Fossils from Paleogene High Arctic deposits provide some of the clearest evidence of greenhouse climates and offer the potential to improve our understanding of Earth system dynamics in a largely ice-free world. One of the most well-known and stunningly-preserved polar forests, Napartulik, crops out of middle Eocene (47.9-37.8 Ma) sediments on eastern Axel Heiberg Island, Nunavut, Canada. An abundance of data from Napartulik suggest mean annual temperatures over 30 °C warmer than today and atmospheric water loads 2× above current levels. Despite decades of intensive paleobotanical, paleoecological, and paleoclimatological work, there are currently no constraints on atmospheric CO2 levels from Napartulik or from any other High Arctic forest site. This lack of CO2 information strongly limits our ability to understand high-latitude climate dynamics during globally-warm times. Here, I test a new gas-exchange based approach for reconstructing paleo-CO2 on living Metasequoia glyptostroboides Hu et Cheng and then apply the model to fossil M. occidentalis (Newberry) Chaney from six forest layers at Napartulik. Reconstructed modern CO2 of 445 ppm (354-558 ppm at 95% confidence) is in good agreement with the current ambient CO2 (396 ppm) and validates model application to Metasequoia Miki. Individual fossil forest layer reconstructions vary between 405-489 ppm with a site mean reconstruction of 437 (337-564 ppm at 95% confidence). These reconstructions provide the first direct evidence for CO2 from a High Arctic polar forest and indicate that the warm, equable conditions present at Napartulik during the middle Eocene were maintained under CO2 concentrations ~1.6× above pre-industrial levels. These results are consistent with high long-term climate sensitivity to CO2, even during times with little ice, suggesting that some fundamental climate feedbacks and/or forcings for enhancing polar warmth during greenhouse periods remain poorly understood.