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Global Climate Evolution During the Last Deglaciation PNAS PLUS Global climate evolution during the last deglaciation PNAS PLUS Peter U. Clarka,1, Jeremy D. Shakunb,1, Paul A. Bakerc, Patrick J. Bartleind, Simon Brewere, Ed Brooka, Anders E. Carlsonf,g, Hai Chengh,i, Darrell S. Kaufmanj, Zhengyu Liug,k, Thomas M. Marchittol, Alan C. Mixa, Carrie Morrillm, Bette L. Otto-Bliesnern, Katharina Pahnkeo, James M. Russellp, Cathy Whitlockq, Jess F. Adkinsr, Jessica L. Bloisg,s, Jorie Clarka, Steven M. Colmant, William B. Curryu, Ben P. Flowerv, Feng Heg, Thomas C. Johnsont, Jean Lynch-Stieglitzw, Vera Markgrafj, Jerry McManusx, Jerry X. Mitrovicab, Patricio I. Morenoy, and John W. Williamsr aCollege of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331; bDepartment of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138; cDivision of Earth and Ocean Sciences, Duke University, Durham, NC 27708; dDepartment of Geography, University of Oregon, Eugene, OR 97403; eDepartment of Geography, University of Utah, Salt Lake City, UT 84112; fDepartment of Geoscience, University of Wisconsin, Madison, WI 53706; gCenter for Climatic Research, University of Wisconsin, Madison, WI 53706; hInstitute of Global Environmental Change, Xi’an Jiaotong University, Xi’an 710049, China; iDepartment of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455; jSchool of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, AZ 86011; kLaboratory for Ocean-Atmosphere Studies, School of Physics, Peking University, Beijing 100871, China; lInstitute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309; mNational Oceanic and Atmospheric Administration National Climatic Data Center, Boulder, CO 80305; nClimate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO 80307; oDepartment of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822; pDepartment of Geological Sciences, Brown University, Providence, RI 02912; qDepartment of Earth Sciences, Montana State University, Bozeman, MT 97403; rDivision of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125; sDepartment of Geography, University of Wisconsin, Madison, WI 53706; tLarge Lakes Observatory and Department Geological Sciences, University of Minnesota, Duluth, MN 55812; uDepartment of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543; vCollege of Marine Science, University of South Florida, St. Petersburg, FL 33701; wSchool of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332; xLamont-Doherty Earth Observatory, Palisades, NY 10964; and yInstitute of Ecology and Biodiversity and Department of Ecological Sciences, Universidad de Chile, Santiago 1058, Chile Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved January 4, 2012 (received for review October 10, 2011) Deciphering the evolution of global climate from the end of the was similar to present, whereas subsequent changes in obliquity Last Glacial Maximum approximately 19 ka to the early Holocene and perihelion caused Northern-Hemisphere seasonality to reach 11 ka presents an outstanding opportunity for understanding the a maximum in the early Holocene. transient response of Earth’s climate system to external and inter- CO2 concentrations started to rise from the LGM minimum nal forcings. During this interval of global warming, the decay of 17 5 Æ 0 5 EARTH, ATMOSPHERIC, approximately . ka (1). The onset of the CO2 rise may AND PLANETARY SCIENCES ice sheets caused global mean sea level to rise by approximately have lagged the start of Antarctic warming by 800 Æ 600 years (1), 80 m; terrestrial and marine ecosystems experienced large distur- but this may be an overestimate (2). CO2 levels stabilized from bances and range shifts; perturbations to the carbon cycle resulted approximately 14.7–12.9 ka, and then rose again from about 12.9– in a net release of the greenhouse gases CO2 and CH4 to the atmo- 11.7 ka, reaching near-interglacial maximum levels shortly there- sphere; and changes in atmosphere and ocean circulation affected after. CH4 concentrations also began to rise starting at approxi- the global distribution and fluxes of water and heat. Here we sum- mately 17.5 ka, with a subsequent abrupt increase at 14.7 ka, an marize a major effort by the paleoclimate research community to abrupt decrease at about 12.9 ka, followed by a rise at approxi- characterize these changes through the development of well- mately 11.7 ka (3). Changes in N2O concentrations appear to dated, high-resolution records of the deep and intermediate ocean follow changes in CH4 (4). The combined variations in radiative as well as surface climate. Our synthesis indicates that the super- forcing due to greenhouse gases (GHGs) is dominated by CO2, position of two modes explains much of the variability in regional but