Increased insolation threshold for runaway greenhouse processes on Earth-like planets

A three-dimensional global climate model shows that the loss of a planet’s oceans through complete vaporization or evaporative escape to space will occur at considerably higher insolation than previously thought, owing to stabilizing atmospheric effects. A longer wait for a 'runaway greenhouse' The Sun is gradually increasing in brightness on a geological timescale. This could lead eventually to a 'runaway greenhouse' effect on Earth, a state that occurs when a planet absorbs more energy from the Sun than it can radiate back to space. The oceans would evaporate and the climate would warm to Venus-like temperatures. Jérémy Leconte et al . use a three-dimensional climate model to show that the threshold for the initiation of a runaway greenhouse is considerably higher than that previously estimated by simplified one-dimensional models. A crucial factor is cooling caused by changes in atmospheric circulation that more than offset cloud-induced warming. This finding is of importance in relation to extrasolar planets, since it extends the size of the habitable zone around other stars. The increase in solar luminosity over geological timescales should warm the Earth’s climate, increasing water evaporation, which will in turn enhance the atmospheric greenhouse effect. Above a certain critical insolation, this destabilizing greenhouse feedback can ‘run away’ until the oceans have completely evaporated 1 , 2 , 3 , 4 . Through increases in stratospheric humidity, warming may also cause evaporative loss of the oceans to space before the runaway greenhouse state occurs 5 , 6 . The critical insolation thresholds for these processes, however, remain uncertain because they have so far been evaluated using one-dimensional models that cannot account for the dynamical and cloud feedback effects that are key stabilizing features of the Earth’s climate. Here we use a three-dimensional global climate model to show that the insolation threshold for the runaway greenhouse state to occur is about 375 W m −2 , which is significantly higher than previously thought 6 , 7 . Our model is specifically developed to quantify the climate response of Earth-like planets to increased insolation in hot and extremely moist atmospheres. In contrast with previous studies, we find that clouds have a destabilizing feedback effect on the long-term warming. However, subsident, unsaturated regions created by the Hadley circulation have a stabilizing effect that is strong enough to shift the runaway g