Increasing fire frequency and severity may shift boreal forests from carbon sinks to carbon sources and amplify climate warming. Analysis indicates that fuel characteristics are important drivers of wildfire carbon emissions across a broad range of North America's boreal forest. The boreal region of North America is an important global carbon reservoir that is warming twice as fast as the global average, associated with intensified fire weather (conditions that influence fire ignition and behaviour), increased wildfire area burned, shorter fire intervals and more severe fires. Uncharacteristically large, severe forest fires consume substantial amounts of above - and below-ground biomass and can cause source-sink shifts in carbon exchange with the atmosphere. Climate-driven changes in fire emissions may serve as a strong, positive feedback to further warming and fire activity, yet controls on emissions are not well constrained. Writing in this issue of Nature Climate Change, Xanthe Walker and colleagues examine potential drivers of carbon emissions from boreal wildfires and find that fuel availability plays a dominant role as compared with fire weather. Many forest types, including boreal forests, are fire-prone and fire-adapted, meaning that fire is an integral and predictable part of their ecological functioning. The amount and rate of carbon release from a fire are directly related to fuel combustion, which depends on the fire's extent and severity as well as pre-disturbance site conditions and productivity. In fire-adapted ecosystems and in the absence of major shifts in climate and disturbance, carbon losses from the fire are balanced by carbon accumulation in live and dead vegetation and soils when integrated over long time periods.