Runaway climate change
Encyclopedia
Runaway climate change describes a theoretical scenario in which the climate system passes a threshold or tipping point
, after which internal positive feedback
effects cause the climate to continue changing without further external forcings. The runaway climate change continues until it is overpowered by negative feedback
effects which cause the climate system to restabilise at a new state.
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
.
The phrase "runaway climate change" is used to describe a theory in which positive feedbacks result in rapid climate change. It is sometimes used in relation to climate change events in climatological literature. It is also 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
.
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. This feedback always operates and is proportional to the fourth power
of temperature. Therefore, while it may be overridden by positive feedbacks for comparatively small temperature changes it will dominate for larger temperature changes. 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. This feedback only operates in a restricted range of temperatures (those for which ice exists, and does not cover the whole surface; once all the ice has melted, the feedback ceases to operate).
Climate feedback effects can be from:
Without climate feedbacks, a doubling in atmospheric carbon dioxide
(CO2) concentration would result in a global average temperature increase of around 1.2°C. Water vapor amount and cloud
s are probably the most important global climate feedbacks. Historical information and global climate model
s 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.
periods, with the Milankovitch cycle providing the initial trigger. This is generally not considered to be a runaway climate change. Another example would be 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 model
s would have to be 'coupled' to a carbon cycle model. Some current global climate models do not include such modelling of methane deposits.
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
:
Soil carbon and climate change: from the Jenkinson effect to the compost-bomb instability
is that "Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change." Note however that this statement is about situations weaker than "runaway 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. Furthermore, there are believed to be another 400 gigatonnes of carbon in methane clathrates in permafrost regions with 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
. Recent years have seen record low Arctic sea ice. 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 of such a release 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.
James E. Hansen has suggested that the Earth could experience a runaway greenhouse effect
and adopt a climate like that of Venus
if fossil-fuel use continues until reserves are exhausted.
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.
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 self-sustaining ice-albedo effect, and that such a condition could be escaped via the accumulation of CO2 from volcanic outgassing.
Tipping point (climatology)
A climate tipping point is a point when global climate changes from one stable state to another stable state, in a similar manner to a wine glass tipping over. After the tipping point has been passed, a transition to a new state occurs...
, after which internal positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
effects cause the climate to continue changing without further external forcings. The runaway climate change continues until it is overpowered by negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
effects which cause the climate system to restabilise at a new state.
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
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
.
Runaway greenhouse effect
A runaway greenhouse effect is not a clearly defined term, but is understood to mean an event analogous to that which is believed to have happened in the early history of Venus, where positive feedback increased the strength of its greenhouse effect until its oceans boiled away...
to describe a situation where the climate deviates catastrophically and permanently from the original state - as happened on Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
.
Related terms
- Tipping Level or tipping pointTipping point (climatology)A climate tipping point is a point when global climate changes from one stable state to another stable state, in a similar manner to a wine glass tipping over. After the tipping point has been passed, a transition to a new state occurs...
- 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 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 feedbackFeedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
effect. If this effect acts in the same direction as the original temperature change, it is a destabilising positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
(e.g. warming causing more warming); and if in the opposite direction, it is a stabilising negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
(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. This feedback always operates and is proportional to the fourth power
Exponentiation
Exponentiation is a mathematical operation, written as an, involving two numbers, the base a and the exponent n...
of temperature. Therefore, while it may be overridden by positive feedbacks for comparatively small temperature changes it will dominate for larger temperature changes. An example of a positive feedback is the ice-albedo feedback
Ice-albedo feedback
Ice-albedo feedback is a positive feedback climate process where a change in the area of snow-covered land, ice caps, glaciers or sea ice alters the albedo. This change in albedo acts to reinforce the initial alteration in ice area...
, in which increasing temperature causes ice to melt, which increases the amount of heat that Earth absorbs. This feedback only operates in a restricted range of temperatures (those for which ice exists, and does not cover the whole surface; once all the ice has melted, the feedback ceases to operate).
Climate feedback effects can be from:
- The same cause as the forcing (e.g. rising methane levels causing more methaneMethaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
to be released) - Another greenhouse gas (e.g. CO2 causing methaneMethaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
release) - On other variables (e.g. ice-albedo feedbackIce-albedo feedbackIce-albedo feedback is a positive feedback climate process where a change in the area of snow-covered land, ice caps, glaciers or sea ice alters the albedo. This change in albedo acts to reinforce the initial alteration in ice area...
)
Without climate feedbacks, a doubling in atmospheric carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
(CO2) concentration would result in a global average temperature increase of around 1.2°C. Water vapor amount and cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
s are probably the most important global climate feedbacks. Historical information and global climate model
Global climate model
A General Circulation Model is a mathematical model of the general circulation of a planetary atmosphere or ocean and based on the Navier–Stokes equations on a rotating sphere with thermodynamic terms for various energy sources . These equations are the basis for complex computer programs commonly...
s indicate a climate sensitivity
Climate sensitivity
Climate sensitivity is a measure of how responsive the temperature of the climate system is to a change in the radiative forcing. It is usually expressed as the temperature change associated with a doubling of the concentration of carbon dioxide in Earth's atmosphere.The equilibrium climate...
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
Cloud feedback
Cloud feedback is the coupling between cloudiness and surface air temperature in which a change in radiative forcing perturbs the surface air temperature, leading to a change in clouds, which could then amplify or diminish the initial temperature perturbation....
