Cloud modulation of shortwave radiation and greenhouse effect forcing has largely been the determining factor in the global warming of the last 45 years. Not CO2.
CO2 forcing and its effect on surface temperatures is detailed in analyses of changes in clear-sky radiation only because all-sky radiation effects that include clouds (and the real-world atmosphere has clouds) overshadow the CO2 impact (Feldman et al., 2015, Harries et al., 2001).
Late 20th Century Climate Forcing
Per satellite observations, from 1985 to 1998 the “background clear-sky OLR [outgoing longwave radiation] was essentially unchanged” (Wang et al., 2002). In other words, any variations in OLR attributed to changes in greenhouse gas concentrations were not detectable.
In contrast, cloud vertical distributions explained 40% of increased tropical outgoing longwave radiation (OLR) and 60% could be explained by the emissivity of clouds, which means OLR changes were “most likely due entirely to changes in tropical cloud characteristics” and “cannot be attributed to increases in greenhouse gas concentrations.”
Furthermore, there was a decrease in reflected shortwave radiation (RSR) of -2.4 W/m² per decade observed from 1985 to 1999, which means there was a +3.6 W/m² increase in solar radiation absorbed by the Earth system during these 14 years. This can easily explain the warming during this period.
Image Source: Wang et al., 2002
Cess and Udelhofen (2003) independently affirmed greenhouse gas forcing could not have been the cause of the 1985-1999 climate changes because the greenhouse effect forcing actually declined during this period.
“…the negative trend of G [greenhouse effect anomalies] indicates that the atmospheric greenhouse effect is temporarily [1985-1999] decreasing, despite the fact that greenhouse gasses are increasing.”
The declining greenhouse effect forcing is consistent with the decreasing trend in cloud cover, which, in turn, may be a manifestation of “internal unforced variability” or “natural variability on decadal time scales.”
And, as Wang and colleagues also noted, Cess and Udelhofen documented a positive trend in absorbed shortwave radiation (“ASW”) from 1985-1999.
Image Source: Cess and Udelhofen, 2003
21st Century Climate Forcing
Song et al. (2016) reported then when all greenhouse effect forcing contributors (CO2, CH4, water vapor, and clouds) are considered (“all-sky”) there was a declining overall greenhouse effect forcing trend (shown in red) from 2003-2014. The increasing trend in greenhouse effect forcing is only realized in an imaginary world with only anthropogenic greenhouse gas forcing (CO2, CH4) and no clouds (“clear-sky”).
And, once again, the “dominant contributor” to the greenhouse effect forcing “hiatus” was the cloud cover variation.
Image Source: Song et al., 2016
Loeb and colleagues (2021) determined the (uncertain) positive trend in Earth’s Energy Imbalance (EEI) from 2005 to 2019, 0.5 W/m² ±0.47 W/m² per decade−1, is “primarily due to an increase in absorbed solar radiation associated with decreased reflection by clouds.”
CERES satellite data indicate clouds and surface albedo account for 89% of the absorbed solar radiation trend in the 21st century, whereas anthropogenic greenhouse gases account for but a tiny fraction of the trends in combined absorbed solar radiation and greenhouse effect forcing (reductions in emitted thermal radiation) during this period.
This very small human emissions/greenhouse gas impact is represented by the red “Other” (“trace gases”) bars in the graph below. In emitted thermal radiation, graph (e) shows the greenhouse gas impact is effectively offset by the cloud influence; both factors are cancelled out by temperature changes. This leaves the increase in absorbed solar radiation shown in graph (d) due to natural variations in clouds and surface albedo (SFC) as the primary driver(s) of top-of-atmosphere (TOA) flux forcing during the last two decades.
Image Source: Loeb et al., 2021
Other scientists (Dübal and Vahrenholt, 2021) have also concluded that the positive TOA net flux (+1.42 W/m²) from increasing downwelling shortwave (SW) facilitated by a drop in cloudiness has been the “major driving effect,” “dominating influence,” and “major heating cause” explaining the 2001-2019 ocean heat content increase (240 ZJ).
The authors note these CERES satellite observations “conflict with the assumption further global warming originates mainly from the LW [longwave] radiation capture caused by greenhouse gases, i.e., a decline in outgoing LW.” In fact, just as other scientists have determined, the LW or greenhouse effect impact has been negative in the 21st century; it has contributed a net cooling influence (-1.1 W/m²) over the last two decades.
Image Source: Dübal and Vahrenholt, 2021
These studies are further supported by a geoscientist (Swift, 2018), who documented there has been a “decreasing greenhouse factor” or a weakening of the greenhouse effect since 1979. In contrast, the “increasing absorbed solar radiation, caused by a lowering of the bond [cloud] albedo, was the driver of increased surface temperature during the period 1979-2015, and also of the increase in ocean heat content.”
Image Source: Swift, 2018
By Kenneth Richard on 8. August 2022