Boreal forests, which make up roughly one‐third of the world's total forested area, provide critical ecosystem services including carbon stocks, climate‐feedbacks, permafrost‐stability, biodiversity, and economic benefits. These services differ markedly between the two boreal forest biomes i.e. the evergreen needle‐leaf forests of North America and Europe, and the summergreen needle‐leaf forests covering much of northern Asia. However, the basic mechanisms that control the distributions of boreal biomes remain poorly understood. The potential future changes in boreal ecosystem services are therefore uncertain, which is a matter of local, regional, and global concern. The GlacialLegacy project will aim to answer the timely questions “Why is northern Asia dominated by larch forests?" and “How will these forests change in the future?”. My new hypothesis is that summergreen and evergreen needle‐leaf forests represent alternative quasistable states that occur today under similar climatic conditions, but were triggered by different environmental conditions and gene pools during the Last Glacial. GlacialLegacy will use coherent empirical and modelling approaches to investigate this hypothesis across the entire Northern Hemisphere. Eventually, these predictions will aim to anticipate potentially critical future ecosystem service changes on a continental scale, thus providing the knowledge base required for adaptation strategies to be prepared.