Climate change feedback

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https://en.wikipedia.org/wiki/Climate_change_feedback

The primary causes and the wide-ranging effects of global warming and resulting climate change. Some effects constitute feedback mechanisms that intensify climate change and move it toward climate tipping points.

Climate change feedback is important in the understanding of global warming because feedback processes may amplify or diminish the effect of each climate forcing, and so play an important part in determining the climate sensitivity and future climate state. Feedback in general is the process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first. Positive (or reinforcing) feedback amplifies the change in the first quantity while negative (or balancing) feedback reduces it.

The term "forcing" means a change which may "push" the climate system in the direction of warming or cooling. An example of a climate forcing is increased atmospheric concentrations of greenhouse gases. By definition, forcings are external to the climate system while feedbacks are internal; in essence, feedbacks represent the internal processes of the system. Some feedbacks may act in relative isolation to the rest of the climate system; others may be tightly coupled; hence it may be difficult to tell just how much a particular process contributes. Forcing can also be driven by socioeconomic factors such as "demand for biofuels or demand for soy bean production." These drivers work as forcing mechanisms by the direct and indirect effects they cause from an individual to a global scale.

Forcings and feedbacks together determine how much and how fast the climate changes. The main positive feedback in global warming is the tendency of warming to increase the amount of water vapor in the atmosphere, which in turn leads to further warming. The main negative feedback comes from the Stefan–Boltzmann law, the amount of heat radiated from the Earth into space changes with the fourth power of the temperature of Earth's surface and atmosphere. Observations and modelling studies indicate that there is a net positive feedback to warming. Large positive feedbacks can lead to effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change.

Positive

Carbon cycle feedbacks

There have been predictions, and some evidence, that global warming might cause loss of carbon from terrestrial ecosystems, leading to an increase of atmospheric CO
2 levels. Several climate models indicate that global warming through the 21st century could be accelerated by the response of the terrestrial carbon cycle to such warming. All 11 models in the C4MIP study found that a larger fraction of anthropogenic CO₂ will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO₂ varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100 ppm. The higher CO₂ levels led to an additional climate warming ranging between 0.1° and 1.5 °C. However, there was still a large uncertainty on the magnitude of these sensitivities. Eight models attributed most of the changes to the land, while three attributed it to the ocean. The strongest feedbacks in these cases are due to increased respiration of carbon from soils throughout the high latitude boreal forests of the Northern Hemisphere. One model in particular (HadCM3) indicates a secondary carbon cycle feedback due to the loss of much of the Amazon Rainforest in response to significantly reduced precipitation over tropical South America. While models disagree on the strength of any terrestrial carbon cycle feedback, they each suggest any such feedback would accelerate global warming.

Observations show that soils in the U.K have been losing carbon at the rate of four million tonnes a year for the past 25 years according to a paper in Nature by Bellamy et al. in September 2005, who note that these results are unlikely to be explained by land use changes. Results such as this rely on a dense sampling network and thus are not available on a global scale. Extrapolating to all of the United Kingdom, they estimate annual losses of 13 million tons per year. This is as much as the annual reductions in carbon dioxide emissions achieved by the UK under the Kyoto Treaty (12.7 million tons of carbon per year).

It has also been suggested (by Chris Freeman) that the release of dissolved organic carbon (DOC) from peat bogs into water courses (from which it would in turn enter the atmosphere) constitutes a positive feedback for global warming. The carbon currently stored in peatlands (390–455 gigatonnes, one-third of the total land-based carbon store) is over half the amount of carbon already in the atmosphere. DOC levels in water courses are observably rising; Freeman's hypothesis is that, not elevated temperatures, but elevated levels of atmospheric CO₂ are responsible, through stimulation of primary productivity.

Tree deaths are believed to be increasing as a result of climate change, which is a positive feedback effect.

Methane climate feedbacks in natural ecosystems.

Wetlands and freshwater ecosystems are predicted to be the largest potential contributor to a global methane climate feedback. Long-term warming changes the balance in the methane-related microbial community within freshwater ecosystems so they produce more methane while proportionately less is oxidised to carbon dioxide.

Arctic methane release

Photo shows what appears to be permafrost thaw ponds in Hudson Bay, Canada, near Greenland. (2008) Global warming will increase permafrost and peatland thaw, which can result in collapse of plateau surfaces.

Warming is also the triggering variable for the release of carbon (potentially as methane) in the arctic. Methane released from thawing permafrost such as the frozen peat bogs in Siberia, and from methane clathrate on the sea floor, creates a positive feedback. In April 2019, Turetsky et al. reported permafrost was thawing quicker than predicted. Recently the understanding of the climate feedback from permafrost improved, but potential emissions from the subsea permafrost remain unknown and are - like many other soil carbon feedbacks - still absent from most climate models.

Thawing permafrost peat bogs

Western Siberia is the world's largest peat bog, a one million square kilometer region of permafrost peat bog that was formed 11,000 years ago at the end of the last ice age. The melting of its permafrost is likely to lead to the release, over decades, of large quantities of methane. As much as 70,000 million tonnes of methane, an extremely effective greenhouse gas, might be released over the next few decades, creating an additional source of greenhouse gas emissions. Similar melting has been observed in eastern Siberia. Lawrence et al. (2008) suggest that a rapid melting of Arctic sea ice may start a feedback loop that rapidly melts Arctic permafrost, triggering further warming. May 31, 2010. NASA published that globally "Greenhouse gases are escaping the permafrost and entering the atmosphere at an increasing rate - up to 50 billion tons each year of methane, for example - due to a global thawing trend. This is particularly troublesome because methane heats the atmosphere with 25 times the efficiency of carbon dioxide" (the equivalent of 1250 billion tons of CO2 per year).

In 2019, a report called " Arctic report card " estimated the current greenhouse gas emissions from Arctic permafrost as almost equal to the emissions of Russia or Japan or less than 10% of the global emissions from fossil fuels.

Hydrates

Methane clathrate, also called methane hydrate, is a form of water ice that contains a large amount of methane within its crystal structure. Extremely large deposits of methane clathrate have been found under sediments on the sea and ocean floors of Earth. The sudden release of large amounts of natural gas from methane clathrate deposits, in a runaway global warming event, has been hypothesized as a cause of past and possibly future climate changes. The release of this trapped methane is a potential major outcome of a rise in temperature; it is thought that this might increase the global temperature by an additional 5° in itself, as methane is much more powerful as a greenhouse gas than carbon dioxide. The theory also predicts this will greatly affect available oxygen content of the atmosphere. This theory has been proposed to explain the most severe mass extinction event on earth known as the Permian–Triassic extinction event, and also the Paleocene-Eocene Thermal Maximum climate change event. In 2008, a research expedition for the American Geophysical Union detected levels of methane up to 100 times above normal in the Siberian Arctic, likely being released by methane clathrates being released by holes in a frozen 'lid' of seabed permafrost, around the outfall of the Lena River and the area between the Laptev Sea and East Siberian Sea.

