Global climate evolution during the last deglaciation 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. Williamss 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 approximately 17.5 Æ 0.5 ka (1). The onset of the CO2 rise may 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 as well as surface climate. Our synthesis indicates that the super- follow changes in CH4 (4). The combined variations in radiative position of two modes explains much of the variability in regional forcing due to greenhouse gases (GHGs) is dominated by CO2, and global climate during the last deglaciation, with a strong but abrupt changes in CH4 and N2O modulate the overall struc- association between the first mode and variations in greenhouse ture, accentuating the rapid increase at 14.7 ka and causing a gases, and between the second mode and variations in the Atlantic slight reduction from 12.9–11.7 ka (Fig. 2D). meridional overturning circulation. Freshwater forcing of the Atlantic meridional overturning circulation (AMOC) is commonly invoked to explain past and pos- uring the interval of global warming from the end of the Last sibly future abrupt climate change (5, 6). During the last deglacia- DGlacial Maximum (LGM) approximately 19 ka to the early tion, the AMOC was likely affected by variations in moisture Holocene 11 ka, virtually every component of the climate system transport across Central America (7), salt and heat transport from underwent large-scale change, sometimes at extraordinary rates, the Indian Ocean (8), freshwater exchange across the Bering Strait as the world emerged from the grips of the last ice age (Fig. 1). (9), and the flux of meltwater and icebergs from adjacent ice sheets This dramatic time of global change was triggered by changes in (6). The first two factors largely represent feedbacks on AMOC insolation, with associated changes in ice sheets, greenhouse gas variability. Freshwater exchange across the Bering Strait began concentrations, and other amplifying feedbacks that produced with initial submergence of the Strait during deglacial sea-level distinctive regional and global responses. In addition, there were rise. Highly variable fluxes from ice-sheet melting and calving and several episodes of large and rapid sea-level rise and abrupt cli- routing of continental runoff (Fig. 2 E–I) also directly forced the mate change (Fig. 2) that produced regional climate signals superposed on those associated with global warming. Consider- able ice-sheet melting and sea-level rise occurred after 11 ka, but Author contributions: P.U.C., J.D.S., Z.L., and B.L.O.-B. designed research; P.U.C., J.D.S., and 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., otherwise the world had entered the current interglaciation with 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., near-pre-Industrial greenhouse gas concentrations and relatively 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. stable climates. Here we synthesize well-dated, high-resolution wrote the paper. ocean and terrestrial proxy records to describe regional and glo- The authors declare no conflict of interest. bal patterns of climate change during this interval of deglaciation. This article is a PNAS Direct Submission. Between the LGM and present, seasonal insolation anomalies 1To whom correspondence may be addressed. E-mail: [email protected] or shakun@ arising from the combined effects of eccentricity, precession, and fas.harvard.edu. obliquity were generally opposite in sign between hemispheres See Author Summary on page 7140 (volume 109, number 19). 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. E1134–E1142 ∣ PNAS ∣ Published online February 13, 2012 www.pnas.org/cgi/doi/10.1073/pnas.1116619109 Downloaded by guest on October 3, 2021 A A -34 LGM OD BA YD PNAS PLUS -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 EARTH, ATMOSPHERIC, 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 AND PLANETARY SCIENCES 0.1 AMOC, but uncertainties in the sources of several key events re- 0 main (SI Appendix). H Flux anomaly (Sv) 16 (%) 3 Results 0.4 8 Sea-Surface Temperatures. We compiled 69 high-resolution proxy 0.3 H1 H0 CaCO I 0 sea-surface temperature (SST) records spanning 20–11 ka that 0.2 SI Appendix provide broad coverage of the global ocean ( ). Our eva- 0.1 luation of SST reconstructions from regional ocean basins suggests Flux anomaly (Sv) that warming trends are smallest at low latitudes (1–3°C)andhigh- 20 18 16 14 12 er at higher latitudes (3–6°C)(SI Appendix, Fig.