Continuous records of δ11B-CO2 covering the Plio-Pleistocene boundary and the Mid Pleistocene Transition show orbital carbon-climate coupling.

The Pliocene-Pleistocene transition and Middle Pleistocene Transition (3.4–2.5 million years ago and ~1.2-0.6 million years ago respectively) represent major shifts in the Earth’s climate, with both being associated with global cooling, sustained and transient changes in ocean circulation, and the development and stabilization of large ice sheets in the northern hemi- sphere. These ice sheets waxed and waned over the last 2.5 million years and are the key mode of climate variability in this ice house world. Knowledge of the relationship of climate and CO₂ on this timescale has to date been hampered by low resolution and imprecise records of CO₂ once beyond the reach of the ice core records. Here we show orbitally resolved and multisite records of CO₂ from boron isotopes across both transitions, and progress towards a highly resolved multi-basin stack of records. We find a persistent relationship between CO₂ and climate state, which implicates CO₂ decrease as a major contributor to both climate transitions, but also highlights non-linear responses in temperature and sea level as well as significant leads and lags on orbital timescales. Our findings confirm that changes in atmospheric CO₂ play a key role in long-term Plio-Pleistocene climate and implicate the repeating transfer of carbon from the atmosphere to the ocean as a key mechanism in major climate transitions of the last 3 million years.