Abstract
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.