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

Climate and the global carbon cycle are a tightly coupled system where changes in climate affect exchange of atmospheric CO 2 with the land biosphere and the ocean, and vice‐versa. In particular, the response of the land biosphere to the ongoing increase in atmospheric CO 2 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 CO 2 and sequesters carbon throughout the 21st century. In a second case, CO 2 fertilization saturates in year 2000; here the land becomes an additional source of CO 2 by 2050. The predicted atmospheric CO 2 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 CO 2 increases by the end of the century.