CO2’s impact on the total net greenhouse effect (GHE) or longwave (LW) forcing trend has been offset by the dominant impact of cloud cover since the 1980s.
It is now widely accepted that changes in clouds, naturally driven by changes in atmospheric circulation, “may be the most important parameter controlling the radiation budget, and, hence, the Earth climate” (Sfîc? et al., 2021). Indeed, in recent decades, the post-1980 +3 W/m² increase in absorbed solar radiation due to the reduction in cloud albedo (Delgado-Bonal et al, 2020) can explain the warming in recent decades.
Image Source: Sfîc? et al., 2021
An overlooked 2020 observational study (Su et al., 2020) suggested that while top of the atmosphere (TOA) shortwave (SW) forcing has risen at a rate of +0.61 W/m² per decade since the early 2000s (because “decreasing cloud optical depth…causes an overall increase in TOA SW flux”), the total longwave (LW) or greenhouse effect (GHE) forcing – which includes the total net forcing impact of CO2 – has declined at a rate of -0.17 W/m² per decade.
Image Source: Su et al., 2020
These trends are consistent with Kato et al., 2018. They found the total downwelling SW forcing (“downward shortwave irradiance”) rose +1.3 W/m² from 2005-2014 (and +2.2 W/m² per decade from 1986-2000). In contrast, the total LW (greenhouse effect) forcing from clouds, water vapor, and CO2 combined to elicit a trend of -0.2 W/m² per decade from 2005-2014.
Image Source: Kato et al., 2018
Cess et al. (2003) also explained total greenhouse effect forcing declines as the absorbed solar radiation increases due to the “major impact” of clouds. Decadal-scale cloud cover changes are “the result of natural variability on decadal time scales,” or “internal unforced variability.”
“Not only is the negative trend in cloud fraction consistent with the positive trend in ASW [absorbed shortwave], it is also consistent with the negative trend in G [greenhouse effect anomaly]” that occurred from 1985-1999.
Image Source: Cess et al., 2003
If the net impact of greenhouse effect forcing has been negative since the 1980s while the trend in absorbed SW forcing has been positive, this explains the warming trends in the last decades and precludes an attribution for anthropogenic greenhouse gas emissions.