Run Away Green House State Might be Easier to Achieve Than Previously Thought

Low simulated radiation limit for runaway greenhouse climates

Authors:

1. Colin Goldblatt (a)
2. Tyler D. Robinson (b)
3. Kevin J. Zahnle (c)
4. David Crisp (d)

Affiliations:

a. School of Earth and Ocean Sciences, University of Victoria, PO Box 3065, Victoria, British Columbia, V8W 3V6, Canada

b. Astronomy Department, University of Washington, Box 351580, Seattle, Washington 98195-1580, USA

c. Space Science and Astrobiology Division, NASA Ames Research Center, MS 245-3, Moffett Field, California 94035, USA

d. Jet Propulsion Laboratory, MS 183-501, 4800 Oak Grove Drive, Pasadena, California 91109, USA

Abstract:

The atmospheres of terrestrial planets are expected to be in long-term radiation balance: an increase in the absorption of solar radiation warms the surface and troposphere, which leads to a matching increase in the emission of thermal radiation. Warming a wet planet such as Earth would make the atmosphere moist and optically thick such that only thermal radiation emitted from the upper troposphere can escape to space. Hence, for a hot moist atmosphere, there is an upper limit on the thermal emission that is unrelated to surface temperature. If the solar radiation absorbed exceeds this limit, the planet will heat uncontrollably and the entire ocean will evaporate—the so-called runaway greenhouse. Here we model the solar and thermal radiative transfer in incipient and complete runaway greenhouse atmospheres at line-by-line spectral resolution using a modern spectral database. We find a thermal radiation limit of 282 W m−2 (lower than previously reported) and that 294 W m−2 of solar radiation is absorbed (higher than previously reported). Therefore, a steam atmosphere induced by such a runaway greenhouse may be a stable state for a planet receiving a similar amount of solar radiation as Earth today. Avoiding a runaway greenhouse on Earth requires that the atmosphere is subsaturated with water, and that the albedo effect of clouds exceeds their greenhouse effect. A runaway greenhouse could in theory be triggered by increased greenhouse forcing, but anthropogenic emissions are probably insufficient.