Requirements for a Satisfactory Model of the Global Carbon Cycle and Current Status of Modeling Efforts

Abstract

The concentration of CO₂ in the atmosphere is rising as a result of emissions into the atmosphere by fossil fuel combustion, deforestation, and expanding agriculture. Experiments with numerical models of the earth’s climatic system indicate that a further increase of the atmospheric CO₂ concentration might change climate on earth significantly. To understand how rapidly such possible changes might occur, we must be able to project the likely rate of change of atmospheric CO₂ due to future CO₂ emissions. For this purpose a better knowledge of the global carbon cycle is required, particularly of how atmospheric CO₂ is exchanging with the terrestrial ecosystems and the oceans. It has been maintained that about half of the CO₂ emitted into the atmosphere has stayed there; that is, the “airborne fraction” of the emissions has been about 50%. The emissions due to deforestation and changing land use need more careful consideration in this context, and the airborne fraction of the total emissions has not been determined accurately as yet. We need to know more precisely what has happened in the past to be able to validate models of the global carbon cycle.