Cultivated plants in the demographic projections of the global carbon budget

Global carbon budgets attribute an incomplete role to cultivated plants due to the exclusion of annual crops, which are not considered to result in net carbon accumulation. Using FAO agricultural and forestry production statistics, this study re-evaluates the amounts of CO2 captured, stored, and restituted by cultivated plants, explicitly including annual crops, forage crops, forest products, and non-marketed plant parts. A stoichiometric and probabilistic approach is developed to describe the temporal dynamics of photosynthetic carbon capture and restitution. The results show that the net balance of cultivated plants has constituted a sink of 3.0 {+/-} 0.7 GtCO2/year on average since 1970, with an average half-life of 10.7 {+/-} 2.9 years. This annual contribution of cultivated plants to atmospheric carbon storage in organic form is sufficient in both quantity and duration to be included in carbon budgets. Among them, annual crops accounted for approximately 27% of total carbon sequestration with retention periods of 6 years, thus invalidating their exclusion from the carbon budgets. The strong correlation observed between the global human population size and the elements of the carbon budget allows for projections up to the end of the 21st century. These projections, adjusted by the United Nations median population scenario, suggest that the carbon sink of cultivated plants would reach zero around the time of the global population peak before reversing. The carbon stocks built by these plants increased by 43 GtC between 1970 and 2024 and would increase by a further 21 GtC by the time of the population peak before declining. Around the same time, the carbon sink constituted by the ocean and unexploited land areas would peak at 25 GtCO2/year, and that constituted by the atmosphere at 24 GtCO2/year, with atmospheric CO2 concentrations reaching 600 ppm. Emissions from fossil fuel combustion would peak at 48 GtCO2/year by this same timeframe before declining. These results highlight the significant role of cultivated plants in the carbon cycle and show that excluding annual crops leads to a substantial underestimation of continental carbon sinks. They call for a revision of carbon accounting methods and a full integration of agriculture and livestock farming into global carbon budgets. Key pointsO_LIAlthough small compared to other carbon sinks, cultivated plants must be included in carbon budgets because they build up a significant carbon stock. C_LIO_LIWhen demography is used to modulate projections, most elements of the carbon budget reach their maximum at the same time as population, while the cultivated plants carbon sink declines and turns a source. C_LI