Runaway terms are occasionally used in relation to climate change events in climatological literature. More generally, uses for these terms are found in the engineering journals, in books, and in the news media. Runaway terms are also used in the planetary sciences to describe the conditions that led to the current greenhouse state of Venus.
Definition
The phrase "runaway climate change" is used to describe a situation in which positive feedbacks result in rapid climate change. It is most commonly used in mass media and popular science literature and by environmental organizations, is occasionally used in the social sciences. It is particularly used in the popular media and by environmentalists with reference to concerns about rapid global warming. Some astronomers use the similar expression runaway greenhouse effect to describe a situation where the climate deviates catastrophically and permanently from the original state - as happened on Venus.
Related terms
* Tipping Level - Climate forcing (greenhouse gas amount) reaches a point such that no additional forcing is required for large climate change and impacts
* Point of No Return - Climate system reaches a point with unstoppable irreversible climate impacts (irreversible on a practical time scale) Example: disintegration of large ice sheet
Feedbacks
The core of the concept of runaway climate change is the idea of a large positive feedback within the climate system. When a change in global temperature causes an event to occur which itself changes global temperature, this is referred to as a feedback effect. If this effect acts in the same direction as the original temperature change, it is a destabilising positive feedback (e.g. warming causing more warming); and if in the opposite direction, it is a stabilising negative feedback (e.g. warming causing a cooling effect). If a sufficiently strong net positive feedback occurs, it is said that a climate tipping point has been passed and the temperature will continue to change until the changed conditions result in negative feedbacks that restabilise the climate.
An example of a negative feedback is that radiation leaving the Earth increases in proportion to the fourth power of temperature, in accordance with the Stefan-Boltzmann law. An example of a positive feedback is the ice-albedo feedback, in which increasing temperature causes ice to melt, which increases the amount of heat that Earth absorbs.
Climate feedback effects can be from:
* The same cause as the forcing (e.g rising methane levels causing more methane to be released)
* Another greenhouse gas (e.g. CO2 causing methane release)
* On other variables (e.g ice-albedo feedback)
Without climate feedbacks, a doubling in atmospheric carbon dioxide concentration would result in a global average temperature increase of around 1.2°C. Water vapor amount and clouds are probably the most important global climate feedbacks. Historical information and global climate models indicate a climate sensitivity of 1.5 to 4.5°C, with a best estimate of 3°C. This is an amplification of the carbon dioxide forcing by a factor of 2.5. Some studies suggest a lower climate sensitivity, but other studies indicate a sensitivity above this range. Partly because of the difficulty in modeling the cloud feedback, the true climate sensitivity remains uncertain.
A 2006 book chapter by Cox et al. considers the possibility of a future runaway climate feedback due to changes in the land carbon cycle:
Here we use a simple land carbon balance model to analyse the conditions required for a land sink-to-source transition, and address the question; could the land carbon cycle lead to a runaway climate feedback? The simple land carbon balance model has effective parameters representing the sensitivities of climate and photosynthesis to CO2, and the sensitivities of soil respiration and photosynthesis to temperature. This model is used to show that (a) a carbon sink-to-source transition is inevitable beyond some finite critical CO2 concentration provided a few simple conditions are satisfied, (b) the value of the critical CO2 concentration is poorly known due to uncertainties in land carbon cycle parameters and especially in the climate sensitivity to CO2, and (c) that a true runaway land carbon-climate feedback (or linear instability) in the future is unlikely given that the land masses are currently acting as a carbon sink.
Examples
There are known examples of the earth's climate producing a large response to small forcings; most obviously CO2 feedback effect is believed to be part of the transition between glacial and interglacial periods, with the Milankovitch cycle providing the initial trigger.. This is not generally considered to be a runaway climate change. Another example is Dansgaard-Oeschger events.
Potentially unstable methane deposits exists in permafrost regions, which are expected to retreat as a result of global warming, and also clathrates, with the clathrate effect probably taking millennia to fully act The potential role of methane from clathrates in near-future runaway scenarios is not certain, as studies show a slow release of methane, which may not be regarded as 'runaway' by all commentators. The clathrate gun runaway effect may be used to describe more rapid methane releases. Methane in the atmosphere has a high global warming potential, but breaks down relatively quickly to form CO2, which is also a greenhouse gas. Therefore, slow methane release will have the long-term effect of adding CO2 to the atmosphere.
In order to model clathrates and other reservoirs of greenhouse gases and their precursors, global climate models would have to be 'coupled' to a carbon cycle model. Some current global climate models do not include such modelling of methane deposits.
Current risk
The scientific consensus in the IPCC Fourth Assessment Report is that "Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change."
Estimates of the size of the total carbon reservoir in Arctic permafrost and clathrates vary widely. It is suggested that at least 900 gigatonnes of carbon in permafrost exists worldwide. Further, there are believed to be around and another 400 gigatonnes of carbon in methane clathrates in permafrost regions alone, and 10,000 to 11,000 gigatonnes worldwide. This is large enough that if 10% of the stored methane were released, it would have an effect equivalent to a factor of 10 increase in atmospheric CO2 concentrations. Methane is a potent greenhouse gas with a higher global warming potential than CO2.
Worries about the release of this methane and carbon dioxide is linked to arctic shrinkage. 2007 had the lowest recorded sea ice area and 2008 had possibly the lowest recorded volume. It has been suggested that rapid melting of the sea ice may initiate a feedback loop that rapidly melts arctic permafrost. Methane clathrates on the sea-floor have also been predicted to destabilise, but much more slowly.
A release of methane from clathrates, however, is believed to be slow and chronic rather than catastrophic. and that 21st-century effects are therefore likely to be 'significant but not catastrophic'. It is further noted that 'much methane from dissociated gas hydrate may never reach the atmosphere', as it can be dissolved into the ocean and be broken down biologically. Other research demonstrates that a release to the atmosphere can occur during large releases. These sources suggest that the clathrate gun effect alone will not be sufficient to cause 'catastrophic' climate change within a human lifetime.
Paleoclimatology
Events that could be described as runaway climate change may have occurred in the past.
Clathrate gun
The clathrate gun hypothesis suggests runaway warming due to a massive release of methane gas from methane clathrates on the seafloor. It has been speculated that the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum were caused by massive clathrate release.
Snowball Earth
Geological evidence shows that ice-albedo feedback caused sea ice advance to near the equator at several points in Earth history. Modeling work shows that such an event would indeed be a result of a runaway ice-albedo effect, and that such a condition could be escaped via the accumulation of CO2 from volcanic outgassing.
From http://en.wikipedia.org/
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