In 2020, the first leak of methane from the sea floor in Antarctica was discovered. The scientists are not sure what caused it. The area where it was found had not warmed yet significantly. It is on the side of a volcano, but it seems that it is not from there. The methane - eating microbes, eat the methane much fewer that was supposed, and the researchers think this should be included in climate models. They also claim that there is much more to discover about the issue in Antarctica Quart of the marine methane is found in the region of Antarctica

Abrupt increases in atmospheric methane

Literature assessments by the Intergovernmental Panel on Climate Change (IPCC) and the US Climate Change Science Program (CCSP) have considered the possibility of future projected climate change leading to a rapid increase in atmospheric methane. The IPCC Third Assessment Report, published in 2001, looked at possible rapid increases in methane due either to reductions in the atmospheric chemical sink or from the release of buried methane reservoirs. In both cases, it was judged that such a release would be "exceptionally unlikely" (less than a 1% chance, based on expert judgement). The CCSP assessment, published in 2008, concluded that an abrupt release of methane into the atmosphere appeared "very unlikely" (less than 10% probability, based on expert judgement). The CCSP assessment, however, noted that climate change would "very likely" (greater than 90% probability, based on expert judgement) accelerate the pace of persistent emissions from both hydrate sources and wetlands.

On 10 June 2019 Louise M. Farquharson and her team reported that their 12-year study into Canadian permafrost had "Observed maximum thaw depths at our sites are already exceeding those projected to occur by 2090. Between 1990 and 2016, an increase of up to 4 °C has been observed in terrestrial permafrost and this trend is expected to continue as Arctic mean annual air temperatures increase at a rate twice that of lower latitudes." Determining the extent of new thermokarst development is difficult, but there is little doubt the problem is widespread. Farquharson and her team guess that about 231,000 square miles (600,000 square kilometers) of permafrost, or about 5.5% of the zone that is permafrost year-round, is vulnerable to rapid surface thawing.

Decomposition

Organic matter stored in permafrost generates heat as it decomposes in response to the permafrost melting. As the tropics get wetter, as many climate models predict, soils are likely to experience greater rates of respiration and decomposition, limiting the carbon storage abilities of tropical soils.

Peat decomposition

Peat, occurring naturally in peat bogs, is a store of carbon significant on a global scale. When peat dries it decomposes, and may additionally burn. Water table adjustment due to global warming may cause significant excursions of carbon from peat bogs. This may be released as methane, which can exacerbate the feedback effect, due to its high global warming potential.

Rainforest drying

Rainforests, most notably tropical rainforests, are particularly vulnerable to global warming. There are a number of effects which may occur, but two are particularly concerning. Firstly, the drier vegetation may cause total collapse of the rainforest ecosystem. For example, the Amazon rainforest would tend to be replaced by caatinga ecosystems. Further, even tropical rainforests ecosystems which do not collapse entirely may lose significant proportions of their stored carbon as a result of drying, due to changes in vegetation.

Forest fires

The IPCC Fourth Assessment Report predicts that many mid-latitude regions, such as Mediterranean Europe, will experience decreased rainfall and an increased risk of drought, which in turn would allow forest fires to occur on larger scale, and more regularly. This releases more stored carbon into the atmosphere than the carbon cycle can naturally re-absorb, as well as reducing the overall forest area on the planet, creating a positive feedback loop. Part of that feedback loop is more rapid growth of replacement forests and a northward migration of forests as northern latitudes become more suitable climates for sustaining forests. There is a question of whether the burning of renewable fuels such as forests should be counted as contributing to global warming. Cook & Vizy also found that forest fires were likely in the Amazon Rainforest, eventually resulting in a transition to Caatinga vegetation in the Eastern Amazon region.

Desertification

Desertification is a consequence of global warming in some environments. Desert soils contain little humus, and support little vegetation. As a result, transition to desert ecosystems is typically associated with excursions of carbon.

Modelling results

The global warming projections contained in the IPCC's Fourth Assessment Report (AR4) include carbon cycle feedbacks. Authors of AR4, however, noted that scientific understanding of carbon cycle feedbacks was poor. Projections in AR4 were based on a range of greenhouse gas emissions scenarios, and suggested warming between the late 20th and late 21st century of 1.1 to 6.4 °C. This is the "likely" range (greater than 66% probability), based on the expert judgement of the IPCC's authors. Authors noted that the lower end of the "likely" range appeared to be better constrained than the upper end of the "likely" range, in part due to carbon cycle feedbacks. The American Meteorological Society has commented that more research is needed to model the effects of carbon cycle feedbacks in climate change projections.

Isaken et al. (2010) considered how future methane release from the Arctic might contribute to global warming. Their study suggested that if global methane emissions were to increase by a factor of 2.5 to 5.2 above (then) current emissions, the indirect contribution to radiative forcing would be about 250% and 400% respectively, of the forcing that can be directly attributed to methane. This amplification of methane warming is due to projected changes in atmospheric chemistry.

Schaefer et al. (2011) considered how carbon released from permafrost might contribute to global warming. Their study projected changes in permafrost based on a medium greenhouse gas emissions scenario (SRES A1B). According to the study, by 2200, the permafrost feedback might contribute 190 (+/- 64) gigatons of carbon cumulatively to the atmosphere. Schaefer et al. (2011) commented that this estimate may be low.

Implications for climate policy

Uncertainty over climate change feedbacks has implications for climate policy. For instance, uncertainty over carbon cycle feedbacks may affect targets for reducing greenhouse gas emissions. Emissions targets are often based on a target stabilization level of atmospheric greenhouse gas concentrations, or on a target for limiting global warming to a particular magnitude. Both of these targets (concentrations or temperatures) require an understanding of future changes in the carbon cycle. If models incorrectly project future changes in the carbon cycle, then concentration or temperature targets could be missed. For example, if models underestimate the amount of carbon released into the atmosphere due to positive feedbacks (e.g., due to melting permafrost), then they may also underestimate the extent of emissions reductions necessary to meet a concentration or temperature target.

