A runaway greenhouse effect occurs when a planet's atmosphere contains greenhouse gas in an amount sufficient to block thermal radiation from leaving the planet, preventing the planet from cooling and from having liquid water on its surface. A runaway version of the greenhouse effect can be defined by a limit on a planet's outgoing longwave radiation, which is asymptotically reached due to higher surface temperatures evaporating water into the atmosphere, increasing its optical depth. This positive feedback loop means the planet cannot cool down through longwave radiation (via the Stefan–Boltzmann law) and continues to heat up until it can radiate outside of the absorption bands of the water vapour. The runaway greenhouse effect is often formulated with water vapour as the condensable species. The water vapour reaches the stratosphere and escapes into space via hydrodynamic escape, resulting in a desiccated planet. This likely happened in the early history of Venus. A 2012 study on climate change indicated that "Earth presently absorbs around 240 W m−2 of solar radiation. Increasing carbon dioxide concentration will make surface warmer with the same outgoing thermal flux. Following this theory, we are not near the threshold of a runaway greenhouse. However, the behaviour of hot, water-vapour-rich atmospheres is poorly understood, and an in-depth study of these is necessary." However, the authors cautioned that "our understanding of the dynamics, thermodynamics, radiative transfer and cloud physics of hot and steamy atmospheres is weak," and that we "cannot therefore completely rule out the possibility that human actions might cause a transition, if not to full runaway, then at least to a much warmer climate state than the present one." A runaway greenhouse effect similar to Venus appears to have virtually no chance of being caused by anthropogenic activities. A 2013 article concluded that a runaway greenhouse effect "could in theory be triggered by increased greenhouse forcing," but that "anthropogenic emissions are probably insufficient." Venus-like conditions on Earth require a large long-term forcing that is unlikely to occur until the Sun brightens by some tens of percents, which will take a few billion years. Earth is expected to experience a runaway greenhouse effect "in about 2 billion years as solar luminosity increases".
History While the term was first coined by Caltech scientist Andrew Ingersoll in a paper that described a model of the atmosphere of Venus, the initial idea of a limit on terrestrial outgoing infrared radiation was published by George Simpson in 1927. The physics relevant to what would later be named the runaway greenhouse effect, was explored by Makoto Komabayashi at Nagoya University. Assuming a water vapor-saturated stratosphere, Komabayashi and Ingersoll independently calculated the limit on outgoing infrared radiation that defines the runaway greenhouse state. That limit is now known as the Komabayashi–Ingersoll limit, to recognize their contributions.
Physics
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