Peatlands store large amounts of carbon and naturally emit methane due to slow decomposition in waterlogged conditions. Climate warming is disrupting this balance, reducing their ability to absorb carbon and increasing their emissions of carbon dioxide and methane. Using ecosystem modelling, this study shows that northern peatlands (>25°N) will likely remain carbon sinks under low-warming conditions (RCP2.6), but under high-warming (RCP8.5), they could become sources of both CO₂ and CH₄ by the mid-21st century. This shift would result in significant climate warming feedback, with peatlands adding to rather than helping offset global warming by the end of the 23rd century.