ANRV309-EA35-09 ARI 19 March 2007 18:4 Wally Was Right: Predictive Ability of the North Atlantic “Conveyor Belt” Hypothesis for Abrupt Climate Change Richard B. Alley Department of Geosciences and PSICE, Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania 16802; email: [email protected] Annu. Rev. Earth Planet. Sci. 2007. 35:241–72 Key Words First published online as a Review in Advance on paleoclimatology, ocean circulation, sea ice, Younger Dryas, February 21, 2007 meridional overturning circulation The Annual Review of Earth and Planetary Sciences is Access provided by Old Dominion University on 04/23/18. For personal use only. online at earth.annualreviews.org Abstract Annu. Rev. Earth Planet. Sci. 2007.35:241-272. Downloaded from www.annualreviews.org This article’s doi: Linked, abrupt changes of North Atlantic deep water formation, 10.1146/annurev.earth.35.081006.131524 North Atlantic sea ice extent, and widespread climate occurred re- Copyright c 2007 by Annual Reviews. peatedly during the last ice age cycle and beyond in response to All rights reserved changing freshwater fluxes and perhaps other causes. This paradigm, 0084-6597/07/0530-0241$20.00 developed and championed especially by W.S. Broecker, has repeat- edly proven to be successfully predictive as well as explanatory with high confidence. Much work remains to fully understand what hap- pened and to assess possible implications for the future, but the foun- dations for this work are remarkably solid. 241 ANRV309-EA35-09 ARI 19 March 2007 18:4 INTRODUCTION The idea of North Atlantic abrupt climate change—freshening of the surface waters leading to a reorganization in oceanic circulation and coupled atmospheric changes with widespread consequences and often abrupt shifts—is now at least 25 years old (Rooth 1982). This rich field of study has especially been led, championed, publi- cized, and developed into a major paradigm of climate change by Prof. Wallace S. Broecker (e.g., Broecker et al. 1985, 1988, 1989, 1990; Broecker & Denton 1989; Broecker 1994, 1997, 1998). The remarkable success of this research program has opened new subdisciplines, including the nascent field of abrupt climate change, pro- vided important insights to climate processes, feedbacks, and sensitivity, and captured public as well as scientific attention. Scientific skeptics do still remain (most notably Wunsch 2003, 2005, 2006), providing important impetus for additional research, but Broecker’s North Atlantic/conveyor paradigm has gained widespread acceptance. For example, the Broecker papers listed above have been cited more than 2000 times as indexed by ISI, and a brief perusal indicates that at least most of those citations are in general agreement. Of particular note is the predictive power of Broecker’s paradigm. Far from being merely a consistent explanation of previously collected data sets, this hypothesis has spawned a range of tests that have been borne out, with high confidence. I summa- rize some of these successful predictions after a quick review of terminology. (The predictive power also shows the intellectual bankruptcy of those groups, such as the proponents of a 6000-year-old Earth, who deride “historical science” as an oxymoron.) BESTIARY The relevant terminology is arcane at best. The summary here may prove useful, but does not provide formal definitions. Greenland ice-core records are often used as a type section of millennial climate change over the last ice age cycle owing to their high time resolution, multiproxy na- ture, and confident paleoclimatic reconstruction. Figure 1 shows the GISP2, central Greenland record over the past 100,000 years (Grootes & Stuiver 1997). The broad sweep of the orbitally paced ice age cycle is evident. The warmth of the previous interglacial approximately 130,000 years ago is variously called marine Access provided by Old Dominion University on 04/23/18. For personal use only. isotope stage (MIS) 5e, the Eemian in Europe, or the Sangamonian in North America; Annu. Rev. Earth Planet. Sci. 2007.35:241-272. Downloaded from www.annualreviews.org Greenland ice of that age and older has been recovered (Suwa et al. 2006), but an intact, ordered record through the complete Eemian has not yet been found, although likely is available given the proper core. The cooling into the most recent ice age (the Weichselian, Wurm, Wisconsinan, Devensian, etc., depending on intellectual or regional heritage) then passed through the cold of MIS 4, the somewhat warmer conditions of stage 3, the last glacial maximum (LGM) in stage 2, and the glacial termination or deglaciation leading to the warmth of stage 1 or the Holocene. The abrupt climate changes were especially prominent during times when the ice age cycling reached intermediate temperatures. The youngest generally recognized abrupt event was the short-lived (roughly