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.