Towards a more complete quantification of the global carbon cycle

The main components of global carbon budget calculations are the emissions from burning fossil fuels, cement production, and net land-use change, partly balanced by ocean CO2 uptake and CO2 increase in the atmosphere. The difference between these terms is referred to as the residual sink, assumed to correspond to increasing carbon storage in the terrestrial biosphere through physiological plant responses to changing conditions (Delta B-phys). It is often used to constrain carbon exchange in global earth-system models. More broadly, it guides expectations of autonomous changes in global carbon stocks in response to climatic changes, including increasing CO2, that may add to, or subtract from, anthropogenic CO2 emissions. However, a budget with only these terms omits some important additional fluxes that are needed to correctly infer Delta B-phys. They are cement carbonation and fluxes into increasing pools of plastic, bitumen, harvested-wood products, and landfill deposition after disposal of these products, and carbon fluxes to the oceans via wind erosion and non-CO2 fluxes of the intermediate breakdown products of methane and other volatile organic compounds. While the global budget includes river transport of dissolved inorganic carbon, it omits river transport of dissolved and particulate organic carbon, and the deposition of carbon in inland water bodies. Each one of these terms is relatively small, but together they can constitute important additional fluxes that would significantly reduce the size of the inferred Delta B-phys. We estimate here that inclusion of these fluxes would reduce Delta B-phys from the currently reported 3.6 GtC yr(-1) down to about 2.1 GtC yr(-1) (excluding losses from land-use change). The implicit reduction in the size of Delta B-phys has important implications for the inferred magnitude of current-day biospheric net carbon uptake and the consequent potential of future biospheric feedbacks to amplify or negate net anthropogenic CO2 emissions.