First exploration of the runaway greenhouse transition with a 3D General Circulation Model

While their detections remain challenging at present, observations of small terrestrial planets will become easier in a near future thanks to continuous improvements of detection and characterisation instruments. In this quest, climate modeling is a key step to understanding their characteristics, atmospheric composition, and possible histories. If a surface water reservoir is present on such a terrestrial planet, an increase in insolation may lead to a dramatic positive feedback induced by water evaporation: the runaway greenhouse. The resulting rise in the global surface temperature leads to the evaporation of the entire water reservoir, separating two very different population of planets: 1) temperate planets with a surface water ocean and 2) hot planets with a puffed atmosphere dominated by water vapor. Therefore, the understanding of the runaway greenhouse is pivotal to assess the different evolution of Venus and the Earth, as well as every similar terrestrial exoplanet. In this work, we use a 3D General Circulation Model (GCM), the Generic-PCM, to study the runaway greenhouse transition, linking temperate and post-runaway states. Our simulations were comprised of two phases. First, assuming initially a liquid surface ocean, there is an evaporation phase, which enriches the atmosphere with water vapor. Second, when the ocean is considered to be entirely evaporated, there is a dry transition phase for which the surface temperature increases dramatically. Finally, the evolution ends with a hot and stable post-runaway state. By describing in detail the evolution of the climate over these two steps, we show a rapid transition of the cloud coverage and of the wind circulation from the troposphere to the stratosphere. By comparing our result to previous studies using 1D models, we discuss the effect of intrinsically 3D processes such as the global dynamics and the clouds, which are key to understanding the runaway greenhouse. We also explore the potential reversibility of the runaway greenhouse that is limited by its radiative unbalance.