Runaway climate change

Runaway climate change or runaway global warming is hypothesized to follow a tipping point in the climate system, after accumulated climate change initiates a reinforcing positive feedback. This is thought to cause the climate to rapidly change until it reaches a new stable condition. These phrases may be used with reference to concerns about rapid global warming. Some astronomers use the expression runaway greenhouse effect to describe a situation where the climate deviates catastrophically and permanently from the original state—as happened on Venus.

Although these terms are rarely used in the peer-reviewed climatological literature, that literature does use the similar phrase "runaway greenhouse effect", which refers specifically to climate changes that cause a planetary body's water to boil off.

The runaway greenhouse effect has several meanings. At the least extreme, this implies global warming sufficient to induce out-of-control amplifying feedbacks, such as ice sheet disintegration and melting of methane hydrates. At the most extreme, a Venus-like planet with crustal carbon baked into the atmosphere and a surface temperature of several hundred degrees, an irreversible climate state.

Between these two is the moist greenhouse, which occurs if the climate forcing is large enough to make water vapour (H₂O) a major atmospheric constituent. In principle, an extreme moist greenhouse might cause an instability with water vapour preventing radiation to space of all absorbed solar energy, resulting in very high surface temperature and evaporation of the ocean. However, simulations indicate that no plausible human-made greenhouse gas (GHG) forcing can cause an instability and baked-crust runaway greenhouse effect.

Conceivable levels of human-made climate forcing could yield the low-end runaway greenhouse. A forcing of 12–16 W m⁻² would require carbon dioxide (CO₂) levels to increase 8–16 times. If the forcing were due only to CO₂ change, this would raise the global mean temperature by 16–24 °C with much larger polar warming. A warming of 16–24 °C produces a moderately moist greenhouse, with water vapour increasing to about 1% of the atmosphere's mass, thus increasing the rate of hydrogen escape to space. If such a forcing were entirely due to CO₂, the weathering process would remove the excess atmospheric CO₂ on a time scale of 10⁴–10⁵ years, well before the ocean was significantly depleted. Venus-like conditions on the Earth require a large long-term forcing that is unlikely to occur until the sun brightens by a few tenths of a percent, which will take a few billion years.

...
Wikipedia