An observational analysis of photometric evidence suggests solar forcing of Earth’s atmosphere could vary by as much as ±4.5 W/m² since 1750, which is “far larger than the IPCC estimate of −0.30 to +0.10 W/m²” (Judge et al., 2020).
A 2017 study suggested the solar activity during the “modern maximum period from 1940 to 2015” is a “relatively rare event, with the previous similarly high levels of solar activity observed 4 and 8 millennia ago” (Yndestad and Solheim, 2017). Variations in solar activity since the 18th century were shown to have ranged between about 1357.5 W/m² and 1362 W/m² (~4.5 W/m²).
In contrast, the total radiative forcing due to the increase in the CO2 concentration since 1750 is suggested to be 1.82 W/m² (Feldman et al., 2015).
Image Source: Yndestad and Solheim, 2017
A new study (Judge et al., 2020) also affirms our highly uncertain estimations of solar forcing variations since 1750 may be “of the order of 3 W/m², far larger than the IPCC estimate of −0.30 to +0.10 W/m²” and also greater than the uncertain IPCC estimates of total anthropogenic forcing (+2.2 ± 1.1 W/m²) since 1750.
Large estimate ranges for solar forcing variability should reduce the certainty that Earth’s radiative forcing has been dominated by anthropogenic activity in recent centuries.