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Peatlands have acted as net CO2sinks over millennia, exerting a global climate cooling effect. Rapid warming at northern latitudes, where peatlands are abundant, can disturb their CO2sink function. Here we show that sensitivity of peatland net CO2exchange to warming changes in sign and magnitude across seasons, resulting in complex net CO2sink responses. We use multiannual net CO2exchange observations from 20 northern peatlands to show that warmer early summers are linked to increased net CO2uptake, while warmer late summers lead to decreased net CO2uptake. Thus, net CO2sinks of peatlands in regions experiencing early summer warming, such as central Siberia, are more likely to persist under warmer climate conditions than are those in other regions. Our results will be useful to improve the design of future warming experiments and to better interpret large-scale trends in peatland net CO2uptake over the coming few decades.