Transition to a Moist Greenhouse with CO2 and solar forcing

Abstract

Water-rich planets such as Earth are expected to become eventually uninhabitable, because liquid water turns unstable at the surface as temperatures increase with solar luminosity. Whether a large increase of atmospheric concentrations of greenhouse gases such as CO2 could also destroy the habitability of water-rich planets has remained unclear. Here we show with three-dimensional aqua-planet simulations that CO2-induced forcing as readily destabilizes the climate as does solar forcing. The climate instability is caused by a positive cloud feedback and leads to a new steady state with global-mean sea-surface temperatures above 330 K. The upper atmosphere is considerably moister in this warm state than in the reference climate, implying that the planet would be subject to substantial loss of water to space. For some elevated CO2 or solar forcings, we find both cold and warm equilibrium states, implying that the climate transition cannot be reversed by removing the additional forcing.

Climate variations on Earth-like circumbinary planets

“…The LEBM lacks the vertical dimension that is necessary to represent the climate instabilities at higher temperatures. Hence, the model is neither able to produce a Runaway Greenhouse2021222324 nor cloud-induced instabilities found in this and two previous studies that lead to a transition from a habitable state to a Moist Greenhouse state1325. This explains why the transition to a hot uninhabitable state occurs so close to stars in ref.…”

“…In this work, we investigate the effect of the full three-body dynamics on the atmosphere of an Earth-like planet in a system similar to Kepler-35, by combining a state-of-the art GCM13 with an analytic orbit propagator for circumbinary planets14. The Kepler-35 binary-system consists of two stars only slightly less luminous than the sun on a mutual orbit with a semimajor axis of 0.176 a.u.…”

Constraints on Climate and Habitability for Earth-like Exoplanets Determined from a General Circulation Model

“…Note that Popp et al (2016) similarly find that ECHAM is significantly more sensitive than CAM4 to increasing stellar flux from the Sun. The striking differences in climate between these simulations highlights the need for detailed model intercomparison to ascertain why the models diverge.…”

“…see Fig. 7a in Popp et al, 2016). We still have much to learn both scientifically and technically, as we apply our 3-D models to the new and exotic atmospheres of extrasolar planets.…”

The habitability of a stagnant-lid Earth

“…Kasting et al (1993b) found that the runaway greenhouse limit for a planet with a water reservoir of one Earth ocean does not depend on the amount of CO 2 in the atmosphere, while the water loss limit occurs at smaller orbital distances if the atmosphere contains more CO 2 . A study by Popp et al (2016) also found that the inner boundary of the HZ as determined by water loss can be strongly influenced by the CO 2 content of the atmosphere. Since we obtain a much smaller water reservoir than one Earth ocean by outgassing from the interior of a stagnant-lid planet, the atmospheres at the inner edge of the HZ are not necessarily dominated by water vapor as is the case in the runaway greenhouse scenarios as modelled by Kasting et al (1993b).…”