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Although progress has been made in recent years in reconstructing the environmental conditions at the Triassic/Jurassic Boundary (TJB), published records of atmospheric CO(2) concentrations have been of low resolution and/or based on multi-taxon estimates. This is addressed here by reconstructing CO(2) concentrations across the TJB using stomatal frequencies of four phylogenetically and ecologically distinct plant groups from two depositionally, geographically and taphonomically separate boundary sections in East Greenland and Northern Ireland, with stomatal proxy methods and regression analysis. The resulting CO(2) records then are compared with an additional existing TJB record from a geological section in Sweden. The final results indicate that pre-TJB (Rhaetian), the CO(2) concentration was approximately 1000 ppm, that it started to rise steeply pre-boundary and had doubled to around 2000-2500 ppm at the TJB. The CO(2) concentration then remained elevated for some time post-boundary, before returning to pre-TJB levels in the Hettangian. These results are in very good accordance with published C-isotope, fire and leaf dissection records, and clearly indicate steeply rising and lingering CO(2) concentration at the TJB., authorCount :3