Onset of millennial climate variability with the intensification of Northern Hemisphere glaciation.

The Quaternary Period (the past 2.58 million years) was characterized by the waxing and waning of large ice sheets in the Northern Hemisphere. Using sediment sequences from the Iberian Margin, we have demonstrated that expansion of Northern Hemisphere ice sheets around 2.7 million years ago was accompanied by the emergence of millennial climate variability (MCV) during glacial periods. The onset of MCV at ~2.7 million years ago was heralded by isolated precursor events, followed by multiple millennial climate oscillations at ~2.5 million years ago. These events coincided with deposition of ice-rafted detritus in the North Atlantic, suggesting a role for marine-terminating ice sheets. Once established, MCV became an intrinsic feature of glacial climates of the Quaternary. Our findings underscore the profound impact Northern Hemisphere glaciation had on climate variability across multiple timescales. Editor's summary: The large continental ice sheets of the Northern Hemisphere gradually expanded during the late Pliocene, but at least one effect of that expansion was abrupt. Hodell et al. showed that millennial climate variability began suddenly around 2.7 million years ago, suggesting that a climatic threshold of some type was passed at that time. Iceberg discharge and the associated freshwater forcing it caused seem to have played a critical role in the onset of that climate pattern, which became an intrinsic feature of the Northern Hemisphere climate during the Quaternary. —Jesse Smith [ABSTRACT FROM AUTHOR]