard drought turns into a flash drought, the effects don’t just add up; they multiply. Combined, they cause the fire danger to skyrocket, setting the perfect stage for fast-spreading and intense wildfires. Short-lived weather anomalies such as flash droughts generate disproportionately severe and extreme fires.Tracking two decades of global firesTo uncover this pattern, we analysed global drought and satellite-based fire data sets from 2002 to 2021. Global wildfires were categorised into four distinct groups: fires occurring under normal (no drought) conditions; fires occurring during flash droughts; fires occurring during isolated standard droughts; and fires occurring where the two drought types overlapped or followed one another. Across every single fire metric we measured, including speed, size, duration and spread, fires burning under combined drought conditions were the most extreme. The median size of these fires was 65% larger than fires in normal conditions, and 21% larger than those occurring during standard droughts alone. They spread 35% faster and burned 19% longer. We also discovered that regions experiencing combined drought conditions endured significantly longer dry spells before a fire actually started. On average, 72 days passed between the onset of a combined drought and the start of a fire. This compared with only 17 days for flash droughts and 56 days for standard droughts. This prolonged period allows severe dryness to develop before fires start.Global fire hotspotsWhile fires that follow standard droughts are widespread across the globe’s arid and semi-arid zones, we found fires following flash droughts and, particularly, combined droughts, clustered in distinct geographic hotspots. The most extreme are heavily concentrated in highly vulnerable communities of plants and animals, including the savannas of South America and Africa, northern Australia, and the western United States. These regions are highly susceptible to rapid-onset heat and moisture stress, making them ground zero for compounding climate extremes. The presence of both slow-moving and flash droughts causes the most extreme fire behaviour, thus amplifying the fire’s intensity.Rethinking wildfire preparationAs the climate continues to heat up, the rate of evaporation increases worldwide. This means the frequency and extent of combined droughts are likely to increase across fire-prone regions around the world.Crucially, we must stop viewing droughts simply as a static state of reduced moisture. Instead, we must recognise them as evolving processes, in which the shift from one type of drought to another can fundamentally change wildfire severity.Unfortunately, many current global drought and fire monitoring systems emphasise real-time conditions, or look at drought severity as an aggregated whole. To better protect communities and nature, these systems urgently need to integrate information on how droughts evolve. This would enable us to better detect emerging hyper-flammable landscapes before the first spark.Abhirup Dikshit receives funding from the Australian Research Council. Jason Evans receives funding from the Australian Research Council.