Editing IPCC:AR6/WGI/Chapter-7 (section)

==== 7.3.4.5 Galactic Cosmic Rays ====

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Variations in the flux of galactic cosmic rays (GCR) reaching the atmosphere are modulated by solar activity and affect new particle formation in the atmosphere through their link to ionization of the troposphere ( [[#Lee--2019|Lee et al., 2019]] ). It has been suggested that periods of high GCR flux correlate with increased aerosol and CCN concentrations and therefore also with cloud properties (e.g., [[#Dickinson--1975|Dickinson, 1975]] ; [[#Kirkby--2007|Kirkby, 2007]] ).

Since AR5, the link between GCR and new particle formation has been more thoroughly studied, particularly by experiments in the CERN CLOUD chamber (Cosmics Leaving OUtdoor Droplets; [[#Dunne--2016|Dunne et al., 2016]] ; [[#Kirkby--2016|Kirkby et al., 2016]] ; [[#Pierce--2017|Pierce, 2017]] ). By linking the GCR-induced new particle formation from CLOUD experiments to CCN, [[#Gordon--2017|Gordon et al. (2017)]] found that the CCN concentration for low-clouds differed by 0.2–0.3% between solar maximum and solar minimum. Combined with relatively small variations in the atmospheric ion concentration over centennial time scales ( [[#Usoskin--2015|Usoskin et al., 2015]] ), it is therefore unlikely that cosmic ray intensity affects present-day climate via nucleation ( [[#Yu--2014|Yu and Luo, 2014]] ; [[#Dunne--2016|Dunne et al., 2016]] ; [[#Pierce--2017|Pierce, 2017]] ; [[#Lee--2019|Lee et al., 2019]] ).

Studies continue to seek a relationship between GCR and properties of the climate system based on correlations and theory. [[#Svensmark--2017|Svensmark et al. (2017)]] proposed a new mechanism for ion-induced increase in aerosol growth rate and subsequent influence on the CCN concentration. The study does not include an estimate of the resulting effect on atmospheric CCN concentration and cloud radiative properties. Furthermore, Svensmark et al. (2009, 2016) find correlations between GCRs and aerosol and cloud properties in satellite and ground-based data. Multiple studies investigating this link have challenged such correlations ( [[#Kristjánsson--2008|Kristjánsson et al., 2008]] ; [[#Calogovic--2010|Calogovic et al., 2010]] ; [[#Laken--2016|Laken, 2016]] ).

AR5 concluded that the GCR effect on CCN is too weak to have any detectable effect on climate and no robust association was found between GCR and cloudiness ( [[#Boucher--2013|Boucher et al., 2013]] ). Published literature since AR5 robustly supports these conclusions with key laboratory, theoretical and observational evidence. There is ''high confidence'' that GCRs contribute a negligible ERF over the period 1750–2019.

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