Continuous Changes in the Thermosphere and Ionosphere over the 20th and 21st Centuries as Simulated by the Whole Atmosphere Community Climate Model - eXtended (WACCM-X)

Although research into the change of the Earth’s climate and its connection to greenhouse gases began many decades earlier, the results and conclusions of this research have more recently become widespread and controversial, especially outside the scientific community. Just as in many other areas of scientific research, the realm of surface climate change research is very complex. This is clear when comparing the change of surface temperature with the increase of carbon dioxide during the 20th century. There are periods, especially in the mid-twentieth century, when this proposed connection does not seem robust. This has introduced skepticism among those who are not exposed to the complexities introduced by factors such as the variation of aerosols and atmosphere-ocean interactions, causing many to question, and even disregard, the enormous impact of human activities on climate. But what about this connection of greenhouse gases to climate at levels above the surface? Can these types of studies lend any support to this connection? In the upper atmosphere above 100 kilometers, research has been ongoing for decades looking at changes in observations during the space age. Model studies have examined both simulations with a doubling of greenhouse gases and simulations decades apart with a large difference in greenhouse gases. These modeling and observational studies demonstrate that carbon dioxide (CO2) causes warming in the troposphere but cooling in the upper atmosphere. What is not clear until now is how the upper atmosphere changes over the entire 20th century, especially during periods when the correspondence of greenhouse gases and surface changes is not as convincing. To address this, we use the Whole Atmosphere Community Climate Model - eXtended (WACCM-X), part of the Community Earth System Model (CESM) version 2.0. WACCM-X is being developed at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. We use simulation results from WACCM-X employing a decadal time slice procedure at a fixed solar activity level for the 20th and early 21st century. Our results reveal, over this entire time period, the strong connection between greenhouse gases and the changes in the upper atmosphere, especially temperature and density. This consistent role that greenhouse gases have played in climate changes in the atmosphere is clear in the upper atmosphere even during time periods when this consistency is not as clear in the lower atmosphere. In addition, we take a first look at the future changes in the upper atmosphere through a single model simulation through the 21st and into the early 22nd century with the assumption that greenhouse gases continue to increase at the recent rate. Although an ensemble of simulations is necessary for a rigorous study of changes, we can initially examine this unique single realization and, for the first time, get an idea of continuous future changes in the upper atmosphere if greenhouse gases continue to increase into the next century.