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 similar to 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 delta C-13 value of terrestrial C-3 plant remains, using a method applicable to the entire similar to 400 m.y. history of C-3 photosynthesis on land. Across the past 23 my., CO2 likely ranged between similar to 230 ppmv and 350 ppmv (68% confidence interval: similar to 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 (84th percentile: similar to 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-clay disruption of long-established CO2 trends within Earth's atmosphere.