Quantifying the effects of CO₂-fertilized vegetation on future global climate and carbon dynamics

[1] Climate and the global carbon cycle are a tightly coupled system where changes in climate affect exchange of atmospheric CO₂ with the land biosphere and the ocean, and vice-versa. In particular, the response of the land biosphere to the ongoing increase in atmospheric CO₂ is not well understood. To evaluate the approximate upper and lower limits of land carbon uptake, we perform simulations using a comprehensive climate-carbon model. In one case the land biosphere is vigorously fertilized by added CO₂ and sequesters carbon throughout the 21st century. In a second case, CO₂ fertilization saturates in year 2000; here the land becomes an additional source of CO₂ by 2050. The predicted atmospheric CO₂ concentration at year 2100 differs by 40% between the two cases. We show that current uncertainties preclude determination of whether the land biosphere will amplify or damp atmospheric CO₂ increases by the end of the century.