Other meanings of Runaway greenhouse effect
Planetary climate
The runaway greenhouse effect is a theoretical climate transition in which a planet absorbs more energy from its star than its atmosphere can radiate to space. Water vapor amplifies warming, causing further evaporation until the oceans may largely vaporize. It differs from ordinary anthropogenic global warming and from a merely severe greenhouse climate: the defining feature is a radiative instability that prevents a stable, liquid-water surface from being maintained under the prevailing stellar flux.
The transition begins when absorbed stellar radiation approaches the atmosphere’s maximum sustainable infrared emission to space. As the surface warms, evaporation raises atmospheric water vapor, a powerful greenhouse gas; the added opacity shifts radiation to colder, higher layers that emit little additional energy. If the planet receives more energy than those layers can lose, warming and evaporation reinforce one another. This is a positive feedback, not a separate force of nature. Classical radiative-convective calculations identify an upper limit on outgoing longwave radiation, often called the Simpson–Nakajima limit. Once absorbed flux exceeds that limit, a stable surface equilibrium is unavailable in the idealized model, even though the atmosphere may continue evolving through clouds, circulation, and photochemistry.
A moist greenhouse and a runaway greenhouse are related but distinct states. In a moist greenhouse, the lower atmosphere can remain comparatively temperate while the upper atmosphere becomes rich in water vapor; ultraviolet light can then split water, allowing hydrogen to escape to space over geological time. A runaway greenhouse is the more extreme radiative outcome in which surface warming itself cannot settle into equilibrium at the available stellar flux. The threshold depends on atmospheric pressure, clouds, surface water, rotation, planetary gravity, and the star’s spectrum. Three-dimensional models therefore place the inner edge of the habitable zone differently from simple one-dimensional calculations, especially for slowly rotating planets and planets around cool stars.1
Venus is the Solar System’s canonical example of a world with an extreme greenhouse atmosphere, but its exact climatic history is not a simple direct observation of a completed runaway transition. Venus now has a dense carbon-dioxide atmosphere, sulfuric-acid clouds, and a surface temperature near 735 K; its present state is consistent with a severe greenhouse climate and an effectively dry surface.2 Models of Venus’s early evolution permit different pathways, including long-lived oceans followed by water loss, and the evidence does not uniquely determine when or how any runaway phase occurred.3 Earth is not presently close to this threshold: increasing carbon dioxide causes substantial warming, but does not imply that modern emissions will trigger an unstoppable ocean-vaporizing runaway.
The runaway threshold is a property of the whole planet–star system, not a fixed temperature printed on a thermometer. Clouds can either cool or warm depending on their altitude, coverage, particle properties, and the spectrum of incoming light; this makes the inner edge of a habitable zone model-dependent.1 Atmospheric escape also complicates the endpoint: hydrogen loss can permanently remove water, while oxygen may be absorbed by rocks rather than accumulating in the air. The phrase is sometimes used loosely for any strong greenhouse warming, but scientific studies distinguish a runaway greenhouse from the faint young Sun problem, ordinary climate feedbacks, and the long-term carbon cycle. Exoplanet surveys use these distinctions when assessing whether a hot planet could retain surface water.4
Thresholds vary with atmospheric composition, clouds, rotation, gravity, surface conditions, and stellar spectrum; simplified one-dimensional limits should not be treated as universal temperatures.
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