On climate response to changes in the cosmic ray flux and radiative budget

[1] We examine the results linking cosmic ray flux (CRF) variations to global climate change. We then proceed to study various periods over which there are estimates for the radiative forcing, temperature change and CRF variations relative to today. These include the Phanerozoic as a whole, the Cretaceous, the Eocene, the Last Glacial Maximum, the 20th century, as well as the 11-yr solar cycle. This enables us to place quantitative limits on climate sensitivity to both changes in the CRF, and the radiative budget, F, under equilibrium. Under the assumption that the CRF is indeed a climate driver, the sensitivity to variations in the globally averaged relative change in the tropospheric ionization I is consistently fitted with μ ≡ - (dT_global/dI) ≈ 7.5 ± 2°K. Additionally, the sensitivity to radiative forcing changes is λ ≡ dT_global/dF = 0.35 ± 0.09°KW^-1m², at the current temperature, while its temperature derivative is undetectable with (dλ/dT)₀ = -0.01 ± 0.04 m²W^-1. If the observed CRF/climate link is ignored, the best sensitivity obtained is λ = 0.54 ± 0.12°KW^-1m² and (dλ/dT)₀ = -0.02 ± 0.05 m²W^-1. Note that this analysis assumes that different climate conditions can be described with at most a linear function of T; however, the exact sensitivity probably depends on various additional factors. Moreover, λ was mostly obtained through comparison of climate states notably different from each other, and thus only describes an average sensitivity. Subject to the above caveats and those described in the text, the CRF/climate link therefore implies that the increased solar luminosity and reduced CRF over the previous century should have contributed a warming of 0.47 ± 0.19°K, while the rest should be mainly attributed to anthropogenic causes. Without any effect of cosmic rays, the increase in solar luminosity would correspond to an increased temperature of 0.16±0.04°K.