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Asynchrony of Antarctic and Greenland Climate Change During the Last Glacial Period 8 T

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Asynchrony of Antarctic and Greenland Climate Change During the Last Glacial Period 8 T articles Asynchrony of Antarctic and Greenland climate change during the last glacial period 8 T. Blunier*, J. Chappellaz², J. Schwander*,A.DaÈllenbach*, B. Stauffer*,T.F.Stocker*, D. Raynaud², J. Jouzel³², H. B. Clausen§, C. U. Hammer§ & S. J. Johnsen§k * Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland ² CNRS Laboratoire de Glaciologie et GeÂophysique de l'Environnement (LGGE), BP 96, 38402 St Martin d'HeÁres Cedex, Grenoble, France ³ Laboratoire des Sciences du Climat et de l'Environnement, UMR CEA-CNRS 1572, CEA Saclay, Orme des Merisiers, 91191 Gif sur Yvette, France § Department of Geophysics (NBIfAPG), University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen O, Denmark k Science Institute, Department of Geophysics, University of Iceland, Dunhaga 3, Is-107 Reykjavik, Iceland ........................................................................................................................................................................................................................................................ A central issue in climate dynamics is to understand how the Northern and Southern hemispheres are coupled during climate events. The strongest of the fast temperature changes observed in Greenland (so-called Dansgaard±Oeschger events) during the last glaciation have an analogue in the temperature record from Antarctica. A comparison of the global atmospheric concentration of methane as recorded in ice cores from Antarctica and Greenland permits a determination of the phase relationship (in leads or lags) of these temperature variations. Greenland warming events around 36 and 45 kyr before present lag their Antarctic counterpart by more than 1 kyr. On average, Antarctic climate change leads that of Greenland by 1±2.5 kyr over the period 47±23 kyr before present. Greenland ice-core isotopic records1,2 have revealed 24 mild periods event. This leads to a cooling of surface air temperatures in the south (interstadials) of 1±3 kyr duration during the last glaciation1 known and a corresponding warming in the north. Charles et al.9 used as Dansgaard±Oeschger (D±O) events3, where temperature stable-isotope ratios of benthic and planktonic foraminifera in a increased by up to 15 8C compared with full glacial values4,5. The single core from the Southern Ocean to argue that the Northern temperature over Greenland is essentially controlled by heat Hemisphere climate ¯uctuations lagged those of the Southern released from the surface of the North Atlantic Ocean6. This heat Hemisphere by ,1.5 kyr. They conclude that a direct link of high- is brought to the North Atlantic by the thermohaline circulation latitude climate is not promoted by deep-ocean variability. which is driven by the North Atlantic Deep Water (NADW) Conclusive information regarding the phase lag of climate events formation7. NADW strongly in¯uences the deep circulation on a can come only from high-resolution palaeoarchives which have global scale. Therefore climate change related to the North Atlantic either absolute or synchronized timescales with uncertainties smal- thermohaline circulation is not restricted to the North Atlantic ler than 500 yr (for example, to distinguish the Younger Dryas from region but has global implications8. Data correlating with D±O the Antarctic Cold Reversal19, hereafter YD and ACR). This pre- events have been found, for examples in the southern Atlantic9, the requisite is at present satis®ed only in polar ice cores. Here we use Paci®c Ocean10 and probably also the Indian Ocean11. On the fast variations in methane during the last glaciation for a synchro- continents, related events were found in North America where D± nization that permits a better estimate of the relative timing of O events are manifested as wetter climate periods12,13. Also, the events in the two hemispheres. hydrological cycle in the tropics appears to have changed in parallel The CH4 records from two Antarctic ice cores (Byrd station 808 S, with D±O events. This is indicated by changes in atmospheric 1208 W; Vostok 78.478 S, 106.808 E) and one Greenland core (GRIP methane concentration, the main source of which is wetlands ice core, Summit 72.588 N, 37.648 W) have been used to establish situated in the tropics during the last glaciation14. coherent timescales for the Antarctic cores with respect to the GRIP 18 Temperature variations in Greenland inferred from the d Orecord timescale over the period from 50 kyr BP to the Holocene epoch. The are characterized by fast warmings and slow coolings during the last timescale for the Greenland reference core (GRIP) has been 18 20 glaciation. Antarctic temperature variations (from d O and dD obtained by stratigraphic layer counting down to 14,450 yr BP with