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Cause and Consequences of a Major Holocene Abrupt Climate Change

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Cause and Consequences of a Major Holocene Abrupt Climate Change ARTICLE IN PRESS Quaternary Science Reviews 24 (2005) 1123–1149 The 8k event: cause and consequences of a major Holocene abrupt climate change Richard B. Alleya,Ã, Anna Maria A´ gu´ stsdo´ ttira,b aDepartment of Geosciences and EMS Earth and Environmental Systems Institute, Pennsylvania State University, Deike Building, University Park, PA 16802, USA bSoil Conservation Service, Gunnarsholt, IS-851 Hella, Iceland Received 9 August 2004; accepted 23 December 2004 Abstract A prominent, abrupt climate event about 8200 years ago brought generally cold and dry conditions to broad northern-hemisphere regions especially in wintertime, in response to a very large outburst flood that freshened the North Atlantic. Changes were much larger than typical climate variability before and after the event, with anomalies up to many degrees contributing to major displacement of vegetative patterns. This ‘‘8k’’ event provides a clear case of cause and effect in the paleoclimatic realm, and so offers an excellent opportunity for model testing. The response to North Atlantic freshening has the same general anomaly pattern as observed for older events associated with abrupt climate changes following North Atlantic freshening, and so greatly strengthens the case that those older events also reflect North Atlantic changes. The North Atlantic involvement in the 8k event helps in estimating limits on climate anomalies that might result in the future if warming-caused ice-melt and hydrologic-cycle intensification at high latitudes lead to major changes in North Atlantic circulation. Few model experiments have directly addressed the 8k event, and most studies of proxy records across this event lack the time resolution to fully characterize the anomalies, so much work remains tobe done. r 2005 Elsevier Ltd. All rights reserved. 1. Introduction predominantly ice-age events characterized by a strong North Atlantic signature and hemispheric to global Abrupt climate change is of great societal interest, extent (e.g., Alley et al., 2003; Broecker, 2003). The with potentially large and far-reaching consequences remarkable ties between cold temperatures in the North (e.g., National Research Council, 2002; Alley et al., Atlantic and drought in Asian and African monsoonal 2003). Abrupt climate change typically occurs when the regions (e.g., Street-Perrott and Perrott, 1990; Overpeck climate system is forced across some threshold, causing et al., 1996; Wang et al., 2001) link concerns about evolution to a new, more-or-less persistent state at a rate drought and the North Atlantic in some cases. determined by the system and faster than the cause. The North Atlantic events, including the Younger Almost any part of the climate system might be Dryas cold interval and the Dansgaard-Oeschger and involved. However, consideration of paleoclimatic Heinrich-Bond events or oscillations (e.g., Broecker, records focuses attention first on regional land-surface 1997), have especially captured attention from the press moisture (e.g., Hodell et al., 1995; Laird et al., 1996; and the public. In the case of the termination of the 1 1 Gasse, 2000), and on those especially large and rapid, Younger Dryas cold event, for example, 3 to 2 of the entire 10,000-year deglacial warming in Greenland was ÃCorresponding author. Tel.: +1 814 863 1700; achieved in order of 10 years (Cuffey et al., 1995; fax: +1 814 863 7823. Severinghaus et al., 1998), with most of the accumula- E-mail address: [email protected] (R.B. Alley). tion-rate change apparently in a single year (Alley et al., 0277-3791/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2004.12.004 ARTICLE IN PRESS 1124 R.B. Alley, A.M. A´gu´stsdo´ttir / Quaternary Science Reviews 24 (2005) 1123–1149 1 1 1993). The scaling of 3 to 2 the deglacial change in about Dryas’’ of Mason et al. (2001), and other terms also may a decade probably applies in many places (Alley, 2000). apply.) Any event for which rate of change is anomalous by two This 8k event involved smaller, shorter-lived, and less to three orders of magnitude merits attention. Older areally extensive climatic anomalies than those asso- events were even larger, with a rapid shift of about 16 1C ciated with older events such as the Younger Dryas cold in Greenland in one case (Lang et al., 1999). interval. Causation, as discussed below, likely had much Debate surrounds the origin of the ice-age events in common with the older events, and involved processes associated with Heinrich-Bond and Dansgaard-Oesch- no longer operative. Nonetheless, this event punctuated ger cycling, as reviewed by Broecker (2003). Some conditions that were similar to or even warmer than workers argue for tropical causes (e.g., Clement et al., recently, it involved processes that might possibly be 2001). A pacing role for solar variability has been partially mimicked by human forcing in the future, and suggested (e.g., Denton and Karle´ n, 1973; van Geel it may provide useful estimates of limits on the et al., 1999; Alley et al., 2001; Bond et al., 2001). It magnitude of climate changes possible in the future. remains (e.g., Sarnthein et al., 1995; Alley and Clark, The literature on this event is increasing very rapidly. 