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The total electron content (TEC) is important among others for Global Navigation Satellite Systems/GPS signal propagation and applications. However, there is only one comprehensive analysis of TEC trends, and the resulted trends are not consistent with trends in other ionospheric parameters. Here we use the TEC data of Lean et al. (2011) and the JPL35 homogeneous TEC data series derived by Emmert et al. (2017). This analysis results in three main conclusions: (1) Too positive TEC trends by Lean et al. (2011) are caused by data problems in 1995-2001, particularly by too low Center for Orbit Determination data. (2) TEC reveals a weak negative trend at the edge of reliability; no trend or trend break is also possible; longer data series than 1994-2015 is required. (3) About 99% of the total variance of yearly average global TEC values is explained by variability of solar activity.

Plain Language Summary The total electron content (TEC) is a number of free electrons in unit column throughout the ionosphere. It is important as global ionospheric characteristic and also for the GPS signal propagation and applications to positioning. The ionosphere is changing on long-term scale mainly due to the increasing concentration of greenhouse gases in the atmosphere. However, there is only one comprehensive analysis of TEC trends, and the resulted trends are not consistent with trends in other ionospheric parameters. Our analysis shows that (1) too positive TEC trends by Lean et al. (2011) are caused by data problems in 1995-2001, which were not known in 2011. (2) TEC reveals a weak negative trend at the edge of reliability, not a positive trend.