Over the US, the modest change in cloud cover from 1996-2019 predominantly drove the +11.77 W/m² surface solar radiation forcing during 1996-2012 (then -2.35 W/m² from 2013-2019). These “brightening” and “dimming” magnitudes easily overwhelm the values associated with an annual 2 to 2.5 ppm rise in CO2 forcing (0.2 W/m² per decade).
When cloud cover changes by even a percentage point or two from one decade to the next, the magnitude of the associated forcing can be compared directly to the impact of rising CO2 concentrations. And in a new study, Augustine and Hodges (2021) point out:
“Documented magnitudes of brightening are significant and much larger than the projected increase in downwelling longwave radiation (LW↓) expected for a doubling of CO2 since industrial times (~4 W/m²)”
Translated, the total projected forcing associated with a doubling of the CO2 concentration since 1750 (280 ppm to 560 ppm) is only 3.7 W/m² (Seinfeld, 2008).
Image Source: Seinfeld, 2008
On a per-decade basis, a 22 ppm rise in CO2 – realized about every 10 years in recent decades – is associated with a forcing of just 0.2 W/m² (Feldman et al., 2015), but only when the sky is clear of clouds. And cloud-free skies are rarely ever observed in the real world atmosphere, as “less than 10 percent of the sky is completely clear of clouds at any one time” (NASA, 2015).
Image Source: Feldman et al., 2015
In stark contrast to these radiative forcing magnitudes from CO2 changes, small changes in cloud cover can explain 62% of the +11.77 W/m² increase (1996-2012) and -2.35 W/m² decrease (2013-2019) in surface solar radiation over the continental US in the last 20-25 years (Augustine and Hodges, 2021).
Another 3% of the recent brightening/dimming trends over the US are explained by aerosol forcing. And, interestingly, even this forcing is said to be about 3 to 4 times larger than that of the CO2 impact. The 0.05 reduction in aerosol optical depth from 1997-2019 “corresponds to a direct radiative forcing of +1.59 W/m²” (Augustine and Hodges, 2021).
Across Europe, other scientists have suggested the magnitude of surface solar radiation forcing was +10 W/m² per decade from the 1930s to 1950s, -13 W/m² per decade from the 1950s to 1980s, and then +10 W/m² per decade from the 1990s to 2000s.
Image Source: Augustine and Hodges, 2021
Across Brazil, another new study (Zuluaga et al., 2021) indicates the decline in cloud cover has resulted in about a +3 W/m² per decade forcing since 1980.
Image Source: Zuluaga et al., 2021
All of these forcing magnitudes easily dwarf the impact from CO2 concentration increases in cloud-free skies and affirm “clouds may be the most important parameter controlling the radiation budget, and, hence, the Earth climate” (Sfîcă et al., 2021).