evidence: Thus, to explain the warming with GHG forcing alone requires feedbacks to account for a forcing of Δf − ΔfG ≈ 2.67 W m−2, that is, a (positive) feedback factor of aG = Δf/ΔfG ≈ 2.1. To explain 60% of the warming with solar forcing requires a feedback factor of aS = 0.6 × Δf/ΔfS ≈ 12.8 and a feedback factor of aG = 0.4 × Δf/ΔfG ≈ 0.8 for greenhouse gases. In other words, the fit of Scafetta (2009) requires a huge positive feedback for the solar forcing but a negative feedback for the greenhouse forcing. Thus, this fit raises much bigger problems than it solves and makes no sense physically. [...] Meehl GA, Washington WM, Wigley TML, Arblaster JM, Dai A (2003) Solar and greenhouse gas forcing and climate response in the twentieth century. J Clim 16:426–444. doi:10.1175/15200442