Book 2.indb - US Climate Change Science Program

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The U.S. Climate Change Science Program Chapter 4The cause of thereduced AMOCduring the YoungerDryas has commonlybeen attributedto the routing ofNorth Americanrunoff with aresulting increasein freshwater fluxdraining eastwardthrough the St.Lawrence, which issupported by recentpaleoceanographicevidence.The cause of the reduced AMOC during theYounger Dryas has commonly been attributedto the routing of North American runoff witha resulting increase in freshwater flux drainingeastward through the St. Lawrence River(Johnson and McClure, 1976; Rooth, 1982;Broecker et al., 1989), which is supported byrecent paleoceanographic evidence (Flower etal., 2004; Carlson et al., 2007) (Box 4.3).4.5 Evidence for Changes in the AMOCDuring Stage 3Marine isotope stage 3 (30,000–65,000 yearsago) was a period of intermediate ice volumethat occurred prior to the LGM. This periodof time is characterized by the Dansgaard-Oeschger (D-O) oscillations, which were firstidentified from Greenland ice-core records(Johnsen et al., 1992; Grootes et al., 1993)(Fig. 4.7). These oscillations are similar to theabrupt climate changes during the last deglaciationand are characterized by alternatingwarm (interstadial) and cold (stadial) stateslasting for millennia, with abrupt transitionsbetween states of up to 16 °C occurring overdecades or less (Cuffey and Clow, 1997; Huberet al., 2006). Bond et al. (1993) recognizedthat several successive D-O oscillations ofdecreasing amplitude represented a longerterm (~7,000-yr) climate oscillation whichculminates in a massive release of icebergsfrom the Laurentide Ice Sheet, known as aHeinrich event (Fig. 4.7) (Box 4.3). The D-Osignal seems largely confined to the NorthernHemisphere, while the Southern Hemisphereoften exhibits less abrupt, smaller amplitudemillennial climate changes (Clark et al., 2007),best represented by A-events seen in Antarcticice core records (Fig. 4.7). Synchronizationof Greenland and Antarctic ice core records(Bender et al., 1994, 1999; Sowers and Bender,1995; Blunier et al., 1998; Blunier and Brook,2001; EPICA Community Members, 2006)suggests an out-of-phase “seesaw” relationshipbetween temperatures of the Northern andSouthern Hemispheres, and that the thermalcontrast between hemispheres is greatest at thetime of Heinrich events (Fig. 4.7).By comparison to the deglaciation, there arefewer proxy records constraining millennialscalechanges in the AMOC during stage 3.Most inferences of these changes are based onδ 13 C as a proxy for water-mass nutrient content.A depth transect of well-correlated δ 13 C recordsis required in order to capture temporal changesin the vertical distribution of any given watermass, since the δ 13 C values at any given depthmay not change significantly if the core siteremains within the same water mass.Figure 4.10 illustrates one such time-depthtransect of δ 13 C records from the easternNorth Atlantic that represent changes in thedepth and volume (but not rate) of the AMOCduring an interval (35–48 ka) of pronouncedmillennial-scale climate variability (Fig. 4.7).We emphasize this interval only because itencompasses a highly resolved and well-datedarray of δ 13 C records. The distinguishing featureof these records is a minimum in δ 13 C at thesame time as Heinrich events 4 and 5, indicatingthe near-complete replacement of nutrient-poor,high δ 13 C NADW with nutrient-rich, low δ 13 CAABW in this part of the Atlantic basin. Theinference of a much reduced rate of the AMOCfrom these data is supported by the proxyrecords during the last deglaciation (Fig. 4.9),which indicate a similar distribution of δ 13 C ata time when Pa/Th ratios suggest the AMOChad nearly collapsed by the time of Heinrichevent 1 (see above). Insofar as we understandthe interhemispheric seesaw relationship establishedby ice core records (Fig. 4.7) to reflectchanges in the AMOC and corresponding oceanheat transport (Broecker, 1998; Stocker andJohnsen, 2003), the fact that Heinrich eventsduring stage 3 only occur at times of maximumthermal contrast between hemispheres (coldnorth, warm south) further indicates that someother mechanism was responsible for causingthe large reduction in the AMOC by the time aHeinrich event occurred.While many of the Heinrich stadials show upclearly in these and other δ 13 C records, there isoften no clear distinction between D-O interstadialsand non-Heinrich D-O stadials (Fig. 4.10)(Boyle, 2000; Shackleton et al., 2000; Elliot etal., 2002). While some δ 13 C and Nd records doshow millennial-scale variability not associatedwith the Heinrich events (Charles et al., 1996;Curry et al., 1999; Hagen and Keigwin, 2002;Piotrowski et al., 2005), there are many challengesthat have impeded the ability to firmlyestablish the presence or absence of coherent142
