Future Climate Change in the Thermosphere under Varying Solar Activity
Conditions.
Abstract
Increasing carbon dioxide concentrations in the mesosphere and lower
thermosphere are increasing radiative cooling in the upper atmosphere,
leading to thermospheric contraction and decreased neutral mass
densities at fixed altitudes. Previous studies of the historic neutral
density trend have shown a dependence upon solar activity, with larger
F10.7 values resulting in lower neutral density reductions. To
investigate the impact on the future thermosphere, the Whole Atmosphere
Community Climate Model with ionosphere and thermosphere extension
(WACCM-X) has been used to simulate the thermosphere under increasing
carbon dioxide concentrations and varying solar activity conditions.
These neutral density reductions have then been mapped onto the Shared
Socioeconomic Pathways (SSPs) published by the Intergovernmental Panel
on Climate Change (IPCC). The neutral density reductions can also be
used as a scaling factor, allowing commonly used empirical models to
account for CO$_2$ trends. Under the“best case’ SSP1-2.6 scenario,
neutral densities reductions at 400 km altitude peak (when CO$_2$ =
474 ppm) at a reduction of 13 to 30\% (under high and
low solar activity respectively) compared to the year 2000. Higher
CO$_2$ concentrations lead to greater density reductions, with the
largest modelled concentration of 890 ppm resulting in a 50 to 77
\% reduction at 400 km, under high and low solar
activity respectively.