Global Long‐Term Trends in Ionospheric TEC From GNSS Observations With Local‐Time Dependence

Long‐term ionospheric changes are expected as a consequence of thermospheric cooling driven by increasing greenhouse gas concentrations. We analyze global ionospheric total electron content (TEC) derived from GNSS observations for 2000–2024 using the high‐resolution ISEE data set. Solar‐driven variability is removed with a two‐step regression approach using multiple solar activity proxies. For the first time, trends are resolved for all local times. A persistent global decline in TEC is found throughout the diurnal cycle when solar flux proxies are used, with the strongest negative trends near noon and at low latitudes. Global mean trends reach about −0.12 TECU/year at noon when filtered with F30, while daily mean trend values are approximately −0.06 TECU/year. Nighttime trends are weaker but remain negative. Even though reduced ionization may improve GNSS positioning for some users, it alters the propagation environment for communication and surveillance systems that rely on ionospheric reflection or refraction.