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RESEARCH | REPORTS ACKNOWLEDGMENTS Youth and Family Services, and Youth Guidance. I thank Chicago Public Schools, Chicago Department of Family and Support Services, This project was supported by award B139634411 from the U.S. Schools partners J. Foreman, K. Klein, J. Loudon, and S. Norris; U.S. Department of Justice, or U.S. Department of Labor. Department of Labor and grant 2012-MU-FX-0002 from the Office of Chief R. Tracy, T. Lavery, and the Chicago Police Department; R. Goerge Juvenile Justice and Delinquency Prevention, Office of Justice and Chapin Hall; R. Ander, S. Coussens, J. Davis, G. Cusick, M. Egan, SUPPLEMENTARY MATERIALS Programs, U.S. Department of Justice. The study received approval H. Pollack (Chicago PI), R. Harris, N. Hess, A. Métivier, and J. Rountree www.sciencemag.org/content/346/6214/1219/suppl/DC1 from the University of Chicago’s IRB (protocol 12-1112) with secondary for project support; and K. Charles, J. Guryan, C. Loeffler, J. Ludwig, Materials and Methods approval from the University of Pennsylvania’sIRB(protocol818707).I J. MacDonald, E. Owens, S. Raudenbush, S. Sotelo, and D. Tannenbaum Supplementary Text gratefully acknowledge the City of Chicago and Mayor R. Emanuel, the for helpful comments. Replication data are posted at the University of Tables S1 to S8 work of the Chicago Department of Family Support Services, especially Michigan’s ICPSR data depository (http://doi.org/10.3886/E18627V1); References (53–96) E. Diaz, J. Axelrod, A. Fernandez, and J. Welch, and the organizations see supplementary materials section 1.5 for details. All content is that implemented the program: Phalanx Family Services, Sinai the responsibility of the author and does not represent the official 23 June 2014; accepted 7 November 2014 Community Institute, Saint Sabina Employment Resource Center, SGA position or policies of the Chicago Police Department, Chicago Public 10.1126/science.1257809 PALEOCLIMATE solation at these latitudes should have reduced precipitation (22). Here, we analyze transient simulations of the Coherent changes of southeastern climate evolution from the LGM to the early Hol- ocene (11 ka) with a global coupled atmosphere– ocean–sea ice–land general circulation model equatorial and northern African (CCSM3) to assess possible mechanisms for the abrupt, synchronous onset of the AHP in NA and rainfall during the last deglaciation SEA. The model has a latitude-longitude resolu- tion of ~3.75° in the atmosphere and ~3° in the Bette L. Otto-Bliesner,1* James M. Russell,2 Peter U. Clark,3 Zhengyu Liu,4,5 ocean and includes a dynamic global vegetation Jonathan T. Overpeck,6 Bronwen Konecky,3,7 Peter deMenocal,8 Sharon E. Nicholson,9 module (supplementary text). The model success- 4 5 fully captures the large-scale observed modern Feng He, Zhengyao Lu features of African climate, including seasonal During the last deglaciation, wetter conditions developed abruptly ~14,700 years ago in shifts of winds, the intertropical convergence on December 5, 2014 southeastern equatorial and northern Africa and continued into the Holocene. Explaining the zone (ITCZ), and precipitation to the summer abrupt onset and hemispheric coherence of this early African Humid Period is challenging due hemispheres (figs. S2 and S3). To characterize to opposing seasonal insolation patterns. In this work,we use a transient simulation with a climate the regional precipitation responses during the model that provides a mechanistic understanding of deglacial tropical African precipitation deglaciation, we examine model changes in the changes. Our results show that meltwater-induced reduction in the Atlantic meridional NA region defined by 11.1° to 18.6°N and 5.6° to overturning circulation (AMOC) during the early deglaciation suppressed precipitation in both 20.6°E and in the SEA region defined by 0° to regions. Once the AMOC reestablished, wetter conditions developed north of the equator in 7.4°S and 24.4° to 43.1°E (see supplementary response to high summer insolation and increasing greenhouse gas (GHG) concentrations, text and fig. S7 for sensitivity of model results whereas wetter conditions south of the equator were a response primarily to the GHG increase. to the definitions of the regions). The prescribed forcings and boundary con- www.sciencemag.org ditions for the full-forcing simulation (TraCE) he future response of African rainfall to in- imum (LGM) (~21,000 years ago, or 21 ka) were include orbitally forced insolation changes, in- creasing greenhouse gas (GHG) concentra- rapidly replaced by a much wetter interval, re- creasing atmospheric concentrations of the tions is a critical socio-economic issue, with ferred to as the African Humid Period (AHP), long-lived GHGs, and retreating ice sheets and implications for water resources, agriculture, starting ~14.7 ka over much of Africa. Over North associated meltwater release to the oceans (23, 24) and potential conflict (1), but uncertainties Africa (NA), the start of the AHP has been (fig. S1). We also explore the individual