Future Decreases in Thermospheric Neutral Density in Low Earth Orbit due to Carbon Dioxide Emissions

Increasing carbon dioxide causes cooling in the upper atmosphere and a secular decrease in atmospheric density over time. With the use of the Whole Atmospheric Community Climate Model with thermosphere and ionosphere extension (WACCM-X), neutral thermospheric densities up to 500 km have been modeled under increasing carbon dioxide concentrations. Only carbon dioxide and carbon monoxide concentrations are changed between simulations, and solar activity is held low at F_10.7 = 70 throughout. Neutral density decreases through to the year 2100 have been modeled using four carbon dioxide emission scenarios produced by the Intergovernmental Panel on Climate Change (IPCC). The years 1975 and 2005 have also been simulated, which indicated a historic trend of −5.8% change in neutral density per decade. Decreases in the neutral density relative to the year 2000 have been given for increasing ground-level carbon dioxide concentrations. WACCM-X shows there has already been a 17% decrease in neutral densities at 400 km relative to the density in the year 2000. This becomes a 30% reduction at the 50:50 probability threshold of limiting warming to 1.5°C, as set out in the Paris Agreement. A simple orbital propagator has been used to show the impact the decrease in density has on the orbital lifetime of objects traveling through the thermosphere. If the 1.5°C target is met, objects in Low Earth Orbit (LEO) will have orbital lifetimes around 30% longer than comparable objects from the year 2000.