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Abstract Peatlands play a critical role in the global carbon (C) cycle, encompassing ∼30% of the 1,500 Pg of C stored in soils worldwide. However, this C is vulnerable to climate and land‐use change. Ecosystem models predict the impact of perturbation on C fluxes based on soil C pools, yet responses could vary markedly depending on soil organic matter (SOM) chemistry. Here, we show that one SOM functional group responds strongly to environmental factors and predicts the risk of carbon dioxide (CO₂) release from peatlands. The molecular composition of SOM in 125 peatlands differed markedly at the global scale due to variation in temperature, land‐use, vegetation, and nutrient status. Despite this variation, incubation of peat from a subset of 11 sites revealed thatO‐alkyl C (i.e., carbohydrates) was the strongest predictor of aerobic CO₂production. This explicit link provides a simple parameter that can improve models of peatland CO₂fluxes.