one century) cold interval approximately 242 Alley ANRV309-EA35-09 ARI 19 March 2007 18:4 −20 Holo- Wisconsinan-Weichselian-Wurm Eem cene MIS1 MIS2 MIS3 MIS4 MIS5 1 −30 21 B 19 23 12 17 L LGM 20 8 Y 8 11 14 16 GISP2 3 15 22 18 4 6 10 5 7 13 2 −40 9 −32 O (per mil) 18 δ −34 0 1 5 −50 −36 4 A7 Ice age terms 6 A6 Byrd 2 3 A2 A3 A4 Marine isotopic stages −38 A1 O (per mil) Warm D-O events 18 A5 δ Occasional terms −40 −60 Heinrich events Antarctic warm events −42 020406080100 Age (kabp) Figure 1 Ice-isotopic records (a proxy for temperature, with less-negative values indicating warmer conditions) from GISP2, Greenland (Grootes & Stuiver 1997) and Byrd Station, Antarctica, as synchronized by Blunier & Brook (2001), with various climate-event terminology indicated. Ice age terms are shown in blue (top); the classical Eemian/Sangamonian is slightly older than shown here, as is the peak of marine isotope stage (MIS, shown in purple) 5, known as 5e. Referring specifically to the GISP2 curve, the warm D-O events or stadial events, as numbered by Dansgaard et al. (1993), are indicated in red; D-O event 24 is older than shown here. Occasional terms (L = Little Ice Age, 8 = 8k event, Y = Younger Dryas, B = Bolling-Allerod, and LGM = Last Glacial Maximum) are shown in pink. Heinrich events are numbered in green just below the GISP2 isotopic curve, as placed by Bond et al. (1993). The Antarctic warm events A1–A7, as identified by Blunier & Brook (2001), are indicated for the Byrd record. Access provided by Old Dominion University on 04/23/18. For personal use only. Annu. Rev. Earth Planet. Sci. 2007.35:241-272. Downloaded from www.annualreviews.org 8200 years before present (8.2 kabp, where present is conventionally taken as the year 1950, and all ages are given in calendar years), often called the 8k event. The coldest time of the ice age was approximately 24 kabp (often called the LGM, although that term is also applied to the few millennia from 24 kabp to approximately 19 or 17 kabp, and often refers to the time of largest ice). The deglacial warming from the LGM to the Holocene was interrupted by the cooling into Heinrich event H1 (sometimes called the Oldest Dryas) approximately 16.8 kabp (Hemming 2004), and the abrupt jump to warmer conditions approximately 14.7 kabp at the start of the Bolling-Allerod warmth (which was interrupted by the Older Dryas and the Inter- Allerod Cold Periods). Cooling into the Younger Dryas (YD) followed, giving more www.annualreviews.org • North Atlantic “Conveyor Belt” Hypothesis 243 ANRV309-EA35-09 ARI 19 March 2007 18:4 than a millennium of cold, and then the abrupt warming at the end of the YD about 11.6 kabp, which may be taken as the start of the Holocene. Older than the LGM, numerous large, abrupt climate jumps are evident. This millennial variability reached a notable fraction of the full glacial-interglacial ampli- tude (often roughly one half). Typically, a rapid warming (often completed in only 10–100 years) was followed by slow cooling, rapid cooling, and then little change or slow warming. Spacing between successive warmings was variable, but a value near 1500 years is most common (Alley et al. 2001). Broecker dubbed the warm intervals Dansgaard-Oeschger (D-O) events for the pioneering work of these paleoclimatol- ogists (Dansgaard et al. 1984, Oeschger et al. 1984), with the implication that the events were anomalous warm times punctuating typical glacial conditions. Millen- nial warm intervals during the ice age are also referred to as interstadials, separating the glacial stadials. The warm interstadial/D-O events have been numbered from younger to older (Dansgaard et al. 1993), from 1 to 24 over the past 110,000 years, with the Bolling warmth given number 1; aficionados speak with fondness of “IS8” or “IS12” for interstadial or D-O warm interval number 8 or number 12. The idea that the millennial changes are not interruptions of glacial conditions, but instead that glacial-interglacial and stadial-interstadial changes represent separate, although somewhat related, oscillations, has led some workers to refer to D-O oscillations between warm and cold conditions in the Greenland records. Bond et al. (1993) noted that a few successive D-O oscillations trended toward progressively colder conditions, and that the coldest point of such a cycle (a Bond cycle) was marked by an especially cold, long-lasting stadial. Furthermore, these coldest intervals included the Heinrich events in the ocean, short intervals of climatic and oceanographic anomalies, especially of anomalously rapid deposition, in a broad belt across the North Atlantic, of ice-rafted debris sourced largely from Hudson Bay (Broecker 1994, Hemming 2004). [Note that the putative 1500-year cycles in the Holocene, seen especially in records of ice-rafted hematite-stained grains off Greenland (Bond et al.