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

The Quaternary Period (the last 2.58 Ma) was characterized by the waxing and waning of large continental ice sheets in the Northern Hemisphere, driven by slow, cyclic changes in Earth's orbit. Using high-resolution, well-dated sediment sequences from the Iberian Margin, we demonstrate that the expansion of Northern Hemisphere ice sheets around 2.7 Ma was accompanied by the emergence of millennial climate variability (MCV) during glacial periods. The onset of MCV at 2.7 Ma was heralded by isolated single precursor events during Marine Isotope Stages (MIS) G6, G4, G2 and 104, followed by the emergence of multiple millennial climate oscillations at 2.5 Ma in MIS 100, resembling the Dansgaard-Oeschger events of the last glaciation. These events coincided with the deposition of ice-rafted detritus in the high-latitude North Atlantic, suggesting an important role for marine terminating ice sheets in MCV. Once established, MCV became an intrinsic feature of Northern Hemisphere glacial climates of the Quaternary. Our findings underscore the profound impacts that expansion of Northern Hemisphere ice sheets had on climate variability across multiple time scales.