Breakthrough in the understanding of flaming wildfires.

In this article, I give an overview of the recent contribution of Finney et al. (1) to our understanding of how wildfires spread by providing its scientific context and also by putting forward the possible impact on the field. The rise of humanity was intimately bound to fire. Humans first observed flames when fleeing wildland fires, the natural version of the phenomenon that would then become the most important technological achievement of the human race: the mastery of fire for cooking, lighting, settlement, hunting, and warfare (2). Wildfires are important to the natural sciences. Since deep time, the top surface of the Earth’s crust has been the interface where abundant plant organicmattermeets an atmosphere rich in oxygen. This interface is flammable, especially in dry, windy, and hot conditions, and leads to wildfire after an ignition event. Not only has fire contributed to shaping most ecosystems on Earth, but it plays essential roles supporting life through the regulation of atmospheric oxygen, the carbon cycle, and the climate (3, 4). As part of the current anthropogenic age, humans have also modified the fire regimes of many ecosystems and have contributed, for example, to fire exclusion in certain regions (e.g., in the US between the 1910s and 1960s) or to increasing fire frequency and severity through drainage (e.g., peatlands) and possibly through climate change (e.g., Arctic fires). Of note, multiple billions of US dollars are spent annually across the world to fight wildfires for the protection of communities and valuable ecosystems. Despite its central importance to the planet and to humanity, our understanding of fire remains very limited. For example, we still cannot accurately forecast the location of a fire in 30-min time. To quote Hottel (5), “A case can be made for fire being, next to the life processes, the most complex of phenomena to understand.” It comes as no surprise, then, that the discipline of fire science is less mature than other Earth science topics. For instance, a quick look at the literature shows that there are three times more scientific studies published per year on volcanoes than on wildfires. Fire science requires more decades of fruitful research to mature and establish a physics-based understanding of this natural phenomenon. The fate of a flaming wildfire starts with its genesis at ignition, by natural means like a lightning strike, or by anthropogenic means like slash-and-burn. Once ignited, part of the heat released by the flames will drive the spread over connected fuel beds of grass, shrubs, and trees. Another mechanism of propagation is by lofting burning embers that land farther away, but flame spread is more pervasive. The dynamics of spread are such that wildfires accelerate with tail winds, dry weather, or upslopes, and decelerate with head winds, moisture, or downslopes. The most lasting contribution to the science of wildland fires is the pioneering work of Rothermel in 1972 (6). He formulated an empirical model for predicting the spread rate of a wildfire at steady-state. This formulation is ubiquitous and can be found at the core of most wildfire behavior simulations. These simulations are currently in use by forestry agencies and firefighting command centers across the world. For example, Rothermel’s model is part of the USWildland Fire Decision Support System, used in planning of every large and long-duration federal wildland fire incident. However, Rothermel’s formulation is empirical: although it can provide rough predictions of the rate of spread by calibration to laboratory data, it does not explain how fire spreads (or when fire would ignite). Its empirical nature hinders scientific progress and does not allow for improvements to simulations. Until very recently, there was no valid scientific theory of wildfire spread that could replace Rothermel’s model. In this context, we see that the recent work by Finney et al. (1) is a scientific breakthrough. Finney et al. have discovered the long-missing piece of the puzzle to understand wildfire dynamics. Their seminal work puts forward for the first time (to my knowledge) a fundamental, comprehensive, and verifiable theory of flaming wildfire spread. Finney’s theory relates the rate of spread to basic fluid mechanics and heat transfer, and it is strongly supported by laboratory data and field observations across a wide range of scales from 10 cm to 15 m.