A time-dependent atmospheric CO2 budget model is shown to closely match yearly Mauna Loa CO2 concentrations during 1959-2021. The model assumes an anthropogenic CO2 source, and yearly CO2 sink rate proportional to the excess of CO2 over a baseline equilibrium value determined by the Mauna Loa data. The yearly CO2 sink rate is found to be 2.02% of the atmospheric excess above 293.6 ppm, with a downward trend in the sink rate during 1959-2021 that disappears when El Niño – Southern Oscillation (ENSO) activity is empirically accounted for. Significant model departures from observations occurred for three years after the 1991 eruption of Mt. Pinatubo, when increased scattered sunlight enhanced photosynthesis. The latest Energy Information Administration CO2 global emissions estimates to 2050, and extrapolated to 2100, lead to model-projected CO2 concentrations well below the highest Representative Concentration Pathways scenario RCP8.5.