The nature of millennial-scale climate variability during the past two glacial periods

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Periodic iceberg discharges during the last glacial period led to a slowdown of the Atlantic meridional overturning circulation. Sediment records from the Portuguese margin show that similar events punctuated the penultimate glacial period as well, although their duration and broader climatic impacts were modified by different background climate conditions. During the last glacial period, iceberg discharges into the North Atlantic disrupted the meridional overturning circulation, leading to cooling in the Northern Hemisphere and warming in Antarctica1,2. This asymmetric response can be explained by a bipolar see-saw mechanism3,4,5, whereby changes in the strength of the meridional overturning circulation lead to changes in the interhemispheric heat transport. It is unclear, however, to what extent the response of the overturning circulation is a function of freshwater flux and boundary climate conditions4. Here we use foraminiferal isotope and pollen records from the Portuguese margin to reconstruct surface- and deep-water hydrography and atmospheric changes during the last and penultimate glacial periods. When we compare our records with temperature reconstructions from Antarctica6, we find that the bipolar see-saw was a characteristic feature of both glacial periods. However, the comparison also underlines the dependence of the bipolar see-saw on background climate and magnitude of iceberg discharge. It also suggests that an intensified hydrological cycle may lead to a weaker overturning circulation with a smaller disruption threshold and extended North Atlantic stadial durations.