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For the 21st century, carbon cycle models typically project an increase of terrestrial carbon with increasing atmospheric CO2 and a decrease with the accompanying climate change. However, these estimates are poorly constrained, primarily because they typically rely on a limited number of emission and climate scenarios. Here we explore a wide range of combinations of CO2 rise and climate change and assess their likelihood with the climate change responses obtained from climate models. Our results demonstrate that the terrestrial carbon uptake depends critically on the climate sensitivity of individual climate models, representing a large uncertainty of model estimates. In our simulations, the terrestrial biosphere is unlikely to become a strong source of carbon with any likely combination of CO2 and climate change in the absence of land use change, but the fraction of the emissions taken up by the terrestrial biosphere will decrease drastically with higher emissions.

Plain Language Summary The future uptake of carbon dioxide (CO2) by the global terrestrial biosphere impacts the atmospheric CO2 concentration. The response of this uptake to a rise in CO2 concentration and to climate change is explored here for a wide range of future scenarios. We conclude that a future with low CO2 increase and little climate change or with high CO2 increase and strong climate change does not differ greatly in the total uptake of terrestrial CO2 because of the compensating effects of CO2 increase and climate change. However, the response is critically determined by the magnitude of climate change for a given CO2 increase (termed climate sensitivity), and a low climate sensitivity will result in a considerably larger terrestrial uptake of CO2 than a high climate sensitivity.