Unlike much of the Earth, surface temperature in the tropics underwent a systematic and sustained increase during the satellite era. Due to the temperature dependence of surface processes that regulate CO2 emission, that long-term change should exert a similar influence on atmospheric CO2. We develop how this influence would manifest in the evolution of CO2. Observed records are then used to investigate the interdependence of surface temperature and net CO2 emission – the component of emission that determines anomalous CO2.
Thermally-induced emission, especially from tropical land surface, is found to represent much of the observed evolution of net CO2 emission. It accounts for sporadic intensifications of net emission that operate on interannual time scales, notably, during El Nino. Accounted for equally well is the long-term intensification of net emission during the last half century. Jointly, these unsteady components of net emission determine the thermally- induced component of anomalous CO2. It tracks the observed evolution of CO2.
In Part I of this two-pronged study (Salby and Harde, 2021; hereafter SH1), the record of anomalous carbon 14 revealed the effective absorption rate of atmospheric CO2, which is emitted and absorbed at the Earth’s surface. Coupled with the conservation law governing CO2, the observed absorption provided an upper bound on the anthropogenic perturbation of CO2. Only a couple of percent of the observed increase, it is much too small for anthropogenic emission to be responsible for the observed increase of atmospheric CO2. The same conclusion follows from recent simulations of atmospheric carbon (Harde, 2017; 2019; Berry, 2019; Harde and Salby, 2021).