Atmospheric CO during the Mid-Piacenzian Warm Period and the M2 glaciation.

The Piacenzian stage of the Pliocene (2.6 to 3.6 Ma) is the most recent past interval of sustained global warmth with mean global temperatures markedly higher (by 2-3 °C) than today. Quantifying CO levels during the mid-Piacenzian Warm Period (mPWP) provides a means, therefore, to deepen our understanding of Earth System behaviour in a warm climate state. Here we present a new high-resolution record of atmospheric CO using the δB-pH proxy from 3.35 to 3.15 million years ago (Ma) at a temporal resolution of 1 sample per 3-6 thousand years (kyrs). Our study interval covers both the coolest marine isotope stage of the mPWP, M2 (3.3 Ma) and the transition into its warmest phase including interglacial KM5c (centered on ~3.205 Ma) which has a similar orbital configuration to present. We find that CO ranged from [Formula: see text]ppm to [Formula: see text]ppm, with CO during the KM5c interglacial being [Formula: see text]ppm (at 95% confidence). Our findings corroborate the idea that changes in atmospheric CO levels played a distinct role in climate variability during the mPWP. They also facilitate ongoing data-model comparisons and suggest that, at present rates of human emissions, there will be more CO in Earth's atmosphere by 2025 than at any time in at least the last 3.3 million years.