Whole Atmosphere Climate Change: Dependence on Solar Activity

Abstract We conducted global simulations of temperature change due to anthropogenic trace gas emissions, which extended from the surface, through the thermosphere and ionosphere, to the exobase. These simulations were done under solar maximum conditions, in order to compare the effect of the solar cycle on global change to previous work using solar minimum conditions. The Whole Atmosphere Community Climate Model‐eXtended was employed in this study. As in previous work, lower atmosphere warming, due to increasing anthropogenic gases, is accompanied by upper atmosphere cooling, starting in the lower stratosphere, and becoming dramatic, almost 2 K per decade for the global mean annual mean, in the thermosphere. This thermospheric cooling, and consequent reduction in density, is less than the almost 3 K per decade for solar minimum conditions calculated in previous simulations. This dependence of global change on solar activity conditions is due to solar‐driven increases in radiationally active gases other than carbon dioxide, such as nitric oxide. An ancillary result of these and previous simulations is an estimate of the solar cycle effect on temperatures as a function of altitude. These simulations used modest, five‐member, ensembles, and measured sea surface temperatures rather than a fully coupled ocean model, so any solar cycle effects were not statistically significant in the lower troposphere. Temperature change from solar minimum to maximum increased from near zero at the tropopause to about 1 K at the stratopause, to approximately 500 K in the upper thermosphere, commensurate with the empirical evidence, and previous numerical models.