Abrupt Climate ChangeFigure 4.10. (a) The GISP2 δ 18 O record (Grootes et al., 1993; Stuiver and Grootes,2000). Times of Heinrich events 4 and 5 identified (H4 and H5). (b) Time-varying δ 13 C, aproxy for distribution of deep water masses, as a function of depth in the eastern NorthAtlantic based on four δ 13 C records at water depths of 1,099 m (Zahn et al., 1997), 2,161 m(Elliot et al., 2002), 2,637 m (Skinner and Elderfield, 2007), and 3,146 m (Shackleton etal., 2000). Control points from four cores used for interpolation are shown (black dots).More negative δ 13 C values correspond to nutrient-rich Antarctic Bottom Water (AABW),whereas more positive δ 13 C values correspond to nutrient-poor North Atlantic DeepWater (see Fig. 4.8). Also shown by the thick gray line is a proxy for Heinrich events,with peak values corresponding to Heinrich events H5 and H4 (Stoner et al., 2000) (notethat scale for this proxy is not shown). During Heinrich events H5 and H4, nutrient-richAABW displaces NADW to shallow depths in the eastern North Atlantic basin. (c) TheByrd δ 18 O record (Johnsen et al., 1972), with the time scale synchronized to the GISP2time scale by methane correlation (Blunier and Brook, 2001). ka, thousand years. A1,A2, Antarctic warm events.143
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TABLE OF CONTENTSAuthor Team for th
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The U.S. Climate Change Science Pro
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PREFACEReport Motivation and Guidan
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1CHAPTERAbrupt Climate ChangeIntrod
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Abrupt Climate Change-35Arctic temp
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Abrupt Climate ChangeFigure 1.2. Po
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Abrupt Climate Changewell below sea
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Abrupt Climate Changeheterogeneous
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Abrupt Climate Changeapart from dro
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Abrupt Climate Change6. Abrupt Chan
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Abrupt Climate Changeintermediate c
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Abrupt Climate Changeper square met
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Abrupt Climate Changeis the norther
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Abrupt Climate ChangeRegardless how
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Abrupt Climate Changecentrations, a
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Abrupt Climate ChangeBox 2.1. Glaci
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Abrupt Climate Changetime. Degree-d
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Abrupt Climate Changetion, and accu
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Abrupt Climate ChangeBox 2.2. Mass
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Abrupt Climate Changeof the glacier
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Abrupt Climate ChangeFigure 2.6. Ra
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Abrupt Climate Changewater) or, mor
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Abrupt Climate ChangeFigure 2.7. Re
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Abrupt Climate Changeresults in a l
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Abrupt Climate Changeis not providi
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Abrupt Climate Changemeasurements c
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Abrupt Climate Changemore than 10,0
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Abrupt Climate Changeocean models h
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Abrupt Climate Changeto atmosphere-
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3CHAPTERHydrological Variability an
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Abrupt Climate Change• The integr
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Abrupt Climate Changethe soil (King
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Abrupt Climate Changeon (Kaplan et
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Abrupt Climate Changeorbital-time-s
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Abrupt Climate ChangeFigure 3.4. (t
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Abrupt Climate Changemodels forced
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Abrupt Climate ChangeBox 3.2. Waves
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Abrupt Climate Changebecause (1) th
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Abrupt Climate ChangeHarrison et al
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Abrupt Climate Changethat even the
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Abrupt Climate Changethat seen afte
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202REFERENCESChapter 1 ReferencesAl
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