Underground stores of rich soil and thick peat were the main fuel behind the record emissions created during Canada’s 2023 wildfire season. Rather than standing trees and leaves, carbon locked beneath the forest floor accounted for 76 per cent of all carbon released by the blazes.According to a study published in the journal Geophysical Research Letters, titled Three-fourths of Carbon Emissions from 2023 Record-Breaking Wildfires in Canada Traced to Soil and Peat Combustion, the fire season burned an estimated 15.1 million hectares of land. This burnt area is seven times larger than Canada’s 20-year average. Across the season, the fires released roughly 554 million tonnes of carbon into the air, making up nearly a quarter of all global wildfire emissions for that year.The findings show a major change in how northern fires burn, proving that hotter weather is exposing ancient underground carbon stores to direct burning.
Centuries of stored carbon lost to underground fires
Canada holds roughly 20 per cent of the world’s soil carbon inside its soils and peatlands. Peatlands cover about 1.1 million square kilometres of the country. This includes massive wetland areas like the Hudson Bay Lowlands, which are the second-largest peatland complex in the world and hold an estimated 30 billion tonnes of carbon.In the past, wet peatlands acted as natural barriers that stopped fires from spreading. However, long heatwaves and dry spells in recent years have lowered water levels, drying out the thick moss and soil. This drying process has turned lands that once blocked fires into massive sources of smouldering fuel.“Canada stewards 20 per cent of the world’s soil organic carbon in its peatlands and soils. Future wildfire risk is not just about losing forests, it’s about releasing these vast stores of carbon that have accumulated underground over millennia into the atmosphere,” explains Alemu Gonsamo, associate professor in the School of Earth, Environment & Society at McMaster University.When fires start during these dry periods, flames do not just burn surface plants. Instead, they dig deep into the dry layers below the ground.“When fires start under these conditions, they can burn deep into peat and soil, releasing carbon that took thousands of years to accumulate in a single fire season,” says Gonsamo.
Swapping from carbon storage to carbon release
Deep burning in peatlands alone produced one-third of all Canadian wildfire carbon emissions in 2023. These underground fires create unique problems for fire crews and forestry teams.“For years we’ve focused mostly on the trees that burn. This study shows we must also pay attention to what’s happening beneath the surface,” says Gonsamo. “While forest management remains important, its impact is limited because many of these fires occur in remote, unmanaged areas. Prolonged hot and dry conditions are allowing smouldering ground fires to persist and releasing significant amounts of stored carbon.“These underground fires can smoulder for weeks or months beneath the surface, making them very hard to put out using water planes or ground crews. As a result, northern lands that stored carbon for thousands of years are now releasing it back into the air.“What we’re seeing is a shift from these landscapes storing carbon to releasing large amounts of it during extreme wildfire seasons,” says Gonsamo.Once this carbon enters the air, it drives global warming further, creating hotter conditions that set the stage for even worse fires in the future.
Climate impacts demand global action
The overall impact of the 2023 Canadian fire season was huge. Measured as carbon dioxide, total emissions from the fires were roughly three times higher than Canada’s total human-made greenhouse gas emissions reported for 2022.Because the main cause of these deep-burning fires is rising global temperatures and dry weather, local firefighting efforts alone cannot stop underground burning.“There is no alternative to reducing global greenhouse gas emissions to slow climate change and curb the growing intensity of wildfire emissions in Canada and elsewhere,” says Gonsamo.“Addressing the root cause of increasingly severe fire seasons requires coordinated global action on climate change. This cannot be done by Canada alone.”
Additional key facts from the study
To measure the true depth of the disaster, the research team, led by McMaster University scientist Zilong Zhong alongside Alemu Gonsamo and Chinyere Ottah, combined satellite tracking, ground-level field measurements, and machine learning models.Their detailed breakdown revealed that underground soil combustion accounted for roughly 412 million tonnes of carbon, while above-ground trees and vegetation contributed just 142 million tonnes. When converted into carbon dioxide, the total volume exceeded two billion tonnes of CO₂.The findings also expose a gap in official climate tracking. Because many of these fires burn in remote, unmanaged northern landscapes, their massive emissions are largely excluded from Canada’s official national greenhouse gas inventories. Yet their atmospheric reach is global, with smoke plumes from the 2023 blazes travelling thousands of kilometres across North America and reaching into Western Europe.
