Climate of the Past Discussions www.clim-past-discuss.net 1814-9340 1814-9359 6 3 2010 10.5194/cpd-6-1163-2010 http://www.clim-past-discuss.net/6/1163/2010/ http://www.clim-past-discuss.net/6/1163/2010/cpd-6-1163-2010.html http://www.clim-past-discuss.net/6/1163/2010/cpd-6-1163-2010.pdf 1163 1207 2010-06-10 Clouds and the Faint Young Sun Paradox C. Goldblatt colin.goldblatt@nasa.gov K. J. Zahnle Space Science and Astrobiology Division, NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA We investigate the role which clouds could play in resolving the Faint Young Sun Paradox (FYSP). Lower solar luminosity in the past means that less energy was absorbed on Earth (a forcing of -50 W m⁻² during the late Archean), but geological evidence points to the Earth being at least as warm as it is today, with only very occasional glaciations. We perform radiative calculations on a single global mean atmospheric column. We select a nominal set of three layered, randomly overlapping clouds, which are both consistent with observed cloud climatologies and reproduce the observed global mean energy budget of Earth. By varying the fraction, thickness, height and particle size of these clouds we conduct a wide exploration of how changed clouds could affect climate, thus constraining how clouds could contribute to resolving the FYSP. Low clouds reflect sunlight but have little greenhouse effect. Removing them entirely gives a forcing of +25 W m⁻² whilst more modest reduction in their efficacy gives a forcing of +10 to +15 W m⁻². For high clouds, the greenhouse effect dominates. It is possible to generate +50 W m⁻² forcing from enhancing these, but this requires making them 3.5 times thicker and 14 K colder than the standard high cloud in our nominal set and expanding their coverage to 100% of the sky. Such changes are not credible. More plausible changes would generate no more that +15 W m⁻² forcing. 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