abrupt changes in CH4 and N2O modulate the overall struc- and global climate during the last deglaciation, with a strong ture, accentuating the rapid increase at 14.7 ka and causing a association between the first mode and variations in greenhouse slight reduction from 12.9–11.7 ka (Fig. 2D). gases, and between the second mode and variations in the Atlantic Freshwater forcing of the Atlantic meridional overturning meridional overturning circulation. circulation (AMOC) is commonly invoked to explain past and pos- sibly future abrupt climate change (5, 6). During the last deglacia- uring the interval of global warming from the end of the Last tion, the AMOC was likely affected by variations in moisture DGlacial Maximum (LGM) approximately 19 ka to the early transport across Central America (7), salt and heat transport from Holocene 11 ka, virtually every component of the climate system the Indian Ocean (8), freshwater exchange across the Bering Strait underwent large-scale change, sometimes at extraordinary rates, (9), and the flux of meltwater and icebergs from adjacent ice sheets as the world emerged from the grips of the last ice age (Fig. 1). (6). The first two factors largely represent feedbacks on AMOC This dramatic time of global change was triggered by changes in variability. Freshwater exchange across the Bering Strait began insolation, with associated changes in ice sheets, greenhouse gas with initial submergence of the Strait during deglacial sea-level concentrations, and other amplifying feedbacks that produced rise. Highly variable fluxes from ice-sheet melting and calving and distinctive regional and global responses. In addition, there were routing of continental runoff (Fig. 2 E–I) also directly forced the several episodes of large and rapid sea-level rise and abrupt cli- AMOC, but uncertainties in the sources of several key events re- mate change (Fig. 2) that produced regional climate signals main (SI Appendix). superposed on those associated with global warming. Consider- able ice-sheet melting and sea-level rise occurred after 11 ka, but otherwise the world had entered the current interglaciation with Author contributions: P.U.C., J.D.S., Z.L., and B.L.O.-B. designed research; P.U.C., J.D.S., and near-pre-Industrial greenhouse gas concentrations and relatively J.X.M. performed research; P.U.C., J.D.S., A.E.C., H.C., Z.L., B.L.O.-B., J.F.A., J.L.B., J.C., S.M.C., W.B.C., B.P.F., F.H., T.C.J., J.L.-S., V.M., J.M., P.I.M., and J.W.W. analyzed data; and P.U.C., stable climates. Here we synthesize well-dated, high-resolution J.D.S., P.A.B., P.J.B., S.B., E.B., A.E.C., D.S.K., T.M.M., A.C.M., C.M., K.P., J.M.R., and C.W. ocean and terrestrial proxy records to describe regional and glo- wrote the paper. bal patterns of climate change during this interval of deglaciation. The authors declare no conflict of interest. Between the LGM and present, seasonal insolation anomalies This article is a PNAS Direct Submission. arising from the combined effects of eccentricity, precession, and 1To whom correspondence may be addressed. E-mail: [email protected] or shakun@ obliquity were generally opposite in sign between hemispheres fas.harvard.edu. C (Fig. 2 ), whereas variations in annual-average insolation were This article contains supporting information online at www.pnas.org/lookup/suppl/ symmetrical about the equator. At the LGM, seasonal insolation doi:10.1073/pnas.1116619109/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1116619109 PNAS Early Edition ∣ 1of9 Downloaded by guest on September 26, 2021 A A -34 LGM OD BA YD -36 -38 O (per-40 mil) 18 δ -42 B -42 ACR -400 -44 -46 -420 D (per mil) -48 O (per mil) δ 18 -440 -50 δ 2 2 ) 0.01 10.1 3 975 11 m /m C -2 465 B 450 435 0 Insolation (W420 m D -1 ) -2 -50 E -2 (W m -75 -3 Radiative forcing -100 ) Sea level (m) 20 Fig. 1. (A) Climate simulation of the Last Glacial Maximum 21,000 y ago F -125 -1 using the National Center for Atmospheric Research Community Climate 10 kyr System Model, version 3.0 (141). Sea-surface temperatures are anomalies re- LIS 2 0 km lative to the control climate. Also shown are continental ice sheets (1,000-m MWP-1a 5 G 0.3 SIS Area change contours) (149) and leaf-area index simulated by the model (scale bar shown). -10 (10 (B) Same as A except for 11 ka. 0.2 19-ka MWP 0.1 Results 0 Flux anomaly (Sv) Sea-Surface Temperatures. We compiled 69 high-resolution proxy H 16 (%) sea-surface temperature (SST) records spanning 20–11 ka that 3 8 provide broad coverage of the global ocean (SI Appendix). Our eva- 0.4 0.3 H1 H0 CaCO luation of SST reconstructions from regional ocean basins suggests I 0 that warming trends are smallest at low latitudes (1–3°C)andhigh- 0.2 er at higher latitudes (3–6°C)(SI Appendix, Fig.
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