Cloud feedback

Warming is expected to change the distribution and type of clouds. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. Low clouds tend to trap more heat at the surface and therefore have a positive feedback, while high clouds normally reflect more sunlight from the top so they have a negative feedback. These details were poorly observed before the advent of satellite data and are difficult to represent in climate models. Global climate models were showing a near-zero to moderately strong positive net cloud feedback, but the effective climate sensitivity has increased substantially in the latest generation of global climate models. Differences in the physical representation of clouds in models drive this enhanced climate sensitivity relative to the previous generation of models.

A 2019 simulation predicts that if greenhouse gases reach three times the current level of atmospheric carbon dioxide that stratocumulus clouds could abruptly disperse, contributing to additional global warming.

Gas release

Release of gases of biological origin may be affected by global warming, but research into such effects is at an early stage. Some of these gases, such as nitrous oxide released from peat or thawing permafrost, directly affect climate. Others, such as dimethyl sulfide released from oceans, have indirect effects.

Ice-albedo feedback

Aerial photograph showing a section of sea ice. The lighter blue areas are melt ponds and the darkest areas are open water; both have a lower albedo than the white sea ice. The melting ice contributes to ice-albedo feedback.

When ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues. During times of global cooling, additional ice increases the reflectivity which reduces the absorption of solar radiation which results in more cooling in a continuing cycle. Considered a faster feedback mechanism.

1870–2009 Northern hemisphere sea ice extent in million square kilometers. Blue shading indicates the pre-satellite era; data then is less reliable. In particular, the near-constant level extent in Autumn up to 1940 reflects lack of data rather than a real lack of variation.

Albedo change is also the main reason why IPCC predict polar temperatures in the northern hemisphere to rise up to twice as much as those of the rest of the world, in a process known as polar amplification. In September 2007, the Arctic sea ice area reached about half the size of the average summer minimum area between 1979 and 2000. Also in September 2007, Arctic sea ice retreated far enough for the Northwest Passage to become navigable to shipping for the first time in recorded history. The record losses of 2007 and 2008 may, however, be temporary. Mark Serreze of the US National Snow and Ice Data Center views 2030 as a "reasonable estimate" for when the summertime Arctic ice cap might be ice-free. The polar amplification of global warming is not predicted to occur in the southern hemisphere. The Antarctic sea ice reached its greatest extent on record since the beginning of observation in 1979, but the gain in ice in the south is exceeded by the loss in the north. The trend for global sea ice, northern hemisphere and southern hemisphere combined is clearly a decline.

Ice loss may have internal feedback processes, as melting of ice over land can cause eustatic sea level rise, potentially causing instability of ice shelves and inundating coastal ice masses, such as glacier tongues. Further, a potential feedback cycle exists due to earthquakes caused by isostatic rebound further destabilising ice shelves, glaciers and ice caps.

The ice-albedo in some sub-arctic forests is also changing, as stands of larch (which shed their needles in winter, allowing sunlight to reflect off the snow in spring and fall) are being replaced by spruce trees (which retain their dark needles all year).

Water vapor feedback

If the atmospheres are warmed, the saturation vapor pressure increases, and the amount of water vapor in the atmosphere will tend to increase. Since water vapor is a greenhouse gas, the increase in water vapor content makes the atmosphere warm further; this warming causes the atmosphere to hold still more water vapor (a positive feedback), and so on until other processes stop the feedback loop. The result is a much larger greenhouse effect than that due to CO₂ alone. Although this feedback process causes an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer. Climate models incorporate this feedback. Water vapor feedback is strongly positive, with most evidence supporting a magnitude of 1.5 to 2.0 W/m²/K, sufficient to roughly double the warming that would otherwise occur. Water vapor feedback is considered a faster feedback mechanism.

Negative

Blackbody radiation

As the temperature of a black body increases, the emission of infrared radiation back into space increases with the fourth power of its absolute temperature according to Stefan–Boltzmann law. This increases the amount of outgoing radiation as the Earth warms. The impact of this negative feedback effect is included in global climate models summarized by the IPCC. This is also called the Planck feedback.

Carbon cycle

Le Chatelier's principle

Following Le Chatelier's principle, the chemical equilibrium of the Earth's carbon cycle will shift in response to anthropogenic CO₂ emissions. The primary driver of this is the ocean, which absorbs anthropogenic CO₂ via the so-called solubility pump. At present this accounts for only about one third of the current emissions, but ultimately most (~75%) of the CO₂ emitted by human activities will dissolve in the ocean over a period of centuries: "A better approximation of the lifetime of fossil fuel CO₂ for public discussion might be 300 years, plus 25% that lasts forever". However, the rate at which the ocean will take it up in the future is less certain, and will be affected by stratification induced by warming and, potentially, changes in the ocean's thermohaline circulation.

Chemical weathering

Chemical weathering over the geological long term acts to remove CO₂ from the atmosphere. With current global warming, weathering is increasing, demonstrating significant feedbacks between climate and Earth surface. Biosequestration also captures and stores CO₂ by biological processes. The formation of shells by organisms in the ocean, over a very long time, removes CO₂ from the oceans. The complete conversion of CO₂ to limestone takes thousands to hundreds of thousands of years.

Net primary productivity

Net primary productivity changes in response to increased CO₂, as plants photosynthesis increased in response to increasing concentrations. However, this effect is swamped by other changes in the biosphere due to global warming.

Lapse rate

The atmosphere's temperature decreases with height in the troposphere. Since emission of infrared radiation varies with temperature, longwave radiation escaping to space from the relatively cold upper atmosphere is less than that emitted toward the ground from the lower atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height. Both theory and climate models indicate that global warming will reduce the rate of temperature decrease with height, producing a negative lapse rate feedback that weakens the greenhouse effect. Measurements of the rate of temperature change with height are very sensitive to small errors in observations, making it difficult to establish whether the models agree with observations.

Feedback loops from the book Al Gore (2006). An inconvenient truth.

Impacts on humans

The graphic at right suggests that the overall effect of climate change upon human numbers and development will be negative. If this is so, then the century-scale prospects for climate change is that Earth's biosphere may adjust to a new, but radically different, equilibrium if large numbers of humans cannot survive future conditions.

Greenhouse and icehouse Earth

From Wikipedia, the free encyclopedia

 

Timeline of the five known great glaciations, shown in blue. The periods in between depict greenhouse conditions.

