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© Nature Publishing Group. We thank O. Sherwood for providing coral data. M.E.M. acknowledges support for this work from the ATM program of the National Science Foundation (grant ATM-0902133)., Possible changes in Atlantic meridional overturning circulation (AMOC) provide a key source of uncertainty regarding future climate change. Maps of temperature trends over the twentieth century show a conspicuous region of cooling in the northern Atlantic. Here we present multiple lines of evidence suggesting that this cooling may be due to a reduction in the AMOC over the twentieth century and particularly after 1970. Since 1990 the AMOC seems to have partly recovered. This time evolution is consistently suggested by an AMOC index based on sea surface temperatures, by the hemispheric temperature difference, by coral-based proxies and by oceanic measurements. We discuss a possible contribution of the melting of the Greenland Ice Sheet to the slowdown. Using a multi-proxy temperature reconstruction for the AMOC index suggests that the AMOC weakness after 1975 is an unprecedented event in the past millennium (p > 0.99). Further melting of Greenland in the coming decades could contribute to further weakening of the AMOC., National Science Foundation (NSF), EE.UU., Depto. de Física de la Tierra y Astrofísica, Fac. de Ciencias Físicas, TRUE, pub

© 2011 American Meteorological Society. We gratefully acknowledge the support of the Climate Program Office at the U.S. National Oceanic and Atmospheric Administration, and the International Geosphere Biosphere, Past Global Changes (PAGES) Program Office in Bern, Switzerland. We are also grateful for the support of the Luso-American Foundation (FLAD), the Portuguese Foundation for Science and Technology, and the Institut Dom Luiz, University of Lisbon for their help in conducting the symposium that set the background for this assessment. Ricardo Trigo and David Barriopedro received support from Portuguese Foundation for Science and Technology (FCT) with project MEDIATIC (PTDC/AAC-CLI/103361/208). We also acknowledge the helpful reviews provided by R. Seager and an anonymous reviewer and thank H. Goosse for kindly making available to us one of the figures., Developing accurate reconstructions of past climate regimes and enhancing our understanding of the causal factors that may have contributed to their occurrence is important for a number of reasons; these include improvements in the attribution of climate change to natural and anthropogenic forcing, gaining a better appreciation for the range and magnitude of low-frequency variability and previous climatic regimes in comparison with the modern instrumental period, and developing greater insights into the relationship between human society and climatic changes. This paper examine up-to-date evidence regarding the characteristics of the climate in medieval times (A.D. 950-1400). Long and high-resolution climate proxy records reported in the scientific literature, which form the basis for the climate reconstructions, have greatly expanded in the last few decades, with greater numbers of sites that now cover more areas of the globe. Some comparisons with the modern climate record and discussion of potential mechanisms associated with the patterns of medieval climate are presented here, but our main goal is to provide the reader with some appreciation of the richness of past natural climate variability in terms of its spatial and temporal characteristics., Climate Program Office at the U.S. National Oceanic and Atmospheric Administration, International Geosphere Biosphere, Past Global Changes (PAGES) Program Office in Bern, Switzerland, Luso-American Foundation (FLAD), Portuguese Foundation for Science and Technology (FCT), Institut Dom Luiz, University of Lisbon, Depto. de Física de la Tierra y Astrofísica, Fac. de Ciencias Físicas, TRUE, pub