ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-10941-2010 Cosmic rays linked to rapid mid-latitude cloud changes Laken B. A. 1 2 Kniveton D. R. 1 Frogley M. R. 1 Department of Geography, University of Sussex, Falmer, Brighton, England, BN1 9QJ, UK Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain 24 11 2010 10 22 10941 10948 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/10941/2010/acp-10-10941-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/10941/2010/acp-10-10941-2010.pdf

The effect of the Galactic Cosmic Ray (GCR) flux on Earth's climate is highly uncertain. Using a novel sampling approach based around observing periods of significant cloud changes, a statistically robust relationship is identified between short-term GCR flux changes and the most rapid mid-latitude (60°–30° N/S) cloud decreases operating over daily timescales; this signal is verified in surface level air temperature (SLAT) reanalysis data. A General Circulation Model (GCM) experiment is used to test the causal relationship of the observed cloud changes to the detected SLAT anomalies. Results indicate that the anomalous cloud changes were responsible for producing the observed SLAT changes, implying that if there is a causal relationship between significant decreases in the rate of GCR flux (~0.79 <i>GU</i>, where <i>GU</i> denotes a change of 1% of the 11-year solar cycle amplitude in four days) and decreases in cloud cover (~1.9 <i>CU</i>, where <i>CU</i> denotes a change of 1% cloud cover in four days), an increase in SLAT (~0.05 <i>KU</i>, where <i>KU</i> denotes a temperature change of 1 K in four days) can be expected. The influence of GCRs is clearly distinguishable from changes in solar irradiance and the interplanetary magnetic field. However, the results of the GCM experiment are found to be somewhat limited by the ability of the model to successfully reproduce observed cloud cover. These results provide perhaps the most compelling evidence presented thus far of a GCR-climate relationship. From this analysis we conclude that a GCR-climate relationship is governed by both short-term GCR changes and internal atmospheric precursor conditions.

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