Throughout the history of the Earth, the planet's climate has been fluctuating between two dominant climate states: the greenhouse Earth and the icehouse Earth. These two climate states last for millions of years and should not be confused with glacial and interglacial periods, which occur only during an icehouse period and tend to last less than 1 million years. There are five known great glaciations in Earth's climate history; the main factors involved in changes of the paleoclimate are believed to be the concentration of atmospheric carbon dioxide, changes in the Earth's orbit, long-term changes in the solar constant, and oceanic and orogenic changes due to tectonic plate dynamics. Greenhouse and icehouse periods have profoundly shaped the evolution of life on Earth.

Greenhouse Earth

Overview of greenhouse Earth

A "greenhouse Earth" is a period in which there are no continental glaciers whatsoever on the planet, the levels of carbon dioxide and other greenhouse gases (such as water vapor and methane) are high, and sea surface temperatures (SSTs) range from 28 °C (82.4 °F) in the tropics to 0 °C (32 °F) in the polar regions. The Earth has been in a greenhouse state for about 85% of its history.

This state should not be confused with a hypothetical hothouse earth, which is an irreversible tipping point corresponding to the ongoing runaway greenhouse effect on Venus. The IPCC states that "a 'runaway greenhouse effect'—analogous to [that of] Venus—appears to have virtually no chance of being induced by anthropogenic activities."

Causes of greenhouse Earth

There are several theories as to how a greenhouse Earth can come about. The geological record shows CO₂ and other greenhouse gases are abundant during this time. Tectonic movements were extremely active during the more well-known greenhouse ages (such as 368 million years ago in the Paleozoic Era). Because of continental rifting (continental plates moving away from each other) volcanic activity became more prominent, producing more CO₂ and heating up the Earth's atmosphere. Earth is more commonly placed in a greenhouse state throughout the epochs, and the Earth has been in this state for approximately 80% of the past 500 million years, which makes understanding the direct causes somewhat difficult.

Icehouse Earth

Overview of icehouse Earth

An "icehouse Earth" is a period in which the Earth has at least two ice sheets, Arctic and Antarctic (on both poles); these sheets wax and wane throughout shorter times known as glacial periods (with other ice sheets in addition to the 2 polar ones) and interglacial periods (without). During an icehouse Earth, greenhouse gases tend to be less abundant, and temperatures tend to be cooler globally. The Earth is currently in an icehouse stage, that started 34 Ma with the ongoing Late Cenozoic Ice Age. Inside it, the last glacial, Würm, recently ended (110 to 12 ka), still has remnants of non-polar ice sheets (Alps, Himalaya, Patagonia). It will likely be soon followed by another interglacial, similar to the last one, Eemian (130 to 115 ka), when there were forests in North Cape and hippopotamus in the rivers Rhine and Thames. Then glacials and interglacials, of similar lengths as the recent ones, will continue to alternate until the end of the 2 pole ice sheets, meaning the end of the current Icehouse and the start of the next Greenhouse.

Causes of icehouse Earth

The causes of an icehouse state are much debated, because not much is really known about the transitions between greenhouse and icehouse climates and what could make the climate change. One important aspect is clearly the decline of CO₂ in the atmosphere, possibly due to low volcanic activity.

Other important issues are the movement of the tectonic plates and the opening and closing of oceanic gateways. These seem to play a crucial part in icehouse Earths because they can bring cool waters from very deep water circulations that could assist in creating ice sheets or thermal isolation of areas. Examples of this occurring are the opening of the Tasmanian gateway 36.5 million years ago that separated Australia and Antarctica and which is believed to have set off the Cenozoic icehouse, and the creation of the Drake Passage 32.8 million years ago by the separation of South America and Antarctica, though it was believed by other scientists that this did not come into effect until around 23 million years ago. The closing of the Isthmus of Panama and the Indonesian seaway approximately 3 or 4 million years ago may have been a major cause for our current icehouse state. For the icehouse climate, tectonic activity also creates mountains, which are produced by one continental plate colliding with another one and continuing forward. The revealed fresh soils act as scrubbers of carbon dioxide, which can significantly affect the amount of this greenhouse gas in the atmosphere. An example of this is the collision between the Indian subcontinent and the Asian continent, which created the Himalayan Mountains about 50 million years ago.

Glacials and interglacials

Within icehouse states, there are "glacial" and "interglacial" periods that cause ice sheets to build up or retreat. The causes for these glacial and interglacial periods are mainly variations in the movement of the earth around the Sun. The astronomical components, discovered by the Serbian geophysicist Milutin Milanković and now known as Milankovitch cycles, include the axial tilt of the Earth, the orbital eccentricity (or shape of the orbit) and the precession (or wobble) of the Earth's rotation. The tilt of the axis tends to fluctuate between 21.5° to 24.5° and back every 41,000 years on the vertical axis. This change actually affects the seasonality upon the earth, since more or less solar radiation hits certain areas of the planet more often on a higher tilt, while less of a tilt would create a more even set of seasons worldwide. These changes can be seen in ice cores, which also contain information showing that during glacial times (at the maximum extension of the ice sheets), the atmosphere had lower levels of carbon dioxide. This may be caused by the increase or redistribution of the acid/base balance with bicarbonate and carbonate ions that deals with alkalinity. During an icehouse, only 20% of the time is spent in interglacial, or warmer times. Model simulations suggest that the current interglacial climate state will continue for at least another 100,000 years, due to CO
2 emissions - including complete deglaciation of the Northern Hemisphere.

Snowball earth

A "snowball earth" is the complete opposite of greenhouse Earth, in which the earth's surface is completely frozen over; however, a snowball earth technically does not have continental ice sheets like during the icehouse state. "The Great Infra-Cambrian Ice Age" has been claimed to be the host of such a world, and in 1964, the scientist W. Brian Harland brought forth his discovery of indications of glaciers in low latitudes (Harland and Rudwick). This became a problem for Harland because of the thought of the "Runaway Snowball Paradox" (a kind of Snowball effect) that, once the earth enters the route of becoming a snowball earth, it would never be able to leave that state. However, in 1992 Joseph Kirschvink [de] brought up a solution to the paradox. Since the continents at this time were huddled at the low and mid-latitudes, there was less ocean water available to absorb the higher amount solar energy hitting the tropics, and at the same time, increased rainfall due to more land mass exposed to higher solar energy might have caused chemical weathering (removing CO₂ from atmosphere). Both these conditions might have caused a substantial drop in CO₂ atmospheric levels resulting in cooling temperatures, increasing ice albedo (ice reflectivity of incoming solar radiation), further increasing global cooling (a positive feedback). This might have been the mechanism of entering Snowball Earth state. Kirschvink explained that the way to get out of Snowball Earth state could be connected again to carbon dioxide. A possible explanation is that during Snowball Earth, volcanic activity would not halt, accumulating atmospheric CO₂. At the same time, global ice cover would prevent chemical weathering (in particular hydrolysis), responsible for removal of CO₂ from the atmosphere. CO₂ was therefore accumulating in the atmosphere. Once the atmosphere accumulation of CO₂ would reach a threshold, temperature would rise enough for ice sheets to start melting. This would in turn reduce ice albedo effect which would in turn further reduce ice cover, exiting Snowball Earth state. At the end of Snowball Earth, before reinstating the equilibrium "thermostat" between volcanic activity and the by then slowly resuming chemical weathering, CO₂ in the atmosphere had accumulated enough to cause temperatures to peak to as much as 60° Celsius, before eventually settling down. Around the same geologic period of Snowball Earth (debated if caused by Snowball Earth or being the cause of Snowball Earth) the Great Oxygenation Event (GOE) was occurring. The event known as the Cambrian Explosion followed, which produced the beginnings of multi-cellular life. However some biologists claim that a complete snowball Earth could not have happened since photosynthetic life would not have survived underneath many meters of ice without sunlight. However, sunlight has been observed to penetrate meters of ice in Antarctica. Most scientists today believe that a "hard" Snowball Earth, one completely covered by ice, is probably impossible. However, a "slushball earth", with points of opening near the equator, is possible.

