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Extreme Events
Related Links
EPA
CCSP
- Synthesis and Assessment Product 3.3: Climate Extremes
- Synthesis and Assessment Product 3.4: Abrupt Climate Change
NOAA
Climate is defined not simply as average temperature and precipitation but also by the type, frequency and intensity of weather events. Human-induced climate change has the potential to alter the prevalence and severity of extremes such as heat waves, cold waves, storms, floods and droughts. Though predicting changes in these types of events under a changing climate is difficult, understanding vulnerabilities to such changes is a critical part of estimating vulnerabilities and future climate change impacts on human health, society and the environment.
Our current level of understanding, as summarized in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC, 2007), is as follows:
Since 1950, the number of heat waves has increased and widespread increases have occurred in the numbers of warm nights. The extent of regions affected by droughts has also increased as precipitation over land has marginally decreased while evaporation has increased due to warmer conditions. Generally, numbers of heavy daily precipitation events that lead to flooding have increased, but not everywhere. Tropical storm and hurricane frequencies vary considerably from year to year, but evidence suggests substantial increases in intensity and duration since the 1970s.In the extratropics, variations in tracks and intensity of storms reflect variations in major features of the atmospheric circulation, such as the North Atlantic Oscillation.
The IPCC projects the following likely, very likely, or virtually certain changes in extreme events and associated effects between now and 2100 (IPCC, 2007):
| Projected Change | Projected Impacts by Sector | |||
|---|---|---|---|---|
| Agriculture, forestry | Water resources | Human health/ mortality | Industry/settlement/society | |
| Warmer/fewer cold days/nights; warmer/more hot days/nights over most land areas. | Increased yields in colder environments; decreased yields in warmer environments; | Effects on water resources relying on snow melt | Reduced human mortality from decreased cold exposure | Reduced energy demand for heating; increased demand for cooling; declining air quality in cities; reduced effects of snow, ice etc. |
| Warm spells/heat waves: frequency increases over most land areas | Reduced yields in warmer regions due to heat stress at key devel. stages; fire danger increase | Increased water demand; water quality problems, e.g., algal blooms | Increased risk of heat-related mortality | Reduction in quality of life for people in warm areas without air conditioning; impacts on elderly and very young; reduced thermoelectric power production efficiency |
| Heavy precipitation events: frequency increases over most areas | Damage to crops; soil erosion, inability to cultivate land, water logging of soils | Adverse effects on quality of surface and groundwater; contamination of water supply | Deaths, injuries, infectious diseases, allergies and dermatitis from floods and landslides | Disruption of settlements, commerce, transport and societies due to flooding; pressures on urban and rural infrastructures |
| Area affected by drought: increases | Land degradation, lower yields/crop damage and failure; livestock deaths; land degradation | More widespread water stress | Increased risk of food and water shortage and wild fires; increased risk of water- and food-borne diseases | Water shortages for settlements, industry and societies; reduced hydropower generation potentials; potentials for population migration |
| Number of intense tropical cyclones: increases | Damage to crops; windthrow of trees | Power outages cause disruption of public water supply | Increased risk of deaths, injuries, water- and food-borne diseases | Disruption by flood and high winds; withdrawal of risk coverage in vulnerable areas by private insurers |
| Incidence of extreme high sea level: increases | Salinization of irrigation and well water | Decreased freshwater availability due to saltwater intrusion | Increase in deaths by drowning in floods; increase in stress-related disease | Costs of coastal protection versus costs of land-use relocation; also see tropical cyclones above |
It is important to understand that directly linking any one specific extreme event (e.g., a severe hurricane) to human-caused climate change is not possible. However, climate change may increase the probability of some ordinary weather events reaching extreme levels or of some extreme events becoming more extreme. For example, according to NOAA, it is probable that heat waves will become more likely and progressively more intense over the course of decades under current climate change scenarios. Changes in the frequency and intensity of heat waves and other extreme events across North America will be comprehensively assessed in the forthcoming Climate Change Science Program Synthesis and Assessment Product 3.3: Climate Extremes.
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 (NRC, 2002).
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 (Schneider et al., 2007).
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 (NRC, 2002). 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.
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 (Meehl et al., 2007).For more information on this issue,
visit NOAA's Abrupt
Climate Change Web site and see the National Resource Council
report "Abrupt
Climate Change: Inevitable Surprises" 
(a short
4 page summary is also available (PDF, 4 pp., 714 KB, About
PDF) )
In addition, visit the CCSP Web site for more information about ongoing multi-agency research on abrupt climate change. An analysis on this topic is underway and will be completed by 2008.
References
- IPCC, 2007: Climate Change 2007: Impacts, Adaptation, and Vulnerability . Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Parry, Martin L., Canziani, Osvaldo F., Palutikof, Jean P., van der Linden, Paul J., and Hanson, Clair E. (eds.)]. Cambridge University Press, Cambridge, United Kingdom, 1000 pp.
- Jansen, E., J. Overpeck, K.R. Briffa, J.-C. Duplessy, F. Joos, V. Masson-Delmotte, D. Olago, B. Otto-Bliesner, W.R. Peltier, S. Rahmstorf, R. Ramesh, D. Raynaud, D. Rind, O. Solomina, R. Villalba and D. Zhang (2007) Palaeoclimate. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- Meehl, G.A. et al. (2007) Global Climate Projections. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
- Schneider, S.H. et al. (2007) Assessing key vulnerabilities and the risk from climate change. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson, Eds., Cambridge University Press, Cambridge, UK, 779-810.
- NRC, 2002: National Research
Council, Abrupt Climate Change: Inevitable Surprises .
Committee on Abrupt Climate Change.