Thermospheric hydrogen response to increases in greenhouse gases

<p>We investigated thermospheric hydrogen response to increase in greenhouse gases and the dependence of this response to solar activity, using a global mean version of the National Center for Atmospheric Research Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model. We separately doubled carbon dioxide (CO₂) and methane (CH₄) to study the influence of temperature and changes to source species for hydrogen. Our results indicate that both CO₂ cooling and CH₄ changes to the source species for hydrogen lead to predicted increases in the upper thermospheric hydrogen density. At 400 km, hydrogen increases ~30\% under solar maximum and ~25\% under solar minimum responding to doubling of CH₄, indicating that hydrogen response to the source variation due to CH₄ increase is relatively independent of solar activity. On the other hand, hydrogen response to doubling of CO₂ highly depends on solar activity. At 400 km, doubling of CO₂ results in an ~7\% hydrogen increase at solar maximum, whereas it is ~25\% at solar minimum. Consequently, at solar maximum, the predicted ~40\% increase in atomic hydrogen in the upper thermosphere is primarily due to the source variation as a result of doubling of CH₄, whereas at solar minimum, both cooling due to doubling of CO₂ and the source variation due to doubling of CH₄ have commensurate effects, resulting in an approximate 50\% increase in the modeled upper thermospheric hydrogen.</p>