Recent studies may have again complicated the idea of a snowball earth. In October 2011, a team of French researchers announced that the carbon dioxide during the last speculated "snowball earth" may have been lower than originally stated, which provides a challenge in finding out how Earth was able to get out of its state and if it were a snowball or slushball.

Transitions

Causes

The Eocene, which occurred between 53 and 49 million years ago, was the Earth's warmest temperature period for 100 million years. However, this "super-greenhouse" eventually became an icehouse by the late Eocene. It is believed that the decline of CO₂ caused this change, though it is possible that positive feedbacks contributed to the cooling.

The best record we have for a transition from an icehouse to greenhouse period where that plant life existed during the Permian period that occurred around 300 million years ago. 40 million years ago, a major transition took place, causing the Earth to change from a moist, icy planet where rainforests covered the tropics, into a hot, dry and windy location where little could survive. Professor Isabel P. Montañez of University of California, Davis, who has researched this time period, found the climate to be "highly unstable" and "marked by dips and rises in carbon dioxide".

Impacts

The Eocene-Oligocene transition, the latest transition, occurred approximately 34 million years ago, resulting in a rapid global temperature decrease, the glaciation of Antarctica and a series of biotic extinction events. The most dramatic species turnover event associated with this time period is the Grande Coupure, a period which saw the replacement of European tree-dwelling and leaf-eating mammal species by migratory species from Asia.

Research

The science of paleoclimatology attempts to understand the history of greenhouse and icehouse conditions over geological time. Through the study of ice cores, dendrochronology, ocean and lake sediments (varve), palynology, (paleobotany), isotope analysis (such as Radiometric dating and stable isotope analysis), and other climate proxies, scientists can create models of Earth's past energy budgets and resulting climate. One study has shown that atmospheric carbon dioxide levels during the Permian age rocked back and forth between 250 parts per million (which is close to present-day levels) up to 2,000 parts per million. Studies on lake sediments suggest that the "Hothouse" or "super-Greenhouse" Eocene was in a "permanent El Nino state" after the 10 °C warming of the deep ocean and high latitude surface temperatures shut down the Pacific Ocean's El Nino-Southern Oscillation. A theory was suggested for the Paleocene–Eocene Thermal Maximum on the sudden decrease of the carbon isotopic composition of the global inorganic carbon pool by 2.5 parts per million. A hypothesis posed for this drop of isotopes was the increase of methane hydrates, the trigger for which remains a mystery. This increase of methane in the atmosphere, which happens to be a potent, but short-lived, greenhouse gas, increased the global temperatures by 6 °C with the assistance of the less potent carbon dioxide.

List of Icehouse and Greenhouse Periods

• A greenhouse period ran from 4.6 to 2.4 billion years ago.
• Huronian Glaciation – an icehouse period that ran from 2.4 billion years ago to 2.1 billion years ago
• A greenhouse period ran from 2.1 billion to 720 million years ago.
• Cryogenian – an icehouse period that ran from 720 to 635 million years ago, at times the entire Earth was frozen over
• A greenhouse period ran from 635 million years ago to 450 million years ago.
• Andean-Saharan glaciation – an icehouse period that ran from 450 to 420 million years ago
• A greenhouse period ran from 420 million years ago to 360 million years ago.
• Late Paleozoic Ice Age – an icehouse period that ran from 360 to 260 million years ago
• A greenhouse period ran from 260 million years ago to 33.9 million years ago
• Late Cenozoic Ice Age – the current icehouse period which began 33.9 million years ago

Modern conditions

Currently, the Earth is in an icehouse climate state. About 34 million years ago, ice sheets began to form in Antarctica; the ice sheets in the Arctic did not start forming until 2 million years ago.^[8] Some processes that may have led to our current icehouse may be connected to the development of the Himalayan Mountains and the opening of the Drake Passage between South America and Antarctica but climate model simulations suggest that the early opening of the Drake Passage played only a limited role, while the later constriction of the Tethys and Central American Seaways is more important in explaining the observed Cenozoic cooling. Scientists have been attempting to compare the past transitions between icehouse and greenhouse, and vice versa, to understand where our planet is now heading.

Without the human influence on the greenhouse gas concentration, the Earth would be heading toward a glacial period. Predicted changes in orbital forcing suggest that in absence of human-made global warming, the next glacial period would begin at least 50,000 years from now, but due to the ongoing anthropogenic greenhouse gas emissions, the Earth is heading towards a greenhouse Earth period. Permanent ice is actually a rare phenomenon in the history of the Earth, occurring only in coincidence with the icehouse effect, which has affected about 20% of Earth's history.

Tipping points in the climate system

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https://en.wikipedia.org/wiki/Tipping_points_in_the_climate_system 

Possible tipping elements in the climate system.

 

Interactions of climate tipping points (bottom) with associated tipping points in the socioeconomic system (top) on different time scales. 

A tipping point in the climate system is a threshold that, when exceeded, can lead to large changes in the state of the system. Potential tipping points have been identified in the physical climate system, in impacted ecosystems, and sometimes in both. For instance, feedback from the global carbon cycle is a driver for the transition between glacial and interglacial periods, with orbital forcing providing the initial trigger. Earth's geologic temperature record includes many more examples of geologically rapid transitions between different climate states.

