Monday, July 10, 2017

Can the Earth have a runaway greenhouse effect?

Stephen Hawking is in the news, having said that the Earth could experience an accelerating greenhouse effect that renders it uninhabitable, like Venus (e.g., here).

Back in 2013, the Scientific American had this story:  
Fact or Fiction?: We Can Push the Planet into a Runaway Greenhouse Apocalypse
A new study suggests human activity could, in theory, bring about the end of most life on Earth
The new study was this paper in Nature Geoscience:
Low simulated radiation limit for runaway greenhouse climates, Colin Goldblatt, Tyler D. Robinson, Kevin J. Zahnle & David Crisp

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 282Wm−2 (lower than previously reported) and that 294Wm−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.
 This paper has since been cited 43 times according to,  and as far as I can tell by a quick scan is that more detailed modeling shows that the Earth's climate is, thankfully, more stable than previously thought (e.g., instead of tipping over with an increase of just 1.5% in the incident solar radiation, it remains stable though uncomfortably hot with even a 15% increase.)

So Hawking is likely wrong.