The Earth has undergone a significant climate switch from greenhouse to icehouse during the Plio-Pleistocene transition ( PPT) around 2.7-2.4 million years ago (Ma), marked by the intensification of the Northern Hemisphere glaciation ( NHG) ~2.7 Ma. Evidence based on oceanic CO2 [( CO2)aq], supposed to be in close equilibrium with the atmospheric CO2 [( CO2)atm], suggests that the CO2 decline might drive such climate cooling. However, the rarity of direct evidence from [ CO2]atm during the interval prevents determination of the atmospheric CO2 level and further assessment on the impact of its fluctuation. Here, we reconstruct the [ CO2]atm level during 2.77-2.52 Ma based on a new developed proxy of stomatal index on Typha orientalis leaves from Shanxi, North China, and depict the first [ CO2]atm curve over the past 5 Ma by using stomata-based [ CO2]atm data. Comparisons of the terrestrial-based [ CO2]atm and the existed marine-based [ CO2]aq curves show a similar general trend but with different intensity of fluctuations. Our data reveal that the high peak of [ CO2]atm occurred at 2.77-2.52 Ma with a lower [ CO2]aq background. The subsequent sharp fall in [ CO2]atm level might be responsible for the intensification of the NHG based on their general temporal synchronism. These findings shed a significant light for our understanding toward the [ CO2]atm changes and its ecological impact since 5 Ma. [ABSTRACT FROM AUTHOR]