Climate tipping points are of particular interest in reference to concerns about global warming in the modern era. Possible tipping point behaviour has been identified for the global mean surface temperature by studying self-reinforcing feedbacks and the past behavior of Earth's climate system. Self-reinforcing feedbacks in the carbon cycle and planetary reflectivity could trigger a cascading set of tipping points that lead the world into a hothouse climate state.

Large-scale components of the Earth system that may pass a tipping point have been referred to as tipping elements. Tipping elements are found in the Greenland and Antarctic ice sheets, possibly causing tens of meters of sea level rise. These tipping points are not always abrupt. For example, at some level of temperature rise the melt of a large part of the Greenland ice sheet and/or West Antarctic Ice Sheet will become inevitable; but the ice sheet itself may persist for many centuries. Some tipping elements, like the collapse of ecosystems, are irreversible.

Definition

The IPCC AR5 defines a tipping point as an irreversible change in the climate system. It states that the precise levels of climate change sufficient to trigger a tipping point remain uncertain, but that the risk associated with crossing multiple tipping points increases with rising temperature. A broader definition of tipping points is sometimes used as well, which includes abrupt but reversible tipping points.

In the context of climate change, an "adaptation tipping point" has been defined as "the threshold value or specific boundary condition where ecological, technical, economic, spatial or socially acceptable limits are exceeded."

Tipping point behaviour in the climate can also be described in mathematical terms. Tipping points are then seen as any type of bifurcation with hysteresis. Hysteresis is the dependence of the state of a system on its history. For instance, depending on how warm and cold it was in the past, there can be differing amounts of ice present on the poles at the same concentration of greenhouse gases or temperature.

In a study inspired by "mathematical and statistical approaches to climate modelling and prediction", the authors identify three types of tipping points in open systems such as the climate system—bifurcation, noise-induced and rate-dependent. The idea of tipping points in climate science, as indicated by palaeoclimate data and global climate models, suggest that the "climate system may abruptly 'tip' from one regime to another in a comparatively short time."

Bifurcation-induced tipping refers to changes in dynamical systems that occur when a small smooth change made to bifurcation parameters of the system causes an abrupt or sudden topological change in the behavior of the system. In the Atlantic Meridional Overturning Circulation (AMOC), slow passage to the bifurcation parameters—the salinity, temperature and density of water—could cause the AMOC to abruptly collapse when it reaches a critical tipping point. Currents of warm, seawater in the upper layers of the Atlantic flow north, while currents of colder, deep waters from the North Atlantic flow south, like a conveyor belt known as thermohaline circulation. Downwelling occurs when the warmer, higher-density seawater accumulates and sinks beneath the colder, lower density less saline water from glacier melt. An AMOC collapse would occur if downwelling was inhibited. [critical slowing down] (CSD) "occurs because a restoring feedback is weakening as a bifurcation-type tipping point is approached."

Noise-induced tipping refers to transitions due to random fluctuations or internal variability of the system, as in the Dansgaard-Oeschger events during the last glacial period, with 25 occurrences of rapid climate fluctuations.

Rate-induced tipping occurs in an "excitable system"—such as peatlands—when one of the systems parameters is "ramped" through a "steady, slow and monotonic change" eliciting a "large excitable response". In the case of peatlands, the rate-induced tipping point results in an "explosive release of soil carbon from peatlands into the atmosphere"—"compost bomb instability".

Tipping points for global temperature

There are many positive and negative feedbacks to global temperatures and the carbon cycle that have been identified. The IPCC reports that feedbacks to increased temperatures are net positive for the remainder of this century, with the impact of cloud cover the largest uncertainty. IPCC carbon cycle models show higher ocean uptake of carbon corresponding to higher concentration pathways, but land carbon uptake is uncertain due to the combined effect of climate change and land use changes.

The geologic record of temperature and greenhouse gas concentration allows climate scientists to gather information on climate feedbacks that lead to different climate states, such as the Late Quaternary (past 1.2 million years), the Pliocene period five million years ago and the Cretaceous period, 100 million years ago. Combining this information with the understanding of current climate change resulted in the finding that "A 2 °C warming could activate important tipping elements, raising the temperature further to activate other tipping elements in a domino-like cascade that could take the Earth System to even higher temperatures".

The speed of tipping point feedbacks is a critical concern and the geologic record often fails to provide clarity as to whether past temperature changes have taken only a few decades or many millennia of time. For instance, a tipping point that was once feared to be abrupt and overwhelming is the release of clathrate compounds buried in seabeds and seabed permafrost, but that feedback is now thought to be chronic and long term.

Some individual feedbacks may be strong enough to trigger tipping points on their own. A 2019 study predicts that if greenhouse gases reach three times the current level of atmospheric carbon dioxide that stratocumulus clouds could abruptly disperse, contributing an additional 8 degrees Celsius of warming.

Runaway greenhouse effect

The runaway greenhouse effect is used in astronomical circles to refer to a greenhouse effect that is so extreme that oceans boil away and render a planet uninhabitable, an irreversible climate state that happened on Venus. The IPCC Fifth Assessment Report states that "a 'runaway greenhouse effect' —analogous to Venus— appears to have virtually no chance of being induced by anthropogenic activities." Venus-like conditions on the Earth require a large long-term forcing that is unlikely to occur until the sun brightens by a few tens of percents, which will take a few billion years.

While a runaway greenhouse effect on Earth is virtually impossible, there are indications that Earth could enter a moist greenhouse state that renders large parts of Earth uninhabitable if the climate forcing is large enough to make water vapour (H₂O) a major atmospheric constituent. Conceivable levels of human-made climate forcing would increase water vapour 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 well before the ocean was significantly depleted.

Tipping elements

Large scale tipping elements

A smooth or abrupt change in temperature can trigger global-scale tipping points. In the cryosphere these include the irreversible melting of Greenland and Antarctic ice sheets. In Greenland, a positive feedback cycle exists between melting and surface elevation. At lower elevations, temperatures are higher, leading to additional melting. This feedback loop can become so strong that irreversible melting occurs. Marine ice sheet instability could trigger a tipping point in West Antarctica. Crossing either of these tipping points leads to accelerated global sea level rise.

When fresh water gets released as a consequence of Greenland melting, a threshold may be crossed which leads to disruption of the thermohaline circulation. The thermohaline circulation transports heat northward which is important for temperature regulation in the Atlantic region. Risks for a complete shutdown are low to moderate under the Paris agreement levels of warming.

Other examples of possible large scale tipping elements are a shift in El Niño–Southern Oscillation. After crossing a tipping point, the warm phase (El Niño) would start to occur more often. Lastly, the southern ocean, which now absorbs a lot of carbon, might switch to a state where it does not do this anymore.

