Climate Change in the Thermosphere and Ionosphere From the Early Twentieth Century to Early Twenty‐First Century Simulated by the Whole Atmosphere Community Climate Model—eXtended.

Motivated by numerous lower atmosphere climate model hindcast simulations, we performed simulations of the Earth's atmosphere from the surface up through the thermosphere‐ionosphere to reveal for the first time the century scale changes in the upper atmosphere from the 1920s through the 2010s using the Whole Atmosphere Community Climate Model—eXtended (WACCM‐X v. 2.1). We impose solar minimum conditions to get a clear indication of the effects of the long‐term forcing from greenhouse gas increases and changes of the Earth's magnetic field and to avoid the requirement for careful removal of the 11‐year solar cycle as in some previous studies using observations and models. These previous studies have shown greenhouse gas effects in the upper atmosphere but what has been missing is the time evolution with actual greenhouse gas increases throughout the last century, including the period of less than 5% increase prior to the space age and the transition to the over 25% increase in the latter half of the 20th century. Neutral temperature, density, and ionosphere changes are close to those reported in previous studies. Also, we find high correlation between the continuous carbon dioxide rate of change over this past century and that of temperature in the thermosphere and the ionosphere, attributed to the shorter adjustment time of the upper atmosphere to greenhouse gas changes relative to the longer time in the lower atmosphere. Consequently, WACCM‐X future scenario projections can provide valuable insight in the entire atmosphere of future greenhouse gas effects and mitigation efforts. Plain Language Summary: From seeing lower atmosphere computer climate models run for the past century, we decided to do the same for the upper atmosphere using the Whole Atmosphere Community Climate Model‐eXtended (WACCM‐X) for the decades from the 1920s to 2010s. In this higher region, the atmosphere is affected strongly by the Sun and removing the Sun's effect is tricky in previous observation and model studies. We make the Sun's effect small to see only effects from the Earth's magnetic field and greenhouse gases. Earlier studies focused on recent decades show effects of greenhouse gas increases on the upper atmosphere but not for the early decades of the past century with greenhouse gas changes from less than 5% increase prior to the space age and the transition to the over 25% increase in the latter half of the 20th century. We cover this entire period and get results like those in studies before, with especially the temperature change matching very well with the greenhouse gas carbon dioxide change. Because WACCM‐X performs well over the past century, it will be useful to predict what will happen in the century ahead as greenhouse gases increase and humans make efforts to reverse the increase. Key Points: Whole Atmosphere Community Climate Model‐eXtended (WACCM‐X) simulated continuous rate of climate change in the thermosphere/ionosphere over the past century follows closely that of CO2 and CH4Large CO2/thermosphere temperature correlation shows greenhouse gas impact on upper atmosphere climate is consistent over the past centuryConsistency with past century observations and model simulations gives confidence in results of WACCM‐X 21st century climate projections [ABSTRACT FROM AUTHOR]