REVIEW Abrupt Climate Change R. B. Alley,1 J. Marotzke,2 W. D. Nordhaus,3 J. T. Overpeck,4 D. M. Peteet,5 R. A. Pielke Jr.,6 R. T. Pierrehumbert,7 P. B. Rhines,8,9 T. F. Stocker,10 L. D. Talley,11 J. M. Wallace8 Instrumental records reveal detailed, glob- Large, abrupt, and widespread climate changes with major impacts have occurred al information on abrupt, often societally dis- repeatedly in the past, when the Earth system was forced across thresholds. Although ruptive, climate shifts. For example, the abrupt climate changes can occur for many reasons, it is conceivable that human forcing warming that occurred during the 20th cen- of climate change is increasing the probability of large, abrupt events. Were such an tury in many northern regions was concen- event to recur, the economic and ecological impacts could be large and potentially trated in two rapid steps, suggestive of a serious. Unpredictability exhibited near climate thresholds in simple models shows that juxtaposition of human-induced secular trend some uncertainty will always be associated with projections. In light of these uncer- and interdecadal variability due to natural tainties, policy-makers should consider expanding research into abrupt climate change, causes (7). The warming on the Atlantic side improving monitoring systems, and taking actions designed to enhance the adaptability of the Arctic during the 1920s was 4°C or and resilience of ecosystems and economies. more in places (8) (Fig. 1). During the fol- lowing decade, an extended drought often called the Dust Bowl had a lasting impact on limatic records show that large, wide- relevant to, but broader than, the FCCC and the United States (9, 10). Such abrupt-onset, spread, abrupt climate changes have consequently requires a broader scientific and severe regional drought regimes have been Coccurred repeatedly throughout the policy foundation. Here we describe the sci- infrequent in the United States during the geological record. Some mechanisms have entific foundation for a research agenda fo- instrumental period but more common else- been identified that could account for these cused on abrupt climate change, as developed where, including in the Sahel (11). The strong changes, and model simulations of them are in a recent study by an international panel of links in many regions between drought or improving, but the models that are currently the U.S. National Research Council (2), and flood and the El Nin˜o–Southern Oscillation being used to assess human impacts on cli- identify areas in which the possibility of (ENSO) system (12) focus attention on mate do not yet simulate the past changes abrupt climate change has a bearing on the ENSO regime shifts (13). with great accuracy. Although public debate current policy debate about human-induced An abrupt Pacific shift in 1976–1977, regarding climate change has focused on the climate change. perhaps related to ENSO, involved en- climatic consequences of greenhouse-gas hancement of the dominant pattern of at- emissions and their impacts on the planet and What Climate Has Done mospheric circulation (including a deepen- on human societies, scientists and policy- Long-term stabilizing feedbacks have main- ing of the Aleutian Low), an oceanwide makers have given less attention to the pos- tained Earth-surface conditions within the change of surface temperature (warmer in sibility that large climate changes could occur narrow liquid-water window conducive to the tropics and along the coast of the Amer- quickly. Such abrupt climate changes could life for about 4 billion years (3); however, icas, colder to the west at temperate lati- have natural causes, or could be triggered by data indicate that over times of 1 year to 1 tudes) (14), and warming-induced shifts in humans and be among the “dangerous anthro- million years, the dominant feedbacks in the ecosystems along the coast of the Americas pogenic interferences” referred to in the U.N. climate system have amplified climate pertur- (15). On the Atlantic side, the past 30 years Framework Convention on Climate Change bations. For example, global-mean tempera- have witnessed an invasion of low-salinity (FCCC) (1). Thus, abrupt climate change is ture changes of perhaps 5° to 6°C over ice- deep waters that spread over the entire age cycles (4) are generally believed to have subpolar North Atlantic Ocean and the seas resulted from small, globally averaged net between Greenland and Europe (16) in just 1Department of Geosciences and EMS Environment forcing (5). More surprisingly, regional the regions critical for abrupt shifts in the Institute, Pennsylvania State University, University changes over ϳ10 years without major exter- thermohaline circulation, which has been 2 Park, PA 16802, USA. Southampton Oceanography nal forcing were in many cases one-third to implicated in many abrupt climate-change Centre, University of Southampton, Southampton SO14 3ZH, UK. 3Department of Economics, Yale Uni- one-half as large as changes over the events of the past (see below). versity, New Haven, CT 06520, USA. 4Institute for the ϳ100,000-year ice-age