The CO2 greenhouse effect presumably perturbs the Earth’s energy budget by driving trends in downwelling longwave radiation from the top of the atmosphere, which in turn drives changes in surface temperatures. Two scientists (Zeppetello and Battisti, 2019) have observed the opposite: perturbations in surface temperatures are what drive changes in downwelling longwave. Consequently, they conclude downwelling longwave “cannot be considered an independent component of the surface energy budget.”
The anthropogenic global warming (AGW) hypothesis rides on the fundamental assumption that perturbations in the Earth’s energy budget – driven by changes in downwelling longwave radiation from CO2 at the top of the atmosphere (TOA) – are what cause climate change.
According to one of the most frequently referenced papers advancing the position that CO2 concentration changes (and downwelling longwave radiation perturbations) drive surface temperature changes, Feldman et al. (2015) concluded there was a modest 0.2 Wm-2 per decade forcing associated with a CO2 change of 22 ppm during 2000 to 2010.
Importantly, this contribution from CO2 accounts for just 10% of the overall trend in downwelling longwave radiation believed to cause global warming.
Image Source: Feldman et al., 2015, Okulaer, 2015
In a new paper, scientists Zeppetello and Battisti, 2019 have found the assumption that “downward longwave radiation” (DLR) forcing independently causes changes to the Earth’s energy budget – and thus drives surface temperature changes – has it exactly backwards.
The DLR perturbations at the TOA are considered primarily a response to surface temperature changes. Concisely, temperature changes drive downwelling longwave, not the other way around.
When we consider CO2 concentration changes may represent just 10% of the overall contribution to downwelling longwave, and that downwelling longwave changes are a response to temperature changes, this would appear to leave little room for CO2 concentration changes to play more than a bit role in climate forcing.