Abrupt climate change encompasses two extreme results of Arctic warming: abrupt cooling (thermohaline circulation change) and abrupt warming (positive Arctic feedbacks).
The major risks to society and environment from climate change are posed primarily by abrupt and extreme climate phenomena. Potential forms of abrupt change include sudden deglaciation, reorganization of the thermohaline circulation system, and widespread melting of permafrost leading to large-scale shifts in the carbon cycle. Abrupt and extreme phenomena can exceed the thresholds for ecological and societal adaptation through either the rapid rate or magnitude of the associated climate change [IPCC, 2007].
A 2002 comprehensive report, Abrupt Climate Change: Inevitable Surprises by the US National Academy of Sciences (NAS), said, “Available evidence suggests that abrupt climate changes are not only possible but likely in the future, potentially with large impacts on ecosystems and societies.” Nothing learned since has changed this statement.
Recent scientific evidence shows that major and widespread climate changes have occurred with startling speed. For example, roughly half the north Atlantic warming since the last ice age was achieved in only a decade, and it was accompanied by significant climatic changes across most of the globe. Similar events, including local warmings as large as 16°C, occurred repeatedly during the slide into and climb out of the last ice age. Human civilizations arose after those extreme, global ice-age climate jumps. Severe droughts and other regional climate events during the current warm period have shown similar tendencies of abrupt onset and great persistence, often with adverse effects on societies.
Abrupt climate changes were especially common when the climate system was being forced to change most rapidly. Thus, greenhouse warming and other human alterations of the earth system may increase the possibility of large, abrupt, and unwelcome regional or global climatic events. (NAS 2002)
End Younger Dryas Abrupt Warming.
We know abrupt warming and cooling can happen because they show up in the ice cores. A particularly recent (in climate history terms 11,500 years ago) and extreme example is the Younger Dryas abrupt cooling and abrupt warming event. Research finds the warming was 10C over just a few decades and for a period the warming may have been as fast 1C per year.
The cause was methane emissions. Research by Prof Nisbet has shown that NH wetland methane was involved and also methane hydrate may have played a role.
Abrupt warming is covered in the Emergency page. It will happen as a result of the multiple positive Arctic feedbacks that amplify the warming of the Arctic as well as global warming. The imminent loss of the Arctic summer sea ice means the loss of the albedo effect's summer cooling, which will hasten catastrophic emissions of methane and nitrous oxide.
Abrupt climate change of the cooling kind made headlines in 2003 when the Pentagon looked at the risk and published a report, An Abrupt Climate Change Scenario and Its Implications for United States National Security: Imagining the Unthinkable. The following year, the scenario was made into the disaster movie, The Day After Tomorrow.
In scientific references to abrupt climate change, abrupt loss of Arctic sea ice and abrupt global warming are seldom mentioned, though the published science says they can happen.
IPCC 2007 Synthesis Report
3.4 Risk of abrupt or irreversible changes
Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change. (WGII 12.6, 19.3, 19.4, SPM)
Abrupt climate change on decadal time scales is normally thought of as involving ocean circulation changes. Based on current model simulations, it is very likely that the meridional overturning circulation (MOC) of the Atlantic Ocean will slow down during the 21st century; nevertheless temperatures in the region are projected to increase. It is very unlikely that the MOC will undergo a large abrupt transition during the 21st century. Longer-term changes in the MOC cannot be assessed with confidence. (WGI 10.3, 10.7; WGII Figure, Table TS.5, SPM.2)
Impacts of large-scale and persistent changes in the MOC are likely to include changes in marine ecosystem productivity, fisheries, ocean CO2 uptake, oceanic oxygen concentrations and terrestrial vegetation. Changes in terrestrial and ocean CO2 uptake may feed back on the climate system. (WGII 12.6, 19.3, Figure SPM.2)
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.
The rate that industrial GHG emissions are increasing radiative forcing (heating the planet)is unprecedented (IPCC 2007) - never happened before.
The U.S. Climate Change Science Program defines it as: "a change in the climate (for example, in temperature or precipitation) that takes place over a few decades or less, persists for at least a few decades, and causes substantial disruptions in human and natural systems." Abrupt climate changes may occur over a region, a hemisphere, or the entire globe.
There is at least a strong case that the decadal temperature increase from 1850 and the sudden acceleration from 1950 are abrupt by the above decadal definition. From the radiative forcing it looks even clearer.
For avoiding global climate catastrophe it makes more sense to use heat forcing as the abrupt metric or indicator. Radiative forcing simply the rate and amount of heat added to the lower atmosphere from all GHG emissions. Over 10,000 and over 2000 years it looks relatively abrupt.
The IPCC states that global warming "could lead to some effects that are abrupt or irreversible". In an article in Science, Alley et al. said "it is conceivable that human forcing of climate change is increasing the probability of large, abrupt events.
The US is conducting research abrupt warming situations under the Investigation of the Magnitudes and Probabilities of Abrupt Climate Transitions (IMPACTS) Project out of the Lawrence Berkeley National Laboratory. This includes the following topics.
Boreal/Arctic-climate positive feedbacks,
Rapid destabilization of methane hydrates in Arctic Ocean sediments; and
Mega droughts in North America, including the role of biosphere-atmosphere feedbacks.
The background information confirms these are all real risks with continued warming,
What is not addressed is that these would be mutually reinforcing.