Carbon dioxide occurs naturally throughout Earth's atmosphere. In the thermosphere, CO2 is the primary radiative cooling agent and fundamentally affects the energy balance and temperature of this high-altitude atmospheric layer(1,2). Anthropogenic CO2 increases are expected to propagate upward throughout the entire atmosphere, which should result in a cooler, more contracted thermosphere(3-5). This contraction, in turn, will reduce atmospheric drag on satellites and may have adverse consequences for the orbital debris environment that is already unstable(6,7). However, observed thermospheric mass density trends derived from satellite orbits are generally stronger than model predictions(8,9), indicating that our quantitative understanding of these changes is incomplete. So far, CO2 trends have been measured only up to 35 km altitude(10-12). Here, we present direct evidence that CO2 concentrations in the upper atmosphere-probably the primary driver of long-term thermospheric trends-are increasing. We analyse eight years of CO2 and carbon monoxide mixing ratios derived from satellite-based solar occultation spectra. After correcting for seasonal-latitudinal and solar influences, we obtain an estimated global increase in COx (CO2 and CO, combined) concentrations of 23.5 +/- 6.3 ppm per decade at an altitude of 101 km, about 10 ppm per decade faster than predicted by an upper atmospheric model. We suggest that this discrepancy may explain why the thermospheric density decrease is stronger than expected.