Regional tipping elements

Climate change can trigger regional tipping points as well. Examples are the disappearance of Arctic sea ice, the establishment of woody species in tundra, permafrost loss, the collapse of the monsoon of South Asia and a strengthening of the West African monsoon which would lead to greening of the Sahara and Sahel. Deforestation may trigger a tipping point in rainforests (i.e. Savannization in the Amazon rainforest, ...). As rain forests recycle a large part of their rainfall, when a portion of the forest is destroyed local droughts may threaten the remainder. Finally, boreal forests are considered a tipping element as well. Local warming causes trees to die at a higher rate than before, in proportion to the rise in temperature. As more trees die, the woodland becomes more open, leading to further warming and making forests more susceptible to fire. The tipping point is difficult to predict, but is estimated to be between 3–4 °C of global temperature rise.

Cascading tipping points

Crossing a threshold in one part of the climate system may trigger another tipping element to tip into a new state. These are so-called cascading tipping points. Ice loss in West Antarctica and Greenland will significantly alter ocean circulation. Sustained warming of the northern high latitudes as a result of this process could activate tipping elements in that region, such as permafrost degradation, loss of Arctic sea ice, and Boreal forest dieback. This illustrates that even at relatively low levels of global warming, relatively stable tipping elements may be activated.

Timothy Lenton at Exeter University, England and his team of researchers, had first warned in their landmark 7 February 2008 PNAS paper, about the "risks of climate tipping points." In 2008, Lenton and his team "thought the dangers would only arise when global warming exceeded 5 degrees Celsius (9 degrees Fahrenheit) above pre-industrial levels." A new study published in Nature on 27 November 2019 by Lenton and 6 co-authors, warned in language that is "much starker" than the Intergovernmental Panel on Climate Change's forecasts, that risks are "much more likely and much more imminent" and that some "may already have been breached."

Early warning signals

For some of the tipping points described above, it may be possible to detect whether that part of the climate system is getting closer to a tipping point; however, detection can note only that abrupt changes are likely, while predicting when and where they will occur remains difficult. A premier mode of detection for these warning signals is through natural archives like sediments, ice caps, and tree rings, where past changes in climate can be observed. All parts of the climate system are sometimes disturbed by weather events. After the disruption, the system moves back to its equilibrium. A storm may damage sea ice, which grows back after the storm has passed. If a system is getting closer to tipping, this restoration to its normal state might take increasingly longer, which can be used as a warning sign of tipping.

Changes in the Arctic

A 2019 UNEP study indicates that now at least for the Arctic and the Greenland ice sheet a tipping point has already been reached. Because of dewing of permafrost soil, more methane (in addition to other short-lived climate pollutant) could enter the atmosphere earlier than previously predicted and the loss of reflecting ice shields has started a powerful positive feedback loop leading to ever higher temperatures. The resulting accelerating climate instability in the polar region has potential to affect the global climate, outdating previous predictions about the point in the future when global tipping will occur.

A more regional tipping point may have already been reached in the form of a mass recession of Arctic sea ice. According to scientist Ron Lindsay at the University of Washington Applied Physics Laboratory, a tipping point in the Arctic materializes as a positive feedback loop, where "increased summer melt means decreased winter growth and then even more melting the next summer, and so on." The loss of Arctic sea ice, while detrimental to the region, also holds severe consequences for the rest of the globe. Critically important is the role of sea ice in increasing the Earth's albedo, or reflectivity. Sea ice has an albedo level of 0.5 to 0.7, reflecting fifty to seventy percent of incoming energy, while the ocean beneath has an albedo of only .06, reflecting only six percent of incoming energy. As sea ice decreases and exposes the less reflective ocean, albedo decreases across the region. Summer sea ice is of particular importance, reflecting approximately fifty percent of incoming radiation back into space at a time when there is already an increase in daylight in the Arctic. NOAA (National Oceanic and Atmospheric Administration) notes that in September 2019, "the sea ice cover reached its annual summer minimum, tying with 2007 and 2016 for second-smallest on record."

In June 2019, satellite images from around the Arctic showed burning fires that are farther north and of greater magnitude than at any time in the 16-year satellite record, and some of the fires appear to have ignited peat soils. Peat is an accumulation of partially decayed vegetation and is an efficient carbon sink. Scientists are concerned because the long-lasting peat fires release their stored carbon back to the atmosphere, contributing to further warming. The fires in June 2019, for example, released as much carbon dioxide as Sweden's annual greenhouse gas emissions.

Tipping point effects

If the climate tips into a hothouse Earth scenario, some scientists warn of food and water shortages, hundreds of millions of people being displaced by rising sea levels, unhealthy and unlivable conditions, and coastal storms having larger impacts. Runaway climate change of 4–5 °C can make swathes of the planet around the equator uninhabitable, with sea levels up to 60 metres (197 ft) higher than they are today. Humans cannot survive if the air is too moist and hot, which would happen for the majority of human populations if global temperatures rise by 11–12 °C, as land masses warm faster than the global average. Effects like these have been popularized in books like The Uninhabitable Earth and The End of Nature.

Abrupt climate change

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Clathrate hydrates have been identified as a possible agent for abrupt changes.

An abrupt climate change occurs when the climate system is forced to transition to a new climate state at a rate that is determined by the climate system energy-balance, and which is more rapid than the rate of change of the external forcing. Past events include the end of the Carboniferous Rainforest Collapse, Younger Dryas, Dansgaard-Oeschger events, Heinrich events and possibly also the Paleocene–Eocene Thermal Maximum. The term is also used within the context of global warming to describe sudden climate change that is detectable over the time-scale of a human lifetime, possibly as the result of feedback loops within the climate system.

Timescales of events described as 'abrupt' may vary dramatically. Changes recorded in the climate of Greenland at the end of the Younger Dryas, as measured by ice-cores, imply a sudden warming of +10 °C (+18 °F) within a timescale of a few years. Other abrupt changes are the +4 °C (+7.2 °F) on Greenland 11,270 years ago or the abrupt +6 °C (11 °F) warming 22,000 years ago on Antarctica. By contrast, the Paleocene-Eocene thermal maximum may have initiated anywhere between a few decades and several thousand years. Finally, Earth Systems models project that under ongoing greenhouse gas emissions as early as 2047, the Earth's near surface temperature could depart from the range of variability in the last 150 years, affecting over 3 billion people and most places of great species diversity on Earth.

