In the U.S., wildland firefighters are able to stop about 98% of all wildfires before the fires have burned even 100 acres. That may seem comforting, but decades of quickly suppressing fires has had unintended consequences.

Fires are a natural part of many landscapes globally. When forests aren’t allowed to burn, they become more dense, and dead branches, leaves and other biomass accumulate, leaving more fuel for the next fire. This buildup leads to more extreme fires that are even harder to put out. That’s why land managers set controlled burns and thin forests to clear out the undergrowth.

However, fuel accumulation isn’t the only consequence of fire suppression.

Fire suppression also disproportionately reduces certain types of fire. In a new study, my colleagues and I show how this effect, known as the suppression bias, compounds the impacts of fuel accumulation and climate change.

Most wildfires are low-intensity. They ignite when conditions aren’t too dry or windy, and they can often be quickly extinguished.

The 2% of fires that escape suppression are those that are more extreme and much harder to fight. They account for about 98% of the burned area in a typical year.

In other words, trying to put out all wildfires doesn’t reduce the total amount of fire equally – instead, it limits low-intensity fires while extreme fires still burn. This effect is worsened by climate change.

In our study, we used a fire modeling simulation to explore the effects of the fire suppression bias and see how they compared to the effects of global warming and fuel accumulation alone.

Fuel accumulation and global warming both inherently make fires more severe. But over thousands of simulated fires, we found that allowing forests to burn only under the very worst conditions increased fire severity by the same amount as more than a century’s worth of fuel accumulation or 21st-century climate change.

The suppression bias also changes the way plants and animals interact with fire.

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