A 23 m.y. record of low atmospheric CO₂

Current atmospheric CO₂ concentration is known to be higher than it has been during the past ~800 k.y. of Earth history, based on direct measurement of CO₂ within ice cores. A comparison to the more ancient past is complicated by a deficit of CO₂ proxies that may be applied across very long spans of geologic time. Here, we present a new CO₂ record across the past 23 m.y. of Earth history based on the δ¹³C value of terrestrial C₃ plant remains, using a method applicable to the entire ~400 m.y. history of C₃ photosynthesis on land. Across the past 23 m.y., CO₂ likely ranged between ~230 ppmv and 350 ppmv (68% confidence interval: ~170-540 ppm). CO₂ was found to be highest during the early and middle Miocene and likely below present-day levels during the middle Pliocene (84^th percentile: ~400 ppmv). These data suggest present-day CO₂ (412 ppmv) exceeds the highest levels that Earth experienced at least since the Miocene, further highlighting the present-day disruption of long-established CO₂ trends within Earth's atmosphere.