The boreal and Mediterranean-climate regions that account for most of the world’s wildfire activity have, over the past two decades, experienced a documented lengthening of fire seasons, increases in average and extreme burn area, and increases in fire intensity. The trends are not uniform — some regions show flat or declining trajectories — but the overall direction is clear, and the human consequences have grown faster than the area burned.
What changed in the climate signal
Fire weather indices integrate temperature, humidity, wind, and fuel moisture. Across most fire-prone regions of the western United States, southern Europe, southeastern Australia, and the Canadian and Russian boreal, the indices show a consistent shift toward more days with conditions favorable to large-fire growth. The proximate cause is the combination of higher temperatures, lower atmospheric humidity in the warm season, and earlier snowmelt that extends the period during which fuels are available to burn.
The longer fire season also overlaps more frequently with periods of dry lightning — thunderstorms that produce ignitions but little rain — which are the dominant ignition source in the boreal and a major one in the western US.
What changed in the fuel
The climatic signal does not exist in isolation. Twentieth-century fire suppression, particularly in the western United States, allowed fuel loads in many forest types to accumulate well beyond their pre-suppression equilibrium. The result is that when fires do occur, they burn in conditions and at intensities that are outside the historical range. Fires that historically would have been low-intensity surface burns now propagate as crown fires, killing mature trees that previous fire regimes left intact.
The consequence is that some of the “more fire” we are seeing is the deferred fire of decades of suppression, occurring under climatic conditions that make it more dangerous than it would otherwise have been. Untangling the climate signal from the fuel signal in any given region is methodologically difficult and politically contested.
The smoke is the new exposure
Direct fire perimeter exposure affects relatively few people per year. Smoke exposure affects, in some recent fire seasons, hundreds of millions. The 2023 Canadian wildfire season produced air quality events across the eastern United States and parts of Europe that were unprecedented in the recent observational record. PM2.5 concentrations in cities thousands of kilometers from the fires reached levels associated with significant short-term cardiovascular and respiratory mortality risk.
The public-health framing of wildfire is shifting from a regional emergency-response question to a continental air-quality question. The instrumentation, the alert systems, and the building-envelope investments needed to manage chronic episodic smoke exposure are different from those that have historically been built for evacuation and structure protection.
Where management can move the needle
Prescribed burning, mechanical fuel reduction, and revised forest-management practices have a meaningful effect on fire behavior at the landscape scale, but the area where these practices have been applied remains a small fraction of the area where they would be needed. The governance, workforce, and air-quality permitting constraints that limit prescribed-burn programs are at least as binding as the budgets. The fire problem is solvable in principle. It is moving faster than the institutional response.