Observations of increasing carbon dioxide concentration in Earth's thermosphere

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 layer1,2. Anthropogenic CO2 increases are expected to propagate upward throughout the entire atmosphere, which should result in a cooler, more contracted thermosphere3–5. This contraction, in turn, will reduce atmospheric drag on satellites and may have adverse consequences for the orbital debris environment that is already unstable6,7. However, observed thermospheric mass density trends derived from satellite orbits are generally stronger than model predictions8,9, indicating that our quantitative understanding of these changes is incomplete. So far, CO2 trends have been measured only up to 35 km altitude10–12. 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. In the mesosphere (50–90 km altitude) and thermosphere (>90 km), almost all carbon is partitioned between CO2 and CO through ultraviolet photolysis of CO2 to create CO and chemical loss of CO to re-form CO2 (refs 1,13). The photochemistry and atmospheric dynamics introduce solar cycle, seasonal and latitudinal dependences into the partitioning and the overall carbon abundance13,14. The CO2 volume mixing ratio (VMR) is approximately constant up to 65–80 km and falls off rapidly at higher altitudes owing to molecular diffusion and photolysis13. The proportion of carbon in the form of CO increases from less than 3% below 80 km to more than 20% above 100 km (refs 13,14). CO2 cools the mesosphere and thermosphere through collisional excitation (by atomic oxygen) of its bending vibrational mode and subsequent 15 μm band emission, most of which escapes from the thin upper atmosphere1,2. We analysed temporal changes in CO2 and CO VMRs measured by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer15 (ACE-FTS) from April 2004 to September 2011, using the approach described in Methods. Figure 1 shows the CO2