, the true climate sensitivity remains uncertain.
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 interglacialInterglacial
An Interglacial period is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age...
periods, with the Milankovitch cycle providing the initial trigger. This is generally not considered to be a runaway climate change. Another example would be Dansgaard-Oeschger events.
Potentially unstable methane deposits exists in permafrost
Permafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...
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
Global warming potential
Global-warming potential is a relative measure of how much heat a greenhouse gas traps in the atmosphere. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A GWP is calculated over a specific time...
, but breaks down relatively quickly to form CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, 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 model
Global climate model
A General Circulation Model is a mathematical model of the general circulation of a planetary atmosphere or ocean and based on the Navier–Stokes equations on a rotating sphere with thermodynamic terms for various energy sources . These equations are the basis for complex computer programs commonly...
s would have to be 'coupled' to a carbon cycle model. Some current global climate models do not include such modelling of methane deposits.
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
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
:
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 photosynthesisPhotosynthesisPhotosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
to CO2, and the sensitivities of soil respirationSoil respirationSoil respiration refers to the production of carbon dioxide when soil organisms respire. This includes respiration of plant roots, the rhizosphere, microbes and fauna....
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.
Soil carbon and climate change: from the Jenkinson effect to the compost-bomb instability
More recently there has been a suggestion that the release of heat associated with soil decomposition, which is neglected in the vast majority of large-scale models, may be critically important under certain circumstances. Models with and without the extra self-heating from microbial respiration have been shown to yield significantly different results. The present paper presents a mathematical analysis of a tipping point or runaway feedback that can arise when the heat from microbial respiration is generated more rapidly than it can escape from the soil to the atmosphere. This ‘compost-bomb instability’ is most likely to occur in drying organic soils with high porosity covered by an insulating lichen or moss layer. However, the instability is also found to be strongly dependent on the rate of global warming. This paper derives the conditions required to trigger the compost-bomb instability, and discusses the relevance of these to the concept of dangerous rates of climate change. On the basis of simple numerical experiments, rates of long-term warming equivalent to 10°C per century could be sufficient to trigger compost-bomb instability in drying organic soils.
Current risk
The scientific consensus in the IPCC Fourth Assessment ReportIPCC Fourth Assessment Report
Climate Change 2007, the Fourth Assessment Report of the United Nations Intergovernmental Panel on Climate Change , is the fourth in a series of reports intended to assess scientific, technical and socio-economic information concerning climate change, its potential effects, and options for...
is that "Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change." Note however that this statement is about situations weaker than "runaway change".
Estimates of the size of the total carbon reservoir in Arctic permafrost
Permafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...
and clathrates vary widely. It is suggested that at least 900 gigatonnes of carbon in permafrost exists worldwide. Furthermore, there are believed to be another 400 gigatonnes of carbon in methane clathrates in permafrost regions with 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
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
is a potent greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
with a higher global warming potential
Global warming potential
Global-warming potential is a relative measure of how much heat a greenhouse gas traps in the atmosphere. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A GWP is calculated over a specific time...
than CO2.
Worries about the release of this methane and carbon dioxide is linked to arctic shrinkage
Arctic shrinkage
Ongoing changes in the climate of the Arctic include rising temperatures, loss of sea ice, and melting of the Greenland ice sheet. Projections of sea ice loss suggest that the Arctic ocean will likely be free of summer sea ice sometime between 2060 and 2080, while another estimate puts this date at...
. Recent years have seen record low Arctic sea ice. It has been suggested that rapid melting of the sea ice may initiate a feedback loop that rapidly melts arctic permafrost
Permafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...
. 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 of such a release 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.
James E. Hansen has suggested that the Earth could experience a runaway greenhouse effect
Runaway greenhouse effect
A runaway greenhouse effect is not a clearly defined term, but is understood to mean an event analogous to that which is believed to have happened in the early history of Venus, where positive feedback increased the strength of its greenhouse effect until its oceans boiled away...
and adopt a climate like that of Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
if fossil-fuel use continues until reserves are exhausted.
Paleoclimatology
Events that could be described as runaway climate change may have occurred in the past.Clathrate gun
The clathrate gun hypothesis suggests an abrupt climate changeAbrupt climate change
An abrupt climate change occurs when the climate system is forced to transition to a new state at a rate that is determined by the climate system itself, and which is more rapid than the rate of change of the external forcing...
due to a massive release of methane gas from methane clathrates on the seafloor. It has been speculated that the Permian-Triassic extinction event
Permian-Triassic extinction event
The Permian–Triassic extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 Ma ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras...
and the Paleocene-Eocene Thermal Maximum
Paleocene-Eocene Thermal Maximum
The most extreme change in Earth surface conditions during the Cenozoic Era began at the temporal boundary between the Paleocene and Eocene epochs . This event, the Paleocene–Eocene Thermal Maximum , was associated with rapid global...
were caused by massive clathrate release.
Snowball Earth
Geological evidence shows that ice-albedo feedbackIce-albedo feedback
Ice-albedo feedback is a positive feedback climate process where a change in the area of snow-covered land, ice caps, glaciers or sea ice alters the albedo. This change in albedo acts to reinforce the initial alteration in ice area...
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 self-sustaining ice-albedo effect, and that such a condition could be escaped via the accumulation of CO2 from volcanic outgassing.