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Vol. 9, No. 1 January 1999 INSIDE • GSA Strategic Plan, p. 9 • Cordilleran Section Meeting, GSA TODAY p. 18 • Executive Director Position, A Publication of the Geological Society of America p. 26 What If the Conveyor Were to Shut Down? Reflections on a Possible Outcome of the Great Global Experiment W. S. Broecker, Lamont-Doherty Earth Observatory, Palisades, NY 10964 ABSTRACT Suggestions that the ongoing greenhouse buildup might induce a shut- D.O. EVENTS down of the ocean’s thermohaline cir- ANTIPHASED D.O. culation raise the questions as to how Y.D. ONLY Earth’s climate would change if such an event were to occur. The answer pre- ferred by the popular press is that con- 18O/16O 0 ditions akin to those that characterized Y. D. the Younger Dryas—the last kiloyear cold snap—would return. But this 20 YEARS AGO D.O. extreme scenario is an unlikely one, for 3 40 10 models suggest that in order to force a COLDER WARMER conveyor shutdown, Earth would have to undergo a 4 to 5 °C greenhouse warming. Hence, the conditions at the onset of the shutdown would be very different from those that preceded the ANTARCTIC ICE CORES Younger Dryas. Thus, it is unlikely that new climate conditions would be nearly Figure 1. Map showing locations where abrupt climate changes (i.e., Dansgaard-Oeschger events) have been documented in records kept in marine sediments or polar ice (red and blue dots). Yellow dots so severe. Unfortunately, because no show those locations where the last of these events (i.e., Younger Dryas) is recorded by major advances atmospheric model to date has been of mountain glaciers. While for most of the globe, these events are in phase, in parts of the Southern able to create the observed large and Ocean and of the Antarctic ice cap, they are clearly antiphased. This switch in phasing at high southern latitudes appears to reflect a seesawing of deep-ocean ventilation between the northern Atlantic and the abrupt changes in climate state of perimeter of the Antarctic continent. Earth’s atmosphere, we lack even the crudest road map. However, as was the case for each of the abrupt changes ogy correct, then indeed a shutdown of that during the Younger Dryas, Canada recorded in Greenland’s ice, if the con- the conveyor would have awesome conse- and Scandinavia still had sizable ice caps. veyor were to shut down, climate would quences. Iceland would become one large The second is that the abrupt part of the likely flicker for several decades before ice cap. Ireland’s climate would be trans- warming at the close of the Younger Dryas locking into its new state. The conse- formed to that of Spitzbergen. Winters in brought climate only about halfway to its quences to agricultural production of Scandinavia would become so cold that interglacial state (Severinghaus et al., these flickers would likely be profound. tundra would replace its forests. The Baltic 1998). The other half of the transition was Sea would be permanently ice covered, as more gradual, reflecting perhaps the INTRODUCTION would much of the ocean between Green- post–Younger Dryas retreat of the residual Past shutdowns of the Atlantic land and Scandinavia. Further, the impacts ice caps in Canada and Scandinavia. Ocean’s conveyor circulation appear to of such a mode change would not be lim- Finally, modeling studies (Manabe and have played a key role in triggering the ited to the northern Atlantic basin; rather, Stouffer, 1993; Stocker and Schmittner, large and abrupt global climate changes they would extend to all parts of the globe 1997) that forecast a greenhouse-induced that punctuated the last period of glacia- (see Fig. 1). Rainfall patterns would dra- conveyor shutdown do so only after a sub- tion including the millennial duration matically shift. Temperatures would fall. stantial global warming (4 to 5 °C) has Younger Dryas (Broecker and Denton, The atmosphere would become dustier. occurred. Hence, the global climate condi- 1990). Modeling studies suggest that the Finally, the transition to this new state tions prevailing at the time of the shut- ongoing greenhouse warming and conse- would be completed in decades, and very down would be substantially warmer than quent strengthening of the hydrologic likely during this transition period, cli- those that existed just before the onset of cycle might trigger yet another such shut- mate would flicker. the Younger Dryas. For these reasons, the down (Manabe and Stouffer, 1993; Stocker But is it realistic to believe that a shut- analogy to the conditions that prevailed and Schmittner, 1997). To most science down of the conveyor a century or so during the Younger Dryas surely consti- writers, this result has been construed as from now would produce the conditions tutes a worst case scenario. implying that conditions similar to those that characterized the last glacial period? that prevailed during the Younger Dryas The answer is very likely “no,” for several cold event would return. Were this anal- reasons. The first has to do with the fact Conveyor continued on p. 2 IN THIS ISSUE GSA TODAY January Vol. 9, No. 1 1999 What If the Conveyor Were to Shut Down? Washington Report ....................... 