Abrupt Climate Change
Abrupt climate change has a specific definition and should not be confused with climate changes that occur slowly or individual extreme events that affect relatively small areas. Abrupt climate change refers to sudden (on the order of decades), large changes in some major component of the climate system, with rapid, widespread effects. The potential for abrupt climate changes cannot be predicted with confidence; however, abrupt climate changes are an important consideration because, if triggered, they could occur so quickly and unexpectedly that human or natural systems would have difficulty adapting to them.
Abrupt climate changes occur when a threshold in the climate system is crossed – a trigger that causes the climate to rapidly shift from one state to a new, different one. Crossing thresholds in the climate system may lead to large and widespread consequences.
These triggers can be forces that are “external” or “internal” to the climate system. Examples of these triggers include:
- changes in the Earth’s orbit
- a brightening or dimming of the sun
- melting or surging ice sheets
- strengthening or weakening of ocean currents
- emissions of climate-altering gases and particles into the atmosphere
- More than one of these triggers can operate simultaneously, since all components of the climate system are linked.
Scientific data show that abrupt changes in the climate at the regional scale have occurred throughout history and are characteristic of the Earth’s climate system. During the last glacial period, abrupt regional warmings (likely up to 16°C within decades over Greenland) and coolings occurred repeatedly over the North Atlantic region (Jansen et al., 2007). These warmings likely had some large-scale effects such as major shifts in tropical rainfall patterns and redistribution of heat within the climate system but it is unlikely that they were associated with large changes in global mean surface temperature.
Our state of knowledge is not yet sufficient to predict the timing of the future abrupt climate changes or pinpoint their effects. However, the National Academies of Sciences did conclude that anthropogenic forcing could increase the risk of abrupt climate change:
…greenhouse warming and other human alterations of the Earth system may increase the possibility of large, abrupt, and unwelcome regional or global climatic events. The abrupt changes of the past are not fully explained yet, and climate models typically underestimate the size, speed, and extent of those changes. Hence, future abrupt changes cannot be predicted with confidence, and climate surprises are to be expected.
- National Research Council, 2002
Changes in weather patterns can result from abrupt changes that might occur spontaneously due to interactions in the atmosphere-ice-ocean system, or from the crossing of a threshold from slow external forcing (as described above) (Meehl et al., 2007). In a warming climate, changes in the frequency and amplitudes of these patterns might not only evolve rapidly, but also trigger other processes that lead to abrupt climate change (NRC, 2002). Examples of these patterns include the El Niño Southern Oscillation (ENSO) and the North Atlantic Oscillation/Arctic Oscillation (NAO/OA).
Scientists have investigated the possibility of an abrupt slowdown or shutdown of the Atlantic meridional overturning circulation (MOC) triggered by greenhouse gas forcing. The MOC transfers large quantities of heat to the North Atlantic and Europe, so an abrupt change in the MOC could have important implications for the climate of this region (Meehl et al., 2007). However, according to Meehl et al. (2007), the probability of an abrupt change in (or shutdown of) the MOC is low: “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.” The slowdown in the MOC projected by most models is gradual, so the resulting decrease in heat transport to the North Atlantic and Europe would not be large enough to reverse the warming that results from the increase in greenhouse gases.
The rapid disintegration of the Greenland Ice Sheet (GIS), which would raise sea levels 7 meters, is another commonly discussed abrupt change. Although models suggest the complete melting of the GIS would only require sustained warming in the range 1.9°C to 4.6°C (relative to the pre-industrial temperatures), it is expected to be a slow process that would take many hundreds of years to complete (Meehl et al., 2007).
A collapse of the West Antarctic Ice Sheet (WAIS), which would raise seas 5-6 meters, has been discussed as a low-probability, high-impact response to global warming (NRC, 2002; Meehl et al., 2007). The weakening or collapse of ice shelves, caused by melting on the surface or by melting at the bottom by a warmer ocean, might contribute to a potential destabilization of the WAIS. If ice discharge accelerates, it is possible that sea level could rise faster than projected in the IPCC scenarios. However, there is presently no consensus on the long-term future of the WAIS or its contribution to sea level rise.
