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8.4.1.5 The Effects of Cosmic Rays on Clouds Changing cloud amount or properties modify the Earth’s albedo and
therefore affect climate. It has been hypothesized that cosmic ray ?ux create atmospheric ions which facilitates aerosol nucleation and new particle formation with a further impact on cloud formation (Dickinson, 1975; Kirkby, 2007). High solar activity means a stronger heliospheric magnetic feld and thus a more effcient screen against cosmic rays.

Under the hypothesis underlined above, the reduced cosmic ray ?ux would promote fewer clouds amplifying the warming effect expected from high solar activity. There is evidence from laboratory, feld and modelling studies that ionization from cosmic ray ?ux may enhance aerosol nucleation in the free troposphere (Merikanto et al., 2009; Mirme et al., 2010; Kirkby et al., 2011). However, there is high confdence (medium evidence and high agreement) that the cosmic rayionization mechanism is too weak to in?uence global concentrations of cloud condensation nuclei or their change over the last century or during a SC in a climatically signifcant way (Harrison and Ambaum, 2010; Erlykin and Wolfendale, 2011; Snow-Kropla et al., 2011). A detailed exposition is found in Section 7.4.6.

IPCC, sida 691


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