Runaway greenhouse effect

A runaway greenhouse effect will occur 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.^[1] This positive feedback 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^[2] 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.^[3] This likely happened in the early history of Venus.

Research in 2012 found that almost all lines of evidence indicate that it is unlikely to be possible to trigger a full runaway greenhouse on Earth, merely by adding greenhouse gases to the atmosphere.^[4] 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".^[4]

A runaway greenhouse effect similar to Venus appears to have virtually no chance of being caused by people.^[5] A 2013 article concluded that runaway greenhouse "could in theory be triggered by increased greenhouse forcing", but that "anthropogenic emissions are probably insufficient".^[6] 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.^[7] Earth is expected to experience a runaway greenhouse effect "in about 2 billion years as solar luminosity increases".^[4]