When wildfire smoke settled over New York, Chicago, and much of the northeastern United States this month, President Donald Trump offered a familiar explanation. Canada, he argued, had failed to manage its forests properly. The smoke, in this telling, was the product of bad policy north of the border, and perhaps even worthy of tariffs.

It is an intuitively appealing story because it fits the way most people think about forests. If forests are burning, then the problem must be the trees. If the trees are the problem, then better forest management should solve it. But what if the trees aren't actually the main thing burning?

A new study published in Geophysical Research Letters suggests that this intuition is fundamentally wrong. Researchers from McMaster University found that roughly three-quarters of the carbon emissions released during Canada's record-breaking 2023 wildfire season came not from trees or surface vegetation, but from carbon that had been stored underground in forest soils for decades, and in some cases centuries.

Much of the public conversation about wildfire still imagines forests as oversized campfires. Trees ignite, flames spread through the canopy, smoke rises into the atmosphere, and firefighters eventually extinguish the blaze. Forest management fits neatly into that mental model. Thin the forest, remove deadwood, conduct prescribed burns, and there is simply less fuel available.

That logic works in many ecosystems, particularly in forests where vegetation itself accounts for most of the combustible material.

Canada's boreal forest is different.

The visible forest is only part of the system. Beneath it lies an enormous reservoir of accumulated organic matter made up of peat, moss, decomposing vegetation, and carbon-rich soils. Collectively, these soils contain far more carbon than the trees growing above them. Under normal conditions they remain wet enough that they burn only minimally. But hotter summers, prolonged droughts, lower water tables, and thawing permafrost are changing those conditions. As the ground dries, the forest floor itself becomes fuel.

The researchers estimate that Canada’s 2023 wildfires released approximately 554 million tonnes of carbon, with about 76% originating below ground. Independent estimates from the European Union’s Copernicus and researchers at NASA arrive at remarkably similar totals, placing Canada’s wildfire emissions somewhere between 470 and 640 million tonnes of carbon, showing that Canada’s boreal forests have become one of the world’s largest episodic sources of carbon emissions. Canada’s wildfires emitted carbon on a scale comparable to the annual fossil fuel emissions of many industrialized nations.

Forest management matters. Prescribed burns reduce fuel loads. Mechanical thinning can lower fire intensity near communities. Better planning can improve resilience and reduce risk around critical infrastructure. None of that is controversial. But those tools were never designed to solve the problem of hundreds of billions of tonnes of carbon stored beneath remote boreal forests stretching across northern Canada.

Once those soils become sufficiently dry, they become extraordinarily difficult to protect. They can smolder for weeks or even months, releasing carbon dioxide, methane, and the fine particulate pollution that eventually drifts into cities thousands of kilometers away. Firefighters often cannot extinguish these underground fires because they are literally burning below the surface.

The political debate often revolves around whether governments are fighting fires aggressively enough or managing forests effectively enough. Those questions increasingly describe only part of the system. Rising temperatures are not simply creating more opportunities for fires to start. They are transforming landscapes that historically resisted combustion into landscapes capable of releasing enormous quantities of ancient carbon. Once that process begins, each severe fire season becomes more than a disaster response problem. It becomes an uncomfortable feedback loop: Fossil fuel emissions warm the climate. A warmer climate dries boreal soils. Drier soils release more carbon when they burn. That additional carbon contributes to further warming, making future fires even more severe.

This is one of the defining characteristics of climate change that is often missed in public debate. We tend to imagine climate impacts as isolated events: a wildfire here, a flood there, a heat wave somewhere else. But the climate system is full of feedbacks. It changes the background conditions in ways that make entirely different phenomena possible.

Researchers involved in the study expect these dynamics to intensify as flash droughts become more common, groundwater levels continue to decline, and permafrost retreats across the north.

If the largest source of wildfire emissions increasingly comes from underground carbon stores, then adaptation strategies must expand beyond suppressing fires after they ignite. They must include protecting peatlands, restoring wetlands, improving early detection systems, incorporating Indigenous fire stewardship, and, ultimately, reducing the greenhouse gas emissions that are making these landscapes combustible in the first place.

The temptation in politics is always to identify a single culprit. It makes for cleaner narratives and simpler solutions.

The science rarely cooperates.

The smoke drifting south from Canada is not primarily evidence that Canada forgot how to manage its forests. It is evidence that one of the planet's largest carbon reservoirs is beginning to behave differently because the climate that sustained it has changed.

And that is a problem no border can contain.