Sun-like Stars Shed Light on Solar Climate Forcing

Recently published, precise stellar photometry of 72 Sun-like stars obtained at the Fairborn Observatory between 1993 and 2017 is used to set limits on the solar forcing of Earth's atmosphere of $\pm$ 4.5 W m$^{-2}$ since 1750. This compares with the +2.2 $\pm$ 1.1 W m$^{-2}$ IPCC estimate for anthropogenic forcing. Three critical assumptions are made. In decreasing order of importance they are: (a) most of the brightness variations occur within the average time-series length of $\approx$17 years; (b) the Sun seen from the ecliptic behaves as an ensemble of middle-aged solar-like stars; and (c) narrow-band photometry in the Str\"omgren $b$ and $y$ bands are linearly proportional to the total solar irradiance. Assumption (a) can best be relaxed and tested by obtaining more photometric data of Sun-like stars, especially those already observed. Eight stars with near-solar parameters have been observed from 1999, and two since 1993. Our work reveals the importance of continuing and expanding ground-based photometry, to complement expensive solar irradiance measurements from space.