21 Reflections on a Possible Outcome of the Congressional Science Fellowship ......... 22 GSA TODAY (ISSN 1052-5173) is published monthly Great Global Experiment ................... 1 New GSA Members, Associates, Fellows . 23 by The Geological Society of America, Inc., with offices at 3300 NPS Internships .......................... 5 Executive Director Position ................ 26 Penrose Place, Boulder, Colorado. Mailing address: P.O. Box GSA Strategic Plan ........................ 9 9140, Boulder, CO 80301-9140, U.S.A. Periodicals postage 1999 Section Meetings ................... 27 paid at Boulder, Colorado, and at additional mailing offices. Environment Matters ..................... 10 SAGE Remarks ............................ 28 Postmaster: Send address changes to GSA Today, Member- About People ............................. 10 Bulletin and Geology Contents ............ 30 ship Services, P.O. Box 9140, Boulder, CO 80301-9140. 1999 Officers and Councilors ............. 11 Appreciation and Thanks .................. 31 Copyright © 1999, The Geological Society of America, Inc. 1999 Research Grants .....................11 Call for Geology Co-Editor ................ 31 (GSA). All rights reserved. Copyright not claimed on content Presidential Address Abstract ............. 12 prepared wholly by U.S. Government employees within the Cole Award Nominations ................. 32 scope of their employment. Permission is granted to individ- GSA Today Student Correspondent ........ 12 1999 GeoVentures ........................ 32 uals to photocopy freely all items other than the science arti- Cady Award Nominations ................. 12 Calendar ................................. 34 cles to further science and education. Individual scientists are Call For Nominations ..................... 13 1999 Division Officers .................... 34 hereby granted permission, without royalties or further Technical Program and requests, to make unlimited photocopies of the science arti- Classifieds ................................ 35 cles for use in classrooms to further education and science, Hot Topics Chairs ........................ 13 GSA Meetings ............................ 36 and to make up to five copies for distribution to associates in GSAF Update ............................. 14 the furtherance of science; permission is granted to make Cordilleran Section Meeting ............... 18 more than five photocopies for other noncommercial, non- profit purposes furthering science and education upon pay- ment of a fee ($0.25 per page-copy) directly to the Copy- right Clearance Center, 222 Rosewood Drive, Danvers, Conveyor continued from p. 1 Allerød climate) was different from today’s MA 01923 USA, phone (978) 750-8400, http://www. and even more different from that which copyright.com; when paying, reference GSA Today, ISSN 1052-5173. Written permission is required from GSA for all If the climate change from Younger would prevail at the time of a greenhouse- other forms of capture, reproduction, and/or distribution of Dryas to present is not an apt analogy to induced conveyor shutdown. Neverthe- any item in this publication by any means, including posting that which would accompany a conveyor less, it is worthwhile to compare the cli- on authors’ or organizational Web sites, except that permission is granted to authors to post the abstracts only of their shutdown, then how might we go about mate of the late Allerød with that of the science articles on their own or their organization’s Web site estimating the consequences of such an Younger Dryas. providing the posting includes this reference: “The full paper event? As noted by some readers of my The contrast between climate was published in the Geological Society of America’s news- magazine, GSA Today, [include year, month, and page num- papers that warned of a possible green- conditions during the warm Allerød and ber if known, where article appears or will appear].” GSA house-induced conveyor shutdown cold Younger Dryas is recorded in four provides this and other forums for the presentation of (Broecker, 1997a, 1997b), I stopped short major ways (see Fig. 2): (1) pollen and bee- diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or polit- of presenting a specific scenario, for I was tle remains in lake and bog sediments tell ical

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