Fig. 1: The natural evolution of atmospheric CO₂ concentration when using different assumptions for volcanic outgassing.

In (a), we show atmospheric CO₂ concentration for the past 200 kyr, and the simulated natural evolution of atmospheric CO₂ for the next 200 kyr. With this, we demonstrate the resulting long-term trend in atmospheric CO₂ evolution due to the carbon cycle imbalance arising between outgassing and weathering (PIeq vs. LGCeq), leading to substantially different climate evolutions over the 200 kyr period. In (c), we display the assumption that the carbon cycle is in balance with average glacial cycle conditions (LGCeq), which necessitates a constant volcanic outgassing of 0.0559 PgC yr⁻¹ that ultimately produces a negative decreasing trend in atmospheric CO₂. Given considerable uncertainties in this value, we simulated future atmospheric CO₂ concentrations for values which are  ±5–10% the constant volcanic outgassing used in LGCeq. In light of this, we also display in (b) the assumption where the carbon cycle is in balance at the pre-industrial time (PIeq). We consider this to be an upper bound on plausible future CO₂ concentrations, which necessitates a constant volcanic outgassing of 0.0706 PgC yr⁻¹. The proxy record (black) shows a reconstruction of atmospheric CO₂ concentration from the EPICA Dome C and Vostok ice cores⁹⁷. A 300-year rolling mean was applied to the simulated data shown here for visibility. Sketches (b, c) were created in part using modified images from the UMCES IAN Media Library under a Creative Commons license https://creativecommons.org/licenses/by-sa/4.0/CC BY-SA 4.0. In these sketches, we show volcanic outgassing must equal one-half atmospheric CO₂ consumption via silicate weathering on timescales  >100,000 years. This is explained with further detail in section S1.1. It should be noted that most of the carbon from volcanic CO₂ outgassing originates from hydrothermal sources (e.g., along mid-ocean ridges), and only a small amount is contributed from sub-aerial volcanoes.