Assessing the sensitivity of fire regimes to climate, atmospheric CO2 and human activity under past and future conditions

Fire regimes have distinct global controls, and how burnt area, wildfire size and wildfire intensity independently respond to changes in climate, vegetation, and human activity remains challenging to quantify. Here, we use robust empirical models of burnt area, fire size and a measure of intensity to explore the global sensitivity of fire regimes to changes in climate, atmospheric CO₂ and human activity under contrasting climate states, specifically at the end of the century under two climate change mitigation scenarios and at the Last Glacial Maximum. Our simulations show a global shift in wildfire patterns by 2100 CE under both low- and high-mitigation scenarios with reduced burnt area in tropical regions but larger and more intense wildfires in extra-tropical regions. Under low mitigation, increases in burnt area worldwide overwhelm the current human-driven declining trend, with fire size and intensity increasingly limited by dryness and vegetation fragmentation. Under different future conditions burnt area continues to increase due to changes fuel availability and dryness, fire intensity is increasingly limited by fuel build-up, and fire size by fuel continuity. These trends differ from those shown in simulations at the last Glacial Maximum, which show decreased burnt area, alongside increased fire size and intensity compared to present, consistent with sedimentary charcoal evidence. The decoupling between different fire properties occurs because of the different temporal and spatial scales on which the controls of burnt area, fire size and fire intensity operate. Under future conditions, the effect of a warming climate and increasing atmospheric CO₂ amplify each other, whereas in cold climate with low atmospheric CO₂, they dampen each other. These findings have immediate implications for the improvement of process-based fire models, which currently do not take the distinctions between these fire properties into account. They also suggest that the current observed patterns of fire regimes today may not hold constant under changing conditions.