To evaluate boreal peatland C losses from warming, novel technologies were used to expose intact bog plots in northern Minnesota to a range of future temperatures (+0°C to +9°C) with and without elevated CO2 (eCO2). After 3 years, warming linearly increased net C loss at a rate of 31.3 g C·m−2·year−1·°C−1. Increasing losses were associated with increased decomposition and corroborated by measures of declining peat elevation. Effects of eCO2 were minor. Results indicate a range of C losses from boreal peatlands 4.5 to 18 times faster than historical rates of accumulation, with substantial emissions of CO2 and CH4 to the atmosphere. A model of peatland C cycle captured the temperature response dominated by peat decomposition under ambient CO2, but improvements will be needed to predict the lack of observable responses to elevated CO2 concentrations thus far.