A late 2021 study finds water vapor and temperature changes accounted for 90% of the changes in clear-sky downwelling longwave or greenhouse effect forcing since the mid-1980s. CO2 forcing assumed a mere bit-player role.
The seminal Feldman et al. (2015) study concluded it takes 10 years and a 22 ppm increase in CO2 to account for just one-tenth of the total longwave or greenhouse effect forcing in recent (2000-2010) climate change trends. The remaining longwave forcing contribution is from water vapor and clouds.
Image Source: Berkeley Lab (press release) and Feldman et al., 2015
Other scientists (Clark et al., 2021) report CO2’s 10% contribution to greenhouse effect forcing can be applied to the period when CO2 rose from 344 to 405 ppm, or 1984 to 2017.
Image Source: Clark et al., 2021
Cloud-radiative forcing dominates in longwave
It should be noted that this proportionately small CO2 effect only applies in a hypothetical or non-real-world atmosphere that has no clouds in it.
In the real-world atmosphere “less than 10 percent of the sky is completely clear of clouds at any one time.”
And when clouds are present they dominate as the driver of greenhouse effect forcing. Quantitatively, the greenhouse effect of clouds is larger than a 100-fold increase in the CO2 concentration (~40,000 ppm).
Image Source: Ramanathan et al., 1989
So if CO2 has one-tenth of the total greenhouse effect forcing impact in an imaginary-world atmosphere that is perpetually cloud-free, and if the real-world atmosphere is cloud-free only about one-tenth of the time, we could conclude that CO2 has a 10% of 10% (1%) total impact in total greenhouse effect forcing.
Water vapor forcing dominates in cloud-free atmospheres
Other scientists have suggested the bit-player role for CO2 in clear-sky-only atmospheres is well less than the 10% outlined above.
In the real-world atmosphere water vapor concentrations can range from 35,000 to 40,000 ppm in the tropics to less than 500 ppm at the poles. Averaged over the globe, there are about 29 water vapor molecules for every 1 CO2 molecule in the atmosphere (Lightfoot and Mamer, 2017).
Image Source: Lightfoot and Mamer, 2017
But we also must consider “water molecules are 1.6 times more effective at warming than CO2 molecules” (Lightfoot and Mamer, 2014).
Image Source: Lightfoot and Mamer, 2014
So, putting it all together, the radiative forcing (RF) effect for CO2 molecules in a clear-sky-only atmosphere is “approximately 2.7% of the total RF of all the GHG [greenhouse gases]” (Lightfoot and Mamer, 2014).
How is it possible climate activists could get away with persuading the public CO2 is Earth’s temperature “control knob” all these years?