Precise interpolar phasing of abrupt climate change during the last ice age

To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by the Nature Publishing Group and can be found at: http://www.nature.com/nature/index.html. Supporting Information can be found at: http://www.nature.com/nature/journal/v520/n7549/full/nature14401.html#/supplementary-informationThe last glacial period exhibited abrupt Dansgaard–Oeschger climatic\ud oscillations, evidence of which is preserved in a variety of\ud Northern Hemisphere palaeoclimate archives¹. Ice cores show that\ud Antarctica cooled during the warm phases of the Greenland\ud Dansgaard–Oeschger cycle and vice versa[superscript 2,3], suggesting an interhemispheric\ud redistribution of heat through a mechanism called the\ud bipolar seesaw[superscript 4–6]. Variations in the Atlantic meridional overturning\ud circulation (AMOC) strength are thought to have been important,\ud but much uncertainty remains regarding the dynamics and\ud trigger of these abrupt events[superscript 7–9]. Key information is contained in\ud the relative phasing of hemispheric climate variations, yet the\ud large, poorly constrained difference between gas age and ice age\ud and the relatively low resolution of methane records from\ud Antarctic ice cores have so far precluded methane-based synchronization\ud at the required sub-centennial precision[superscript 2,3,10]. Here we use a\ud recently drilled high-accumulation Antarctic ice core to show that,\ud on average, abrupt Greenland warming leads the corresponding\ud Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard–Oeschger events, including the Bølling event; Greenland cooling\ud leads the corresponding onset of Antarctic warming by\ud 208 ± 96 years. Our results demonstrate a north-to-south directionality\ud of the abrupt climatic signal, which is propagated to the\ud Southern Hemisphere high latitudes by oceanic rather than atmospheric\ud processes. The similar interpolar phasing of warming and\ud cooling transitions suggests that the transfer time of the climatic\ud signal is independent of the AMOC background state. Our findings\ud confirm a central role for ocean circulation in the bipolar\ud seesaw and provide clear criteria for assessing hypotheses and\ud model simulations of Dansgaard–Oeschger dynamics