Drought and overwintering fires drive consecutive large fire seasons in boreal North America
This study reveals that a severe multi-year drought in boreal North America facilitated overwintering fires, which, combined with subsequent drought conditions, drove unprecedented consecutive extreme fire seasons in 2023–2025 and challenges current fire danger systems that fail to account for multi-year hydrological carryover effects.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Imagine the forests of northern North America as a giant, slow-cooking stew. Usually, this stew simmers quietly for decades, maybe even centuries, before a big fire stirs the pot. But between 2023 and 2025, something went wrong with the recipe. Instead of a gentle simmer, the pot boiled over three years in a row, burning a record-breaking 35.5 million hectares and releasing a massive 1.2 petagrams of carbon.
The paper by Scholten and her team investigates why this happened. They found that the secret ingredient wasn't just hot weather; it was a severe, multi-year drought that acted like a giant sponge, sucking the moisture right out of the ground.
The Great Sponge Squeeze
Think of the soil in these forests as a giant, thirsty sponge. For years, the region has been getting wetter on average, but the authors show that a specific, intense drought hit western and central Canada that was the worst in at least 45 years. By the end of the fire seasons in 2023, 2024, and 2025, the "sponge" (measured as Terrestrial Water Storage) was so dry it was about 4 standard deviations below normal. That's like a sponge that should be dripping wet but is instead as dry as a cracker.
The researchers used a water balance model to figure out why the sponge was so dry. They discovered that the main culprit was a lack of rain (precipitation deficits), which accounted for about 75% to 85% of the water loss. While the heat made the plants drink more water (evapotranspiration), that was only a minor player, contributing about 15% to 25% of the drying. So, it wasn't just that the sun was too hot; the sky simply stopped raining enough.
The "Zombie" Fires
Here is where the story gets spooky. Usually, when winter comes, fires in these forests are supposed to die out, buried under snow. But because the drought had dried out the deep, organic soil (like peatlands) so thoroughly, some fires didn't actually die. They went underground, smoldering slowly through the winter like zombies.
The paper found that in 2024 and 2025, these "overwintering" fires were responsible for 14% to 22% of the total burned area in boreal North America. In the western region specifically, they made up nearly a quarter of the damage. These fires would start in the spring, reigniting from the charred edges of the previous year's burns. In fact, some fires in 2025 were reigniting from burns that happened as far back as 2023, proving they had survived two winters underground.
The Early Warning System
The authors suggest that we can predict these disasters months in advance. They found that if you measure how dry the soil is in May (before the main fire season starts), it's a strong crystal ball for how bad the fire season will be. If the soil is super dry in May, the fire season is likely to be huge. This is a big deal because current fire danger systems often treat each year as a fresh start, ignoring the fact that a dry year can leave a "hangover" that makes the next year even worse.
What It's NOT
The paper explicitly argues against a few common ideas. First, it says that big climate cycles like El Niño or the Pacific Decadal Oscillation (PDO) aren't the main reason for these specific back-to-back fires. The PDO has actually been in a "negative" phase since 2018, which usually means less fire, so it can't explain why we've had three massive years in a row.
Second, the authors challenge the idea that these fires are just random lightning strikes or short-term heatwaves. While heat and lightning are important, the study shows that the carryover effect of the drought and the zombie fires are the real drivers of these consecutive extremes.
The Bottom Line
The study suggests that we are entering a new era where multi-year droughts can override the long-term trend of the north getting wetter. Even if the climate is generally becoming wetter over decades, a few years of severe rain shortages can dry out the landscape enough to create a fire super-cycle. The authors propose that we need to update our fire warning systems to include these "zombie" fires and the deep soil moisture levels, rather than just looking at the weather for the current week. If we can spot the dry soil early, we might be able to catch these underground fires before they wake up and spread.
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