As we gear up for another summer with likely record-breaking wildfire activity, there’s lots of interest in what is driving these increases. In addition to climate change and increased human ignitions, we know that decades of fire suppression have led to fuel accumulation and increasing fire severity. This impact is known as the “fire suppression paradox”—by suppressing fires we make fires harder to put out in the future.

However, in a new simulation study, we show a separate impact of fire suppression that may cause even greater increases in average fire severity than climate change or fuel accumulation.  

Suppression disproportionately removes some types of fire more than others. Most of the fire removed by suppression is low-intensity and occurs under moderate weather conditions when suppression is highly effective. For example, in the U.S., the Forest Service suppresses nearly 99% of fires before they reach 300 acres. The 1% of fires that evade suppression are typically extreme and make up 98% of the burned area. We call this effect the “suppression bias”—by removing less-extreme wildfires, we ensure that the fires that occur are the most extreme.  

In this paper, we were interested in examining the consequences of the suppression bias. We used fire modeling simulations to estimate how it affects patterns of fire, compared to two well understood drivers of increased fire activity—climate change and fuel accumulation. Our study shows that fire suppression makes fires more severe, less diverse, and increase at faster rates under climate change and fuel accumulation.

Though conventional fire suppression reduces the total area burned, it largely removes low- and moderate-intensity fires, leaving extreme fires. As a result, the remaining fires are biased toward much higher average severity. Our research indicates that conventional fire suppression increases average fire severity by an amount comparable to over century's worth of fuel accumulation or climate change effects. In other words, by suppressing low-intensity fires, we guarantee that the remaining fires will be as severe as fires burning in a significantly hotter, drier, and more fuel-loaded future.

The suppression bias alters fire's impact on ecosystems, potentially reshaping evolutionary paths by favoring traits adapted to high-severity fires over those suited for low-severity events. Moreover, the higher proportion of extreme fires reduces seed dispersal and seedling survival, limiting opportunities for new plant establishment and decreasing the ability of ecosystems to adapt to environmental changes like climate change.

Our ability to suppress fires declines significantly as the climate becomes more fire-prone and fuel accumulates. As a result, as climate shifts and fuels increase over time, more fires evade suppression, resulting in a rapid escalation of burned area. Our research shows that conventional suppression amplified burned area increases by a factor of nearly 3, compared to a scenario without suppression. This accelerated pace of change places additional strain on societies already grappling with adapting to climate change.

We show that the suppression bias exerts a significant influence on fire activity and ecological impacts. Integrating the impacts of the suppression bias would enhance our understanding of fire-human dynamics and allow us to better address the wildfire crisis. Safely allowing fires to burn under moderate conditions may be as critical as other necessary interventions, such as mitigating climate change and reducing unintentional human-related ignitions.