contribu- Tamong model projections remain (2–4). African widely recorded in lake-level records (5, 6)and tions of orbital forcing and GHGs during the hydroclimate changed substantially during the proxies of aeolian and fluvial processes pre- deglaciation with two sensitivity experiments: Downloaded from last deglaciation, the most recent time period served in marine sediments from the eastern (i) TraCE orbital-only, where only the orbital during which natural global warming was as- Atlantic Ocean (7–10). At the same time, a near- forcing is allowed to vary, with all other forcings sociated with increases in GHG concentrations. contemporaneous precipitation increase is also kept at their values for 17 ka, and (ii) TraCE GHG- Numerous proxy records from Africa indicate recorded in southeastern equatorial Africa (SEA) only, where only the concentrations of the GHGs that dry conditions during the Last Glacial Max- (to 9°S) by lake-level records (11–14), as well as change, increasing from low concentrations at in pollen and geochemical records from lake 17 ka to close to their pre-industrial concentra- sediments (14 16). tions by 10 ka. In both sensitivity experiments, 1 – Climate and Global Dynamics Division, National Center for Models and data establish that the initial in- the ice sheets and meltwater release are held Atmospheric Research, Boulder, CO 80307-3000, USA. 2Department of Earth, Environmental, and Planetary crease of NA summer monsoonal rainfall occurred constant at 17-ka conditions, and this meltwater Sciences, Brown University, Providence, RI 02912, USA. in response to increasing local insolation associ- maintains a strongly reduced Atlantic meridional 3College of Earth, Ocean, and Atmospheric Sciences, Oregon ated with orbital variations (17), amplified through overturning circulation (AMOC) afterward. 4 State University, Corvallis, OR 97331, USA. Center for feedbacks with the ocean and possibly vegeta- The temporal evolution of the simulated de- Climatic Research and Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, Madison, tion (18–20), but the cause of the abrupt start of glacial precipitation shows good agreement with WI 53706, USA. 5Laboratory for Climate, Ocean and the AHP remains unclear. Proposed triggers in- individual proxy records. TraCE and a proxy Atmosphere Studies, School of Physics, Peking University, clude a nonlinear threshold response to gradu- record of humidity (9)bothshowdryconditions 6 Beijing 100871, P. R. China. Department of Geosciences and ally changing summer insolation (8) and/or the in the central Sahel at the LGM, a decrease in Institute of the Environment, University of Arizona, Tucson, AZ 85721, USA. 7Cooperative Institute for Research in recovery of deep convection in the North Atlantic precipitation at ~17 ka, an abrupt increase at Environmental Sciences, University of Colorado Boulder, following cessation of a Northern Hemisphere the onset of the Bølling-Allerød warm interval, Boulder, CO 80309, USA. 8Department of Earth and meltwater event (21). Similarly, the cause for the an episode of drying during the Younger Dryas Environmental Sciences, Columbia University, New York, NY 10027, synchronous onset of the AHP in the SEA region (YD) (12.9 to 11.7 ka), and an increase during USA. 9Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, FL 32306, USA. has remained enigmatic, as models and theory the early Holocene (Fig. 1B). The total leaf-area *Corresponding author. E-mail: [email protected] suggest that orbital forcing of local summer in- index of simulated vegetation over the Sahel SCIENCE sciencemag.org 5DECEMBER2014• VOL 346 ISSUE 6214 1223 RESEARCH | REPORTS closely parallels changes in precipitation, start- ture balance from proxy data (table S1). The first The secondEOFmodes of the dataand TraCE ing from low values signifying primarily bare EOF mode (EOF1) (48.0% of total variance) of (16.4 and 17.2% of total variance, respectively) ground followed by increasing values starting the proxy data shows an in-phase relation over both indicate an in-phase relation of much of at ~16 ka (Fig. 1C). To the extent that precipita- most of Africa that extends southward to the NA and SEA (Fig. 2, C and D). The character of tion and vegetation cover influence availability latitude of Lake Malawi (9° to 14°S, 34° to 35°E), the data PC2 is similar to that simulated by the of sediment for aeolian transport, the temporal whereas the associated PC1 indicates increas- model, particularly when the model PC2 is sam- evolution of these simulated properties can be ing moisture from the LGM to early Holocene pled at the same resolution as the data to ac- used to explain a record of terrigenous dust flux (Fig. 2A). EOF1 of model precipitation (39.2% count for potential aliasing artifacts (Fig. 2, C sourced from subtropical NA (8)(Fig.1C). of total variance) and its associated PC1 (Fig. 2B) and D), although the model has a more muted Simulated deglacial changes in SEA hydrology reproduce the overall spatial and temporal be- recovery from the YD.

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