Definitions

According to the Committee on Abrupt Climate Change of the National Research Council:

There are essentially two definitions of abrupt climate change:

• In terms of physics, it is a transition of the climate system into a different mode on a time scale that is faster than the responsible forcing.
• In terms of impacts, "an abrupt change is one that takes place so rapidly and unexpectedly that human or natural systems have difficulty adapting to it".

These definitions are complementary: the former gives some insight into how abrupt climate change comes about; the latter explains why there is so much research devoted to it.

General

Possible tipping elements in the climate system include regional effects of global warming, some of which had abrupt onset and may therefore be regarded as abrupt climate change. Scientists have stated, "Our synthesis of present knowledge suggests that a variety of tipping elements could reach their critical point within this century under anthropogenic climate change".

It has been postulated that teleconnections, oceanic and atmospheric processes, on different timescales, connect both hemispheres during abrupt climate change.

The IPCC states that global warming "could lead to some effects that are abrupt or irreversible".

A 2013 report from the U.S. National Research Council called for attention to the abrupt impacts of climate change, stating that even steady, gradual change in the physical climate system can have abrupt impacts elsewhere, such as in human infrastructure and ecosystems if critical thresholds are crossed. The report emphasizes the need for an early warning system that could help society better anticipate sudden changes and emerging impacts.

Scientific understanding of abrupt climate change is generally poor. The probability of abrupt change for some climate related feedbacks may be low. Factors that may increase the probability of abrupt climate change include higher magnitudes of global warming, warming that occurs more rapidly and warming that is sustained over longer time periods.

Climate models

Climate models are currently unable to predict abrupt climate change events, or most of the past abrupt climate shifts. A potential abrupt feedback due to thermokarst lake formations in the Arctic, in response to thawing permafrost soils, releasing additional greenhouse gas methane, is currently not accounted for in climate models.

Possible precursor

Most abrupt climate shifts are likely due to sudden circulation shifts, analogous to a flood cutting a new river channel. The best-known examples are the several dozen shutdowns of the North Atlantic Ocean's Meridional Overturning Circulation during the last ice age, affecting climate worldwide.

• The current warming of the Arctic, the duration of the summer season, is considered abrupt and massive.
• Antarctic ozone depletion caused significant atmospheric circulation changes.
• There have also been two occasions when the Atlantic's Meridional Overturning Circulation lost a crucial safety factor. The Greenland Sea flushing at 75 °N shut down in 1978, recovering over the next decade. Then the second-largest flushing site, the Labrador Sea, shut down in 1997 for ten years. While shutdowns overlapping in time have not been seen during the 50 years of observation, previous total shutdowns had severe worldwide climate consequences.

Effects

A summary of the path of the thermohaline circulation. Blue paths represent deep-water currents, and red paths represent surface currents.

 

The Permian–Triassic extinction event, labelled "P-Tr" here, is the most significant extinction event in this plot for marine genera.

Abrupt climate change has likely been the cause of wide-ranging and severe effects:

• Mass extinctions in the past, most notably the Permian–Triassic extinction event (often referred colloquially to as the Great Dying) and the Carboniferous Rainforest Collapse, have been suggested as a consequence of abrupt climate change.
• Loss of biodiversity: without interference from abrupt climate change and other extinction events, the biodiversity of Earth would continue to grow.
• Changes in ocean circulation such as

• Increasing frequency of El Niño events
• Potential disruption to the thermohaline circulation, such as that which may have occurred during the Younger Dryas event.
• Changes to the North Atlantic oscillation
• Changes in Atlantic Meridional Overturning Circulation (AMOC) which could contribute to more severe weather events.

Climate feedback effects

The dark ocean surface reflects only 6 percent of incoming solar radiation; sea ice reflects 50 to 70 percent.

One source of abrupt climate change effects is a feedback process, in which a warming event causes a change that adds to further warming. The same can apply to cooling. Examples of such feedback processes are:

• Ice–albedo feedback in which the advance or retreat of ice cover alters the albedo ("whiteness") of the earth and its ability to absorb the sun's energy.
• Soil carbon feedback is the release of carbon from soils in response to global warming.
• The dying and the burning of forests by global warming.

Volcanism

Isostatic rebound in response to glacier retreat (unloading) and increased local salinity have been attributed to increased volcanic activity at the onset of the abrupt Bølling-Allerød warming. They are associated with the interval of intense volcanic activity, hinting at an interaction between climate and volcanism: enhanced short-term melting of glaciers, possibly via albedo changes from particle fallout on glacier surfaces.

Past events

The Younger Dryas period of abrupt climate change is named after the Alpine flower, Dryas.

Several periods of abrupt climate change have been identified in the paleoclimatic record. Notable examples include:

• About 25 climate shifts, called Dansgaard-Oeschger cycles, which have been identified in the ice core record during the glacial period over the past 100,000 years.
• The Younger Dryas event, notably its sudden end. It is the most recent of the Dansgaard-Oeschger cycles and began 12,900 years ago and moved back into a warm-and-wet climate regime about 11,600 years ago. It has been suggested that "the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the North Atlantic climate system." A model for this event based on disruption to the thermohaline circulation has been supported by other studies.
• The Paleocene-Eocene Thermal Maximum, timed at 55 million years ago, which may have been caused by the release of methane clathrates, although potential alternative mechanisms have been identified. This was associated with rapid ocean acidification
• The Permian–Triassic Extinction Event, in which up to 95% of all species became extinct, has been hypothesized to be related to a rapid change in global climate. Life on land took 30 million years to recover.
• The Carboniferous Rainforest Collapse occurred 300 million years ago, at which time tropical rainforests were devastated by climate change. The cooler, drier climate had a severe effect on the biodiversity of amphibians, the primary form of vertebrate life on land.

There are also abrupt climate changes associated with the catastrophic draining of glacial lakes. One example of this is the 8.2-kiloyear event, which is associated with the draining of Glacial Lake Agassiz. Another example is the Antarctic Cold Reversal, c. 14,500 years before present (BP), which is believed to have been caused by a meltwater pulse probably from either the Antarctic ice sheet or the Laurentide Ice Sheet. These rapid meltwater release events have been hypothesized as a cause for Dansgaard-Oeschger cycles.

A 2017 study concluded that similar conditions to today's Antarctic ozone hole (atmospheric circulation and hydroclimate changes), ∼17,700 years ago, when stratospheric ozone depletion contributed to abrupt accelerated Southern Hemisphere deglaciation. The event coincidentally happened with an estimated 192-year series of massive volcanic eruptions, attributed to Mount Takahe in West  Antarctica.