Whole Atmosphere Climate Change: Dependence on Solar Activity

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Thursday, May 9, 2019

Solomon and colleagues conducted global simulations of temperature change due to emissions of trace gases due to human activity, that extended from the surface, throughout the atmosphere, to space. These simulations were done under conditions of high solar activity, in order to compare the effect of the solar cycle on global change to previous work using low solar activity.

Graph of global simulations of temperature change due to emissions of trace gases
Model calculations of the anthropogenic and solar cycle variation in global mean annual mean temperature. (a) Temperature profiles as a function of altitude. Blue: 1972 to 1976 ensemble, solar minimum; red: 2001 to 2005 ensemble, solar minimum; cyan: 1972 to 1976 ensemble, solar maximum; ma-genta: 2001 to 2005 ensemble, solar maximum. (b) Temperature change between the two epochs. Blue: solar minimum conditions; red: solar maximum conditions. (c) Temperature change with solar activity. Black: solar max - solar min for the 1972 to 1976 ensemble; green: solar max - solar min for the 2001 to 2005 ensemble. (d) Same as (c) but with an expanded altitude scale to show the lower and middle atmosphere. Changes of less than 0.2 degree, below ~10 km, are not considered statistically significant for these modest ensemble sizes, and also note that lower troposphere temperatures are largely controlled by the use of measured sea-surface temperatures as a lower boundary condition.

The Whole Atmosphere Community Climate Model–eXtended was employed in this study. As in previous work, lower atmosphere warming, due to increasing emissions of greenhouse gases, is accompanied by upper atmosphere cooling, starting in the lower stratosphere, and becoming dramatic, almost 2 K per decade on average, above 100 km altitude. This upper atmosphere cooling, and consequent reduction in density, is less than the almost 3 K per decade for low solar activity conditions calculated in previous simulations. This dependence of global change on solar activity is due to solar-driven increases in other gases other that cool the thermosphere, so greenhouse gases such as carbon dioxide have less effect. An ancillary result of these and previous simulations is an estimate of the solar cycle effect on temperatures as a function of altitude. There weren’t enough repeated runs to tell if there were any solar cycle effects in the lower atmosphere, but at higher altitudes, temperature change from solar minimum to maximum increased from near-zero at about 15 km, to approximately 1 K at about 50 km, to approximately 500 K at about 400 km, commensurate with observations and previous modeling studies.

Publication Name: Journal of Geophysical Research Space Physics
First HAO Author's Name: Stanley C. Solomon