Clouds and the Faint Young Sun Paradox

Abstract

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<sup>−2</sup> 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<sup>−2</sup> whilst more modest reduction in their efficacy gives a forcing of +10 to +15 W m<sup>−2</sup>. For high clouds, the greenhouse effect dominates. It is possible to generate +50 W m<sup>−2</sup> 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<sup>−2</sup> forcing. Thus neither fewer low clouds nor more high clouds can provide enough forcing to resolve the FYSP. Decreased surface albedo can contribute no more than +5 W m<sup>−2</sup> forcing. Some models which have been applied to the FYSP do not include clouds at all. These overestimate the forcing due to increased CO<sub>2</sub> by 20 to 25% when <i>p</i>CO<sub>2</sub> is 0.01 to 0.1 bar

The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth

“…On the other hand, hazes can also act as cloud condensation nuclei, enhancing cloud formation (Hasenkopf et al , 2011). This might lead to cooling of the planet or even warming depending on cloud particle size and the altitude—and therefore temperature—of the cloud layer (Goldblatt and Zahnle, 2011). A complete treatment of the impact of ice-albedo feedback, haze deposition, haze circulation, and cloud feedbacks is left to future General Circulation Model (GCM) studies better equipped to deal with these inherently 3-D issues.…”

“…This includes the effect of clouds, which is standard in this 1-D treatment (Kopparapu et al , 2013) and is the albedo that reproduces the average temperature of present-day Earth (288 K) with modern atmospheric conditions. Of course, the true cloud distribution on Archean Earth is unknown, and clouds may have had important climatic effects on our early planet (Goldblatt and Zahnle, 2011). The solar zenith angles (SZAs) used in the climate and photochemical models were chosen to best represent globally averaged behavior of the physics in each specific model, which Segura et al (2003) found as SZA = 45° in the photochemical model and SZA = 60° in the climate model.…”

WATER LOSS FROM TERRESTRIAL PLANETS WITH CO2-RICH ATMOSPHERES

“…Clouds play a key role in the climates of Earth, past and present (Goldblatt & Zahnle 2010;Hartmann et al 1986) and Venus (Titov et al 2007). However, their effects are extremely hard to predict in general, due to continued uncertainty in microphysical and small-scale convective processes.…”

Reduced albedo on early Mars does not solve the climate paradox under a faint young Sun

“…Surface albedo in this model is therefore considered as a tunable parameter that is adjusted in order to allow the model to reproduce an Earth-like climate under present-day conditions. This approach, as well as possible alternatives, have been discussed (Goldblatt & Zahnle 2010;Rondanelli & Lindzen 2010), but three dimensional general circulation models are ultimately required to properly constrain the effects of partial cloud cover.…”