Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: results from an atmosphere-ocean general circulation model

摘要

Inclusion of fundamental ecological interactions between carbon and nitrogencycles in the land component of an atmosphere-ocean general circulationmodel (AOGCM) leads to decreased carbon uptake associated with CO₂fertilization, and increased carbon uptake associated with warming of theclimate system. The balance of these two opposing effects is to reduce thefraction of anthropogenic CO₂ predicted to be sequestered in landecosystems. The primary mechanism responsible for increased land carbonstorage under radiatively forced climate change is shown to be fertilizationof plant growth by increased mineralization of nitrogen directly associatedwith increased decomposition of soil organic matter under a warming climate,which in this particular model results in a negative gain for theclimate-carbon feedback. Estimates for the land and ocean sink fractions ofrecent anthropogenic emissions are individually within the range ofobservational estimates, but the combined land plus ocean sink fractionsproduce an airborne fraction which is too high compared to observations.This bias is likely due in part to an underestimation of the ocean sinkfraction. Our results show a significant growth in the airborne fraction ofanthropogenic CO₂ emissions over the coming century, attributable inpart to a steady decline in the ocean sink fraction. Comparison toexperimental studies on the fate of radio-labeled nitrogen tracers intemperate forests indicates that the model representation of competitionbetween plants and microbes for new mineral nitrogen resources isreasonable. Our results suggest a weaker dependence of net land carbon fluxon soil moisture changes in tropical regions, and a stronger positive growthresponse to warming in those regions, than predicted by a similar AOGCMimplemented without land carbon-nitrogen interactions. We expect that thebetween-model uncertainty in predictions of future atmospheric CO₂concentration and associated anthropogenic climate change will be reduced asadditional climate models introduce carbon-nitrogen cycle interactions intheir land components.