Forest fires are usually viewed within the context of a single fire season, in which weather conditions and fuel supply can combine to create conditions favourable for fire ignition—usually by lightning or human activity—and spread^1,2,3. But some fires exhibit ‘overwintering’ behaviour, in which they smoulder through the non-fire season and flare up in the subsequent spring^4,5. In boreal (northern) forests, deep organic soils favourable for smouldering⁶, along with accelerated climate warming⁷, may present unusually favourable conditions for overwintering. However, the extent of overwintering in boreal forests and the underlying factors influencing this behaviour remain unclear. Here we show that overwintering fires in boreal forests are associated with hot summers generating large fire years and deep burning into organic soils, conditions that have become more frequent in our study areas in recent decades. Our results are based on an algorithm with which we detect overwintering fires in Alaska, USA, and the Northwest Territories, Canada, using field and remote sensing datasets. Between 2002 and 2018, overwintering fires were responsible for 0.8 per cent of the total burned area; however, in one year this amounted to 38 per cent. The spatiotemporal predictability of overwintering fires could be used by fire management agencies to facilitate early detection, which may result in reduced carbon emissions and firefighting costs.

通常在单一火灾季节的背景下观察森林火灾，其中天气条件和燃料供应可以结合起来创造有利于火灾点燃的条件 - 通常通过闪电或人类活动 - 并蔓延^1,2,3。但一些火灾表现出“越冬”行为，在这种情况下，它们会在非火灾季节闷烧，并在随后的春季^4,5爆发。在寒带（北部）森林中，深层有机土壤有利于闷烧⁶以及加速的气候变暖⁷, 可能为越冬提供异常有利的条件。然而，北方森林越冬的程度以及影响这种行为的潜在因素仍不清楚。在这里，我们表明北方森林中的越冬火灾与炎热的夏季产生大火年和深度燃烧有机土壤有关，这些情况在我们的研究区域最近几十年变得更加频繁。我们的结果基于一种算法，我们使用该算法使用野外和遥感数据集检测美国阿拉斯加和加拿大西北地区的越冬火灾。2002 年至 2018 年间，越冬火灾占总燃烧面积的 0.8%；然而，在一年内，这一比例达到了 38%。