|
|
|
|
Runaway climate change
|
| |
|
| |
The phrase runaway climate change is used to describe a situation in which changes to global climate cause the climate system to pass a tipping point, after which internal positive feedback effects cause climate to rapidly change until it reaches a new, stable condition. The latter is more traditionally used to describe extreme climate change, such as on Venus. The term 'Runaway' is used in relation to climate change events in climatological literature.
Discussion
Ask a question about 'Runaway climate change' Start a new discussion about 'Runaway climate change' Answer questions from other users |
Encyclopedia
The phrase runaway climate change is used to describe a situation in which changes to global climate cause the climate system to pass a tipping point, after which internal positive feedback effects cause climate to rapidly change until it reaches a new, stable condition. The latter is more traditionally used to describe extreme climate change, such as on Venus. The term 'Runaway' is 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.
Definitions and usage
The expression runaway climate change is used in mass media and popular science literature. It is sometimes used in scientific papers and journals (such as social policy texts) The word 'runaway' is occasionally used in climatology in reference to feedback processes.
The UK newspaper 'The Guardian' defines runaway climate change in relation to current climatic conditions as follows
Runaway climate change is a theory of how things might go badly wrong for the planet if a relatively small warming of the earth upsets the normal checks and balances that keep the climate in equilibrium. As the atmosphere heats up, more greenhouse gases are released from the soil and seas. Plants and trees that take carbon dioxide out of the atmosphere die back, creating a vicious circle as the climate gets hotter and hotter.
Astronomers, including Kasting,, 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 (such an extreme incidence is not expected on Earth.). In popular media, the terms runaway climate change, runaway greenhouse effect and runaway global warming may be used interchangeably to describe periods of self-sustaining climate change. References to the term runaway climate change exist in industrial literature, mass media, and by environmentalists.
- 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
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.
Introduction
Once started, runaway climate change will continue until the feedback loop is interrupted. Once the runaway feedback process is underway, reversing the initial 'trigger' event will not undo the process, as it is already self-sustaining. Once a period of runaway climate change has started, the only way for human intervention to stop it is by geoengineering to artificially break the feedback loop.
The main runaway climate change scenarios expected as a result of global warming involve changes to the methane deposits in permafrost, and also clathrates, with the clathrate effect probably taking longer 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. 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.
Current global climate models do not include modelling of methane deposits.
The loss of Artic ice creates a positive ice-albedo feedback, but it does not normally start a 'runaway' change. However, if the ice melts more than normal, methane (another Greenhouse Gas) will be released from permafrost.. It is expected that methane clathrates will also decay, resulting in the release of methane. These positive feedbacks could create runaway climate change if the gain in the feedback is sufficiently high. Studies indicate that the potential effects of these is uncertain because it may take a long time for them to enter the atmosphere. Also, though methane has a high global warming potential, it breaks down relatively quickly to form CO2, which is a weaker greenhouse gas. Therefore, arctic methane release will have the long-term effect of adding CO2 to the atmosphere.
Feedback Mechanisms
When a change in global temperature causes an event to occur which itself changes global temperature, this is referred to as a feedback effect. For example, if a large forest dies due to warmer, drier conditions and then decays or burns, it will release CO2, which will exacerbate the original warming 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 positive feedback effect has sufficient gain then it is possible to induce runaway climate change. This results from the fact that a small input temperature change can cause a larger output temperature change. Each iteration of this 'loop' therefore produces an ever-larger change. If the effects from the second iteration of the loop of effects is larger than the effects of the first iteration of the loop this will lead to a self perpetuating effect. If this occurs and the feedback only ends after producing a major temperature increase, it is called a climate tipping point.
With radiation from the Earth increasing in proportion to the fourth power of temperature, in accordance with the Stefan-Boltzmann law, the feedback effect has to be very strong to cause a runaway effect. An example of a positive feedback mechanism which does not cause runaway climate change is the evaporation of water. An increase in temperature from greenhouse gases may lead to increased water vapour in the atmosphere. Water vapour is a greenhouse gas, which cause further warming is a positive feedback. However, this cannot be a runaway effect or the runaway effect would have occurred long ago. Positive feedback effects are common and can always exist while runaway effects are much rarer and cannot be operating at all times. Specifically, the gain in the system changes - most commonly because the climate is forced out of a stable state into an unstable state, such as by an initial addition of greenhouse gases to the atmosphere, or Milankovitch cycle affecting insolation.
Runaway feedbacks are bound to stop eventually, since infinite or zero temperatures are not observed. They are stopped by factors like a reducing supply of a greenhouse gas, or ice cover reducing to zero.
Climate feedback effects can be on
- The same cause as the forcing (e.g rising methane levels causing more methane to be released)
- Via another greenhouse gas (e.g. CO2 causing methane release)
- On other variables (e.g ice-albedo feedback)
Examples of positive feedback mechanisms for global warming include:
- Loss of sea ice, glaciers and ice caps, exposing darker ocean or rock beneath.
- The clathrate gun effect, which describes the release of methane from ocean stores of methane hydrate (AKA clathrate).
- Release of methane from permafrost due to anaerobic decomposition or clathrate breakdown.
The above positive feedbacks have always existed, yet climate over the last ten thousand years of the Holocene has been quite stable; there has been no runaway effect. There is no guarantee that this will continue to be the case, due to the effects of global warming.
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."
The phenomenon of Arctic shrinkage is leading some scientists to fear that a runaway climate change event may be imminent or may even have started, although other scientists have challenged this.
There is an albedo effect, as white ice is replaced by dark ocean. Rapid Arctic shrinkage is occurring, with 2007 being the lowest ever recorded area and 2008 being possibly the lowest ever recorded volume. This will induce or accelerate other positive feedback mechanisms, such as Arctic methane release from melting permafrost and clathrates. Lawrence et al(2008) suggests that a rapid melting of the sea ice may up a feedback loop that rapidly melts arctic permafrost. However, ocean clathrates are expected to destabilise much more slowly.
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.. Should this estimate of volume be correct or at least too low, and if clathrates are omitted from the analysis completely, then 900 gigatonnes of carbon may potentially be released as methane as a result of human activity. Methane is a potent greenhouse gas with a higher global warming potential than CO2.
The global carbon reservoir in ocean clathrates is estimated in the range 10,000-11,000 gigatonnes. Archer notes that according to the reservoir estimates used in his own paper that:
The hydrate reservoir is so large that if 10% of the methane were released to the atmosphere within a few years, it would have an impact on the Earth’s radiation budget equivalent to a factor of 10 increase
in atmospheric CO2.
However, his paper suggests that the great majority of such a release is likely to be chronic, rather than catastrophic, and that 21st-century effects are therefore likely to be 'significant but not catastrophic'. It is further noted by Kvenvolden that 'much methane from dissociated gas hydrate may never reach the atmosphere', as it can be dissolved into the ocean and broken down biologically. Other research clearly 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 the effects that Archer envisages as 'catastrophic' within a human lifetime. A slow release of methane chiefly affects global warming by increasing the CO2 levels in the atmosphere, rather than by direct action.
Precedents
Events that could be described as runaway climate change may have occurred in prehistoric times. According to the clathrate gun hypothesis a runaway effect could be caused by liberation of methane gas from hydrates by global warming if there are sufficient hydrates close to unstable conditions. It has been speculated that the Permian-Triassic extinction event and the Paleocene-Eocene Thermal Maximum were caused by such a runaway effect.
|
| |
|
|
