Land/ocean absorption dynamics and airborne projection of carbon dioxide under finite fossil-fuel reserves

The paper has been suggested by a pair of observations: 1) the atmospheric growth rate of carbon dioxide is smaller than that ascribed to the emission by fossil fuel combustion; 2) the fossil-fuel reserves are finite. The first observation leads to a simple dynamic model, based on the balance between the landocean absorption and the anthropogenic emissions of CO2, only limited by the depletion of fossil-fuel reserves. The second observation suggests of projecting the historical CO2 emissions in the future, by constraining them to the limit of reserve availability. Similar projections are available in the literature, but either driven by heuristics or by complex simulation packages. Here we provide a transparent and formal method only driven by historical data, their uncertainty and simple models. The method is proven capable of providing emission and concentration projections, which being constrained by finite reserves, may be taken as realistic bounds to forecasting exercises. The dynamics of the land-ocean absorption is proved by simplifying a more complex set of equations describing the CO2 exchange between Earth's reservoirs. The contribution of other greenhouse gases like methane and nitrous oxide is neglected, as their emissions cannot be projected with the paper method. Notwithstanding this limitation, the paper results demonstrate that some of the IPCC projections are overestimated if compared to fossil-fuel physical limits, in agreement with other authors. Copyright (c) 2023 The Authors.