records) show a different pattern with fewer, less pronounced and an uncertainty of 670 yr at 11,550 yr BP (termination of the YD). For rather gradual warming and cooling events over the same period15,16. older ages, the chronology is based on an ice-¯ow model21.This One of the great challenges of climate research is therefore to ®nd timescale is not considered to be absolute; for instance, the model- a mechanism that is able to reconcile the very different dynamical based dating differs from the stratigraphic layer counting in the deeper behaviour of high-latitude Northern and Southern hemispheres part of the core by several thousand years. However, our conclusions and their phase relationship during rapid climate change. A crucial do not depend on the particular timescale, as explained below. test for our understanding of climate change is the relative phase of We are able to show that long-lasting Greenland warming events events in the two hemispheres. Using the global isotopic signal of around 36 and 45 kyr BP (D±O events 8 and 12) lag their Antarctic atmospheric oxygen, Bender et al.17 hypothesized that interstadial counterpart by 2±3 kyr (comparing the starting points of correspond- events start in the north and spread to the south provided that they ing warmings). On average, Antarctic climate change leads that of last long enough (,2 kyr), postulating that northern temperature Greenland by 1±2.5 kyr over the period 47±23 kyr BP.This®nding variations lead those in Antarctica. On the other hand, they did not contradicts the hypothesis that Antarctic warmings are responses to exclude the possibility that corresponding events are out of phase, as events in the Northern Hemisphere17. The observed time lag also calls suggested by Stocker et al.18 on the basis of a simpli®ed ocean± into question a coupling between northern and southern polar regions atmosphere model which shows that heat is drawn from the via the atmosphere, and favours a connection via the ocean. Indeed, Antarctic region when NADW switches on at the onset of a D±O climate models18,22±24 suggest that heat is extracted from the South- Nature © Macmillan Publishers Ltd 1998 NATURE | VOL 394 | 20 AUGUST 1998 739 articles ern Hemisphere when NADW formation switches on. This inter- measurements cover the time period 9±47 kyr BP on Byrd and 30± hemispheric coupling is clearly identi®ed in interstadial events 8 43 kyr BP on Vostok, with a mean sampling resolution of 200 and and 12, as well as during the termination of the last glaciation. 250 yr, respectively. For the synchronization of the CH4 records, a Monte Carlo method was used, searching for a maximal correlation 5 Synchronization of ice-core records between the records . The fast CH4 variations allow us to ®t the Because the porous ®rn layer on the surface of the ice sheet records to 6200 yr. The time period 25±17 kyr BP was excluded from exchanges air with the overlying atmosphere, air, which gets the synchronization because only low CH4 variations appear during enclosed in bubbles at 50±100 m below the surface (present-day that period, making the correlation uncertain. conditions), has a mean age which is younger than the surrounding The uncertainty in Dage for the Antarctic sites is dominated by ice5. Consequently, a timescale for the ice and a timescale for the air the uncertainty in temperature and accumulation. Estimates of the 8 need to be known, both of which depend on local climate char- glacial±interglacial temperature difference at Vostok and Byrd acteristics. For GRIP, the difference between the age of the ice and station range from 7 to 15 8C (refs 29, 30). This range is covered the mean age of the gas at close-off depth (Dage) has been calculated in our Dage calculations for Byrd station (see Methods). Dage for using a dynamic model5 for ®rn densi®cation and diffusional the Byrd core is one order of magnitude smaller than for the Vostok mixing of the air in the ®rn, including the heat transport in the core, and comparable to the GRIP Dage. This makes the synchro- ®rn and temperature dependence of the close-off density. For nization of isotopes more convincing for Byrd/GRIP than for Summit, estimates of the main parametersÐaccumulation rate Vostok/GRIP (see below and Methods). and temperature4,25Ðallow the calculation of Dage with an To validate the resulting timescales, the distinct double peak of 10 uncertainty of only 6100 yr between 10 and 20 kyr BP, increasing the cosmogenic radioisotope Be around 40 kyr BP can be used. The 10 31 32 to 6300 yr back to 50 kyr BP (ref. 5). Temperature was deduced from GRIP Be record is plotted together with that of Byrd on the new the d18O pro®le as suggested from borehole temperature measure- 4 ments for the glacial±interglacial transition , corresponding 1 roughly to a linear dependence with a slope of 0.33½ per 8C. This GRIP - 34 26 MWP la - 35 relation is in contrast to today's relation with a slope of 0.67½ per 2 34 5 6 7 8 910 11 12 13 - 36 18 8C.
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