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 degrees 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 degrees C target is met, objects in Low Earth Orbit (LEO) will have orbital lifetimes around 30% longer than comparable objects from the year 2000. Plain Language Summary The atmosphere extends upwards into the lower regions of space. Here, carbon dioxide causes cooling of the atmosphere and a decrease in atmospheric density. These density reductions have been simulated for increasing CO2 concentrations up to an altitude of 500 km by computationally modeling the Earth's atmosphere. For reference, the International Space Station orbits at around 400 km. Density reductions up to the year 2100 have been given for the four CO2 concentration scenarios published by the Intergovernmental Panel on Climate Change (IPCC). The model has shown there has already been a 17% decrease in atmospheric density at an altitude of 400 km since the year 2000. This will reach a maximum of 30% if the Paris Agreement target to limit global warming to 1.5 degrees C is met. Objects in low Earth orbit travel through the thin, upper atmosphere. A reduction in density at these altitudes means a decrease in the amount of atmospheric drag that orbiting objects experience. This increases the amount of time it takes for their orbit to decay. If the 1.5 degrees C target is met, orbital lifetimes will be 30% longer than those in the year 2000. Key Points altitude Thermospheric neutral density at 500 km altitude lowers by over 80% with a high ground-level carbon dioxide concentration of 890 ppm Meeting the 1.5 degrees C Paris Agreement target limits the reduction in neutral density at 400 km since the year 2000 to around 28% Objects in Low Earth Orbit (LEO) will have orbital lifetimes around 30% longer at the 1.5 degrees C target than comparable objects from the year 2000