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Future climate change due to increased atmospheric CO2 may affect land and ocean efficiency to absorb atmospheric CO2. Here, using climate and carbon three-dimensional models forced by a 1% per year increase in atmospheric CO2, we show that there is a positive feedback between the climate system and the carbon cycle. Climate change reduces land and ocean uptake of CO2, respectively by 54% and 35% at 4 × CO2. This negative impact implies that for prescribed anthropogenic CO2 emissions, the atmospheric CO2 would be higher than the level reached if climate change does not affect the carbon cycle. We estimate the gain of this climate-carbon cycle feedback to be 10% at 2 × CO2 and 20% at 4 × CO2. This translates into a 15% higher mean temperature increase.