The return of cold and snow at the close of the year typically signal the end of the wildfire season. …Zombie fires, sometimes betrayed by a plume of steam emerging from the bubbling ground in the frozen forest, were once a rare occurrence in the boreal regions that stretch across the far north through Siberia, Canada and Alaska. But in a rapidly heating world, they are becoming increasingly common. The overwintering burns are small – and often hard to detect – but they are transforming fires into multi-year events. …“It is a massive problem,” says Lori Daniels, a professor at the University of BC. Current estimates show that only about 15% of the northern hemisphere is underlain by permafrost, yet these frozen soils contain roughly twice as much carbon than is now in the atmosphere. By burning slowly and at a lower temperature, they release vastly more particulate pollution and greenhouse gas emissions than flaming fires.