A rise in atmospheric CO2 is believed to be necessary for the termination of large-scale glaciations. Although the Antarctic Ice Sheet is estimated to have melted from ∼125% to ∼50% its modern size, there is thus far no evidence for an increase in atmospheric CO2 associated with the Mi-1 glacial termination in the earliest Miocene. Here, we present evidence from a high-resolution terrestrial record of leaf physiological change in southern New Zealand for an abrupt increase in atmospheric CO2 coincident with the termination of the Mi-1 glaciation and lasting approximately 20 kyr. Quantitative pCO2 estimates, made using a leaf gas exchange model, suggest that atmospheric CO2 levels may have doubled during this period, from 516±111ppm to 1144±410ppm, and subsequently returned back to 425±53ppm. The 20-kyr interval with high pCO2 estimates is also associated with a period of increased moisture supply to southern New Zealand, inferred from carbon and hydrogen isotopes of terrestrial leaf waxes. The results provide the first high-resolution record of terrestrial environmental change at the Oligocene/Miocene boundary, document a ∼20 kyr interval of elevated pCO2 and increased local moisture availability, and provide insight into ecosystem response to a major orbitally driven climatic transition.