Clouds and the Faint Young Sun Paradox

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 Wm-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 Wm-2 whilst more modest reduction in their efficacy gives a forcing of +10 to +15 Wm-2. For high clouds, the greenhouse effect dominates. It is possible to generate +50 Wm-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 Wm-2 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 Wm-2 forcing. Some models which have been applied to the FYSP do not include clouds at all. These overestimate the forcing due to increased CO2 by 20 to 25% when CO2 is 0.01 to 0.1 bar.

💡 Research Summary

The paper by Goldblatt and Zahnle (2021) investigates whether changes in cloud properties could resolve the Faint Young Sun Paradox (FYSP), the apparent conflict between a ~20 % fainter Sun during the Archean (≈2.5 Ga) and geological evidence for a relatively warm climate with only occasional glaciations. The authors use a single‑column radiative‑transfer model (the AER Rapid Radiative Transfer Model, RR TM) applied to a Global Annual Mean (GAM) atmospheric profile that reproduces present‑day temperature, water‑vapor, ozone, and surface albedo. They construct a three‑layer cloud representation (low, middle, high) that matches observed cloud climatology and the modern Earth’s energy budget. By systematically varying cloud fraction, optical thickness, altitude, and particle size, they explore the full phase space of plausible cloud changes and quantify the resulting radiative forcing at the tropopause.

First, they compare cloudy versus cloud‑free calculations of greenhouse‑gas forcing. Using RR TM and benchmarking against a line‑by‑line model (LBLRTM), they find that a clear‑sky model overestimates the forcing from increased CO₂ by 20–25 % for pCO₂ between 0.01 and 0.1 bar. The overestimate arises because clouds fill the water‑vapor window (8–15 µm) where the surface emits strongly; without clouds, added greenhouse gases absorb more efficiently in that window than they would in a cloudy atmosphere.

Next, they assess the direct impact of cloud modifications. Low clouds primarily reflect shortwave solar radiation, contributing to planetary albedo, while their greenhouse effect is modest. Removing low clouds entirely yields a positive forcing of +25 W m⁻², but such an extreme scenario is unrealistic. More modest reductions—e.g., decreasing low‑cloud fraction by 30 % or increasing droplet size—produce only +10 to +15 W m⁻². High clouds, by contrast, act as longwave absorbers. Enhancing high‑cloud optical depth by a factor of 3.5, cooling them by 14 K, and expanding coverage to 100 % can generate +50 W m⁻², enough to offset the −50 W m⁻² solar deficit. However, the authors argue that such a configuration is physically implausible for the early Earth. More realistic high‑cloud enhancements (e.g., 1.5‑fold thickness, 70 % coverage) produce at most +15 W m⁻². Surface albedo changes (e.g., reducing land reflectivity) add a further ≤+5 W m⁻².

Summing the most optimistic, yet plausible, contributions from low‑cloud reduction, high‑cloud enhancement, and surface albedo yields a maximum forcing of roughly +30 W m⁻², still insufficient to compensate the ~50 W m⁻² shortfall caused by the faint Sun. Consequently, the authors conclude that cloud changes alone cannot resolve the FYSP. Moreover, they highlight that many earlier Archean climate studies omitted clouds entirely, thereby overestimating the warming effect of CO₂ and other greenhouse gases.

The paper also critiques specific cloud‑based hypotheses that have appeared in the literature. Rondanelli and Lindzen (2010) proposed an “iris” mechanism that would increase high‑cloud coverage as temperatures fell; the authors deem the required cloud expansion unrealistic. Rosing et al. (2010) suggested that the lack of biogenic dimethyl sulfide in the Archean would lead to larger cloud droplets and reduced low‑cloud albedo; the resulting forcing, however, falls far short of the needed magnitude. The authors also dismiss cosmic‑ray–cloud links as insufficiently supported.

In summary, Goldblatt and Zahnle provide a rigorous quantitative assessment of cloud radiative effects in a single‑column framework, demonstrate that cloud‑free models misrepresent greenhouse‑gas forcing, and show that even the most favorable cloud adjustments cannot supply the ~50 W m⁻² needed to keep early Earth warm under a faint Sun. Their work implies that additional mechanisms—high CO₂ concentrations, other greenhouse gases, or fundamentally different atmospheric dynamics—must be invoked to solve the Faint Young Sun Paradox.