Abrupt climate change
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. One proposed reason for the observed abrupt climate change is that feedback loops within the climate system both enhance small perturbations and cause a variety of stable states.
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 within a timescale of a few years. Other abrupt changes are the +4 °C on Greenland 11,270 years ago or the abrupt +6 °C 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.
There are essentially two definitions of abrupt climate change:
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.
- 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".
In an article in Science, Richard Alley et al. said "it is conceivable that human forcing of climate change is increasing the probability of large, abrupt events. Were such an event to recur, the economic and ecological impacts could be large and potentially serious."
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—in human infrastructure and ecosystems for example—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.
Lenton et al. investigated tipping elements in the climate system. These were regional effects of global warming, some of which had abrupt onset and may therefore be regarded as abrupt climate change. They found that "Our synthesis of present knowledge suggests that a variety of tipping elements could reach their critical point within this century under anthropogenic climate change."
- 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
Climate feedback effects
One source of abrupt climate change effects is a feedback process, in which a warming event causes a change which leads to further warming. This can also apply to cooling. Example of such feedback processes are:
- Ice-albedo feedback, where the advance or retreat of ice cover alters the 'whiteness' of the earth, and its ability to absorb the sun's energy
- The dying and burning of forests, as a result of global warming
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 clathrate gun effect, although potential alternative mechanisms have been identified. This was associated with rapid ocean acidification
- The Permian-Triassic Extinction Event, also known as the great dying, 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 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 from the Antarctic ice sheet. These rapid meltwater release events have been hypothesized as a cause for Dansgaard-Oeschger cycles,
Abrupt climate shifts since 1976
Had the 1997 El Niño lasted twice as long, the rain forests of the Amazon basin and Southeast Asia could have quickly added much additional carbon dioxide to the air from burning and rotting, with heat waves and extreme weather quickly felt around the world (The "Burn Locally, Crash Globally" scenario.)
Most abrupt climate shifts, however, 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. But there have been a series of less dramatic abrupt climate shifts since 1976, along with some near misses.
- The circulation shift in the western Pacific in the winter of 1976-1977 proved to have much wider impacts.
- Since 1950, El Niños had been weak and short, but La Niñas were often big and long, This pattern reversed after 1977.
- Land temperatures had remained relatively trendless from 1950 to 1976, despite the CO2 rising from 310 to 332 ppm as fossil fuel emissions tripled. Then in 1977 there was a marked shift in observed global mean surface temperature to a rising fever of about 2°C/century.
- The expansion of the tropics from overheating is usually thought to be gradual, but the percentage of the land surface in the two most extreme classifications of drought suddenly doubled in 1982 and stayed there until 1997 when it jumped to triple (after six years, it stepped down to double). While their inceptions correlate with the particularly large El Niňos of 1982 and 1997, the global drought steps far outlast the 13-month durations of those El Niňos.
- There were near-misses for "Burn Locally, Crash Globally" in Amazonia in 1998, 2005, and 2007, each with higher flammability than its predecessor.
- 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 fifty years of observation, previous total shutdowns had severe worldwide climate consequences.
This makes abrupt climate shifts more like a heart attack than like a chronic disease whose course can be extrapolated. Like heart attacks, some abrupt climate shifts are minor, some are catastrophic—and one cannot predict which or when. The recent track record, however, is that there have been several sudden shifts and several near-misses in each decade since 1976.
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 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 this planet would continue to grow.
- Amundsen sea
- Arctic methane release
- Arctic shrinkage
- Clathrate gun hypothesis
- Climate change feedback
- Climate sensitivity
- Effects of global warming
- Global catastrophic risks
- Paleocene–Eocene Thermal Maximum
- Physical impacts of climate change
- Runaway climate change
- Shutdown of thermohaline circulation
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