1999; Stocker 2000) that the older events share For example, the ISI Web of Science listed 165 citations characteristics with each other and with younger events tothe paper by Alley et al. (1997) on the 8k climate including the 8k, including: (1) the climate anomalies anomalies as of August, 2002, and the Barber et al. seem to have been biggest around the north Atlantic; (1999) paper on causation of the event was cited by 61 (2) many of the cold anomalies have followed outburst indexed articles; by June of 2004, these had increased to flooding (Younger Dryas, Preboreal, 8k; Clark et al., 127 citations for Barber et al. and 272 for Alley et al. 2002; Teller et al., 2002; Teller and Leverington, 2004) With the literature large and growing, this review can be or ice-sheet surging (Broecker, 1994a; Alley and nobetter than representative; a comprehensive treat- MacAyeal, 1994; Broecker and Hemming, 2001; ment likely will require an international meeting Hemming, 2004) that freshened the North Atlantic; followed by a coordinated project to collect, collate, and (3) important similarities exist between recon- and evaluate the data. Such recommendations are structed anomaly patterns and patterns expected follow- discussed below. ing a North Atlantic freshening (e.g., Stocker and Broecker, 1994; Fawcett et al., 1997; Manabe and Stouffer, 2000; Rind et al., 2001; Rahmstorf, 2002). 2. Anomaly hunting A potentially reassuring aspect of these remarkable past events is that they were most prominent during All paleoclimatic records showing some climate- times colder and with more ice than recently. Owing to linked variable as a function of time exhibit ‘‘wiggles’’, the large effect of ice cover, and especially of topo- or anomalies, from a mean state or trend. When graphically high as well as reflective continental ice bioturbation or other diffusive processes are sufficiently sheets (e.g., Shinn and Barron, 1989), the ice-age world small and sampling is sufficiently intensive, an anomaly can be viewed as existing in a fundamentally different of any desired age can be found. Furthermore, because mode of operation than more recently. Colder tempera- zero-error ages are very scarce even a few millennia back tures allow more ice, hence a stronger ice-albedo in time, one can often have a choice in correlating feedback, hence greater temperature changes. (In the possible anomalies across many records. Paleoclimatol- warm limit of zero snow and ice, there can be no ice- ogists are of course well-aware of these difficulties, but albedofeedback.) Ice sheets displace jet streams, with they really do complicate determination of anomaly downwind consequences. maps for specific events (e.g., Fairbanks, 1990): are two The major problem with this ‘‘reassuring’’ view of the potentially correlated anomalies observed in records paleoclimatic record is provided by the abrupt event from widely separate places really representative of two about 8200 years ago (all ages are estimated in calendar different events, of one synchronous event, or of one years before 1950 unless otherwise specified). This event time-transgressive event? is often referred to by its age (the ‘‘8k’’ or ‘‘8.2k’’ event). Ice-core records are central in helping solve this It has been known for some time (e.g., Denton and difficulty. An event that is recorded near-simultaneously Karle´ n, 1973), and has been termed the ‘‘Finse event’’ by major changes in ice- and gas-isotopic ratios by Dahl and Nesje (1994) based on data from southern (recording temperature at the core site), annual-layer Norway. We will refer to it as the ‘‘8k’’ event here, thickness (accumulation rate at the core site), wind- pending any official action to formalize a name such as blown materials especially if fingerprinted by source ‘‘Finse’’. (As noted below, the 8k event is possibly (e.g., sea-salt, dust, forest-fire smoke and other materials correlative to the Miser cold oscillation in Alpine pollen from beyond the ice sheet; Mayewski et al., 1994; Taylor diagrams (Wick and Tinner, 1997), and perhaps tothe et al., 1996; Biscaye et al., 1997) and biochemically Mesoglacial of Beget (1983) and the ‘‘younger Younger important trace gases with widespread rather than ARTICLE IN PRESS R.B. Alley, A.M. A´gu´stsdo´ttir / Quaternary Science Reviews 24 (2005) 1123–1149 1125 localized sources, such as methane (Severinghaus et al., widespread, synchronous climate event did occur. The 1998) and nitrous oxide (Sowers et al., 2003), must have pattern of the changes (generally cold, dry and windy occurred near-synchronously across broad regions.
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