The Aptian-Albian (121.4-100.5 Ma) is a known greenhouse period, with global temperatures 10-15°C warmer than pre-industrial conditions. Given this warmth, it is surprising that the most reliable CO2 estimates from this time (from https://paleo-co2.org/) max out at about 1500 ppm. This implies a very high climate sensitivity (>>4 °C per CO2 doubling). We sought to address this discrepancy with a well-vetted paleo-CO2 proxy based on leaf gas-exchange principles (the Franks model) that requires measurements of stomatal density, stomatal size, and leaf δ13C. We applied this proxy to two species of Pseudotorellia Florin (ginkgoalean) from a single stratigraphic level (PSH261) in the Tevshiin Govi Formation at Tevshiin Govi lignite mine in central Mongolia. Dated as 119.7-100.5 Ma based on U-Pb age from coeval Khukhteeg Formation biostratigraphic constraints accommodate an age as young as the end of the Albian (100.5 Ma). Our median estimated CO2 concentration from 42 leaves of P. resinosa  is 3375 ppm (1920-6436 at 95% confidence); estimates from 9 leaves of P. palustris are lower but overlapping (median = 1878 ppm;  1321-3827 at 95% confidence). The primary reason for the high yet variable CO2 from P. resinosa is its very low stomatal density (mean = 13.4 mm-2); when stomatal densities are this low, small variations propagate to large changes in estimated CO2. Indeed, we find that at least 15 leaves are required before the aggregate estimated CO2 approaches that of the full data set (42 leaves); this is more leaves than the conventional recommendation for minimum sampling (5 leaves). Despite the variability within and between species, we can exclude CO2 concentrations below 1300 ppm at high confidence. Analysis of how the individual inputs to the Franks model affect estimated CO2 are consistent with this view. Overall, a CO2 concentration >1300 ppm (and potentially much higher) during a greenhouse time is much more in keeping with our current understanding of climate sensitivity.