As carbon dioxide increases in the earth's atmosphere, plants are stimulated to grow faster, thereby taking up some of the atmospheric increase in CO2. But increased plant growth requires increases in availability of other nutrients needed by plants to support this accelerated growth. Hungate et al. (2003) examine the nitrogen needs of plants in this accelerated growth mode and ask whether the required nitrogen will be available to plants. If not, then perhaps previous studies have over-estimated the ability of the biosphere to soak up some of the increased CO2 from burning fossil fuels. Their estimates indicate 1.2 to 6.0 Pg of nitrogen might be available to support this accelerated growth. Ecosystem models, however, suggest that may need as much as 37 Pg of nitrogen. If insufficient nitrogen is available, then atmospheric accumulation of CO2 might be more than is estimated by models used in the 2001 IPCC report. The authors also point out that similar analyses need to be made for other plant nutrients, such as potassium and phosphorus.
Hungate, B. A., J. S. Dukes, M. R. Shaw, Y. Luo, and C. B. Field, 2003: Nitrogen and climate change. Science, 302, 1512-1513.