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

# Onset of millennial climate variability with the intensification of Northern Hemisphere glaciation *Published: 2026 Apr 2* 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. DOI: 10.1126/science.ady7970