Ice cores from the Allan Hills, Antarctica show relatively stable atmospheric CO2 and CH4 levels over the last 3 million years

The role greenhouse gases play in the evolution of Earth's climate over the last 3 million years is uncertain beyond the continuous ice core record (800,000 years). Here, we present new snapshots of carbon dioxide (CO ₂ ) and methane (CH ₄ ) between 3.1 and 0.4 million years ago (Ma) from shallow ice cores drilled in the Allan Hills Blue Ice Area (BIA). In the oldest ice (>1 Ma), mixing and thinning have attenuated the glacial-interglacial variability, and we reconstruct long-term averages. The data indicate that CO ₂ and CH ₄ levels in the early Pleistocene are within the range of variability observed for the last 0.8 Ma. Across the Pleistocene, no significant change in mean CH ₄ is observed but we find a small, 25 ppm decline in CO ₂ from 2.9 to 1.2 Ma followed by stable mean CO ₂ across the mid-Pleistocene Transition. In late Pliocene samples (ranging from 2.8-3.1 Ma), trapped air is impacted by the addition of CO ₂ from respired organic matter. Corrections using the stable carbon isotopes of CO ₂ indicate that atmospheric CO ₂ in these late Pliocene samples is within the range measured in the early Pleistocene (<300 ppm). Observed changes in greenhouse gases are small relative to both local and global cooling observed in the same ice cores (Shackleton et al., 2024a, b) and independent records from marine sediments, suggesting that other components of Earth’s climate system contributed to global cooling over the last 3 million years.