Natural, present day and anthropic fluxes

A common source of confusion regarding the global biogeochemical carbon cycle and its interpretation is the difference between the notions of present day, natural and anthropogenic cycles. Using a simplified representation of the global carbon cycle, reduced to only three compartments (atmosphere, ocean and terrestrial vegetation), this article aims at illustrating the meaning of these three important concepts.  

The Present-day cycle represents the current amounts of carbon in all reservoirs of the global carbon cycle and the current magnitude of the fluxes (i.e. transfer of mater) connecting these reservoirs. In other terms, the present-day carbon cycle corresponds to carbon stocks and fluxes that can be directly observed and measured on the field.

The Natural cycle, also often called ‘pre-anthropogenic cycle’, represents the assumed state of the global carbon cycle prior to its perturbation by human activities. It is common practice, as done in the IPCC reports, to use the beginning of the industrial revolution (~1750) as reference for this natural state because it is associated with the start of large-scale emissions of CO₂ into the atmosphere by industries. It should be kept in mind that human activities already began disturbing the global carbon cycle before the industrial revolution through land-use change and deforestation but at a slower rate. The quantification of natural stocks and fluxes of carbon often required assumptions regarding the dynamics of the global carbon cycle in a pristine state and is generally assumed to be at steady-state. 

The Anthropogenic cycle corresponds to the impact of human activities on the global carbon cycle (i.e. the new fluxes introduced or modified by human activities as well as the subsequent changes in the stocks of the various reservoirs). These fluxes and changes in stocks are sometimes referred to as ‘anthropic perturbation’. They can be calculated as the difference between the present-day cycles and the natural cycle. Some fluxes like the emissions of CO₂ into the atmosphere through the consumption of fossil fuels are only anthropogenic. However, the modification of a compartment of the global carbon cycle (i.e. the atmosphere through fossil fuel burning for example) also affects other connected compartment by the modification of the fluxes between them (this is the propagation of the anthropic perturbation through the global carbon cycle). This is why only ~50% of the carbon currently emitted into the atmosphere by human activities remains in the atmosphere.

Simplified representation of the global carbon cycle with its natural state (1750) in black and the anthropic perturbation in red (all stocks are in Pg C and fluxes in Pg C yr^-1). All values are rounded but based on IPCC, 2021. 

In the simplified representation of the global carbon cycle above, the present-day carbon cycle can be calculated by summing up the natural and anthropogenic cycles. For instance, the pre-industrial (i.e. natural) amount of C in the atmosphere was 600 Pg C and human activities increased this stock by 280 Pg C over a period of 270 years, which means that the current amount of C in the atmosphere is 880 Pg C (as of 2020). Note however that anthropogenic fluxes have changed since 1750 and the flux of 9.5 Pg C yr^-1 entering the atmosphere through fossil fuel consumption correspond to current estimated value but does not correspond to the average flux over the 1750-2020 period. This is why, for example Terrestrial vegetation and soils are currently receiving a net flux of carbon of 1 Pg C yr^-1 (+2.5 resulting from the increase of the atmospheric stock -1.5 associated with land-use change) but have seen their stock decrease since 1750.