cycles (4, 6). The instrumental record is becoming more Study of Planet Earth, University of Arizona, Tucson, “Technically, an abrupt climate change valuable as it is lengthened, but is insufficient to AZ 85721, USA. 5Lamont Doherty Earth Observatory occurs when the climate system is forced to have sampled the full range of climatic behav- of Columbia University, Palisades, NY 10964, USA, and NASA Goddard Institute for Space Studies, New cross some threshold, triggering a transition ior. Paleoclimatic records from the Holocene York, NY 10025, USA. 6Center for Science and Tech- to a new state at a rate determined by the (the current, 10,000-year interglacial warm pe- nology Policy Research, Cooperative Institute for Re- climate system itself and faster than the riod) show larger abrupt changes in regional search in Environmental Sciences (CIRES), University cause” (2, p. 14). Even a slow forcing can climate than recorded instrumentally. These in- 7 of Colorado, Boulder, CO 80309, USA. Department trigger an abrupt change, and the forcing may clude apparently abrupt shifts in past hurricane of the Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA. 8Department of Atmospheric be chaotic and thus undetectably small. For frequency (17), changes in flood regimes, and Sciences and 9Department of Oceanography, Univer- human concerns, attention is especially fo- especially prominent droughts (10) (Fig. 2). sity of Washington, Seattle, WA 98195, USA. 10Cli- cused on persistent changes that affect sub- Examples include episodic desiccation of lakes mate and Environmental Physics, Physics Institute, continental or larger regions, and for which in African (18) and Asian (19) monsoonal ar- University of Bern, 3012 Bern, Switzerland. 11The Scripps Institution of Oceanography, University of ecosystems and economies are unprepared or eas, remobilization of dunes on the U.S. high California–San Diego, La Jolla, CA 92093, USA. are incapable of adapting. plains, the multidecadal drought implicated in www.sciencemag.org SCIENCE VOL 299 28 MARCH 2003 2005 R EVIEW the collapse of classic ciently thick that its surface Mayan civilization (20), and becomes high enough and the multicentennial drought cold enough that melting is associated with the fall of unlikely. Persistence also the Akkadian empire (21). may arise from the wind- Shifts in drought regimes driven circulation of the appear to have often been oceans, stratospheric circu- abrupt (10). lation and related chemistry Many paleoclimatic (31), or other processes. records, and especially those For the DO oscillations, from high latitudes, show the thermohaline circulation that ice-age events were of the oceans is implicated even larger and more wide- in the persistence. In the spread than those of the Ho- presently most likely hy- locene or of previous inter- pothesis, warm, salty water glacials (6). Regional cli- flowing into the North At- mate changes of as much as lantic densifies as it cools 8° to 16°C(6, 22) occurred and then sinks. However, repeatedly in as little as a precipitation and runoff decade or less (Fig. 3). The from surrounding land data do not yet exist to draw masses supply more fresh quantitatively reliable, glob- water to the North Atlantic al anomaly maps of any ma- Fig. 1. Observed, zonally averaged, land-surface air-temperature anomalies (°C) as a than is removed by evapora- jor climate variables for function of latitude and time (7), together with the temperature record for the same tion. Failure of sinking interval from Upernavik, Greenland (72°47ЈN, 56°10ЈW, on the northwest coast of these changes, but effects Greenland) (8). Global instrumental coverage is just sufficient to capture the rapid, would allow freshening to were clearly hemispheric to concentrated warming at high northern latitudes in the 1920s, which is shown more decrease surface density, global (4) and included dramatically at sites such as Upernavik. preventing further sinking changes in tropical wetlands and the associated inflow of (23) and the Asian monsoon (24). Cold, dry, ing solar radiation (29). The DO oscillations warm waters [e.g. (4, 6)]. and windy conditions generally occurred to- were especially prominent during the orbitally Whereas triggers, amplifiers, and sources gether, although antiphase behavior occurred in mediated cooling into and warming out of the of persistence are easily identified, globaliz- parts of the Southern Ocean and Antarctica (6). ice age. Triggers may be fast (e.g., outburst ers that spread anomalies across large regions These jumps associated with the Dansgaard- floods from glacier-dammed lakes), slow (con- or even the whole Earth are less obvious. Oeschger (DO) oscillation (25) were especially tinental drift, orbital forcing), or somewhere General circulation models (GCMs) forced prominent during the cooling into and warming between (human-produced greenhouse gases), by hypothesized causes of abrupt climate out of ice ages, but persisted into the early part and may even be chaotic; multiple triggers also changes often simulate some regional chang- of the current Holocene warm period (Fig.