Analysis and attribution of climate change in the upper atmosphere from 1950 to 2015 simulated by WACCM-X

Monitoring climatic changes in the thermosphere and ionosphere and understanding their causes is important for practical purposes. To support this effort and facilitate comparisons between observations and model results, a long transient simulation with the Whole Atmosphere Community Climate Model eXtension (WACCM‐X) from 1950 to 2015 was conducted. This simulation used realistic variations in solar and geomagnetic activity, main magnetic field changes, and trace gas emissions, including CO2, thereby including all known drivers of upper atmosphere climate change. Analysis of the full 1950‐2015 interval with a standard multi‐linear regression approach demonstrated difficulties in removing solar cycle effects sufficiently to obtain reliable trends. Results improved when an (F10.7a)2 was included in the regression model, in addition to terms for F10.7a, KP, and the trend itself. Comparisons with previous studies and analysis of spatial variations in trend estimates confirmed that the increase in CO2 concentration is the main driver of trends in thermosphere temperature and density, but at high (magnetic) latitudes effects of main magnetic field changes play a role as well, especially in the Northern hemisphere. Spatial patterns of trends in hmF2, NmF2, and total electron content indicate a superposition of CO2 and geomagnetic field effects, with the latter dominating trends in the region of ~50°S‐20°N, ~60°W‐20°E. Additional model experiments to investigate the indirect dynamical effects of climate change in the lower atmosphere (100 km) suggested that these effects are small and insignificant. However, current model limitations could mean that these effects are underestimated