The oceans mediate the response of global climate to natural and anthropogenic forcings. Yet for the past 2,000 years — a key interval for understanding the present and future climate response to these forcings — global sea surface temperature changes and the underlying driving mechanisms are poorly constrained. Here we present a global synthesis of sea surface temperatures for the Common Era (ce) derived from 57 individual marine reconstructions that meet strict quality control criteria. We observe a cooling trend from 1 to 1800 ce that is robust against explicit tests for potential biases in the reconstructions. Between 801 and 1800 ce, the surface cooling trend is qualitatively consistent with an independent synthesis of terrestrial temperature reconstructions, and with a sea surface temperature composite derived from an ensemble of climate model simulations using best estimates of past external radiative forcings. Climate simulations using single and cumulative forcings suggest that the ocean surface cooling trend from 801 to 1800 ce is not primarily a response to orbital forcing but arises from a high frequency of explosive volcanism. Our results show that repeated clusters of volcanic eruptions can induce a net negative radiative forcing that results in a centennial and global scale cooling trend via a decline in mixed-layer oceanic heat content.
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We thank the many scientists who made their published data sets available via public data repositories. T. Kiefer, L. von Gunten and C. Telepski from the IGBP PAGES-IPO provided organizational and logistical support. V. Masson-Delmotte, C. Giry, S. P. Bryan, S. Stevenson, D. Colombaroli, B. Horton, J. Tierney and the Ocean2k HR Working Group are thanked for early input to the project design and methodology. G. Lohmann assisted with model output. A. Mairesse assisted with model figures. L. Skinner and D. Reynolds are thanked for discussions on age models. We are grateful to the 75 volunteers who constructed the Ocean2k metadatabase (see Supplementary Information for full list of names). We acknowledge the World Climate Research Programme's Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups (listed in Supplementary Table S4) for producing and making available their model output. For CMIP, the US Department of Energy's Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. We acknowledge support from PAGES, a core project of IGBP financially supported by the US and Swiss National Science Foundations (NSFs) and NOAA; Australian Research Council (ARC) Discovery Project grant DP1092945 (H.V.M., S.J.P.), ARC Future Fellowship FT140100286 grant (H.V.M.), AINSE Fellowship grant (H.V.M.) and the research contributes to ARC Australian Laureate Fellowship FL120100050; US NSF awards NSF/ATM09-02794 (M.N.E.) and NSF/ATM0902715 (M.N.E), and Royal Society of New Zealand Marsden Fund grant 11-UOA-027 (M.N.E.); F.R.S-FNRS (Belgium; H.G.); French National Research Agency (ANR) under ISOBIOCLIM grant (G.L.); European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement number 243908, Past4Future 'Climate change — Learning from the past climate' contribution no. 81 (H.G., B.M., P.G.M., M.-S.S.); CSIC-Ramón y Cajal post-doctoral programme RYC-2013-14073 (B.M.), Clare Hall College, Cambridge, Shackleton Fellowship (B.M.) and Red CONSOLIDER GRACCIE CTM2014-59111-REDC (B.M.); US Geological Survey's Climate and Land Use Change Research and Development Program and the Volcano Science Center (J.A.A.); Ralph E. Hall Endowed Award for Innovative Research (D.W.O.); Danish Council for Independent Research Natural Science OCEANHEAT project 12-126709/FNU (M.-S.S.); LEFE/INSU/NAIV project (M.-A.S.); NSF of China grant 41273083 (K.S.) and Shanhai Fund grant 2013SH012 (K.S.); UTIG Ewing-Worzel Fellowship (K.T.); Swedish Research Council grant 621-2011-5090 (H.L.F.); and from a Marie Curie Intra-European Fellowship for Career Development (V.E.).