Analysis and Attribution of Climate Change in the Upper Atmosphere From 1950 to 2015 Simulated by WACCM‐X

Abstract

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 CO 2 , thereby including all known drivers of upper atmosphere climate change. Analysis of the full 1950-2015 interval with a standard multilinear 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, K P , and the trend itself. Comparisons with previous studies and analysis of spatial variations in trend estimates confirmed that the increase in CO 2 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 h m F 2 , N m F 2 , and total electron content indicate a superposition of CO 2 and geomagnetic field effects, with the latter dominating trends in the region of ∼50-20 • N, ∼60 • W to 20 • E. Additional model experiments to investigate the indirect dynamical effects of climate change in the lower atmosphere (<50 km) on the upper atmosphere (>100 km) suggested that these effects are small and insignificant. However, current model limitations could mean that these effects are underestimated. Plain Language Summary A simulation with a state-of-the-art global model of the atmosphere from the surface up to about 500-km altitude was done for the period 1950-2015 to study climate change in the upper atmosphere (above 100-km altitude). The simulation included realistic variations in solar and geomagnetic activity, the Earth's magnetic field, and trace gas emissions, including CO 2. Solar and geomagnetic activity variations occur naturally as part of the ∼11-year solar cycle and have a large effect on the upper atmosphere. Still, by removing these effects as much as possible, we showed that the increase in CO 2 concentration is the main cause of cooling in the upper atmosphere, while effects of magnetic field changes also play a role near the poles, especially in the Northern Hemisphere. Long-term trends in the charged portion of the upper atmosphere, the ionosphere, are caused by both CO 2 and magnetic field effects, with the latter being most important in the region of ∼50 • S to 20 • N, ∼60 • W to 20 • E. Additional model experiments to investigate effects of climate change in the lower atmosphere (<50 km) on the upper atmosphere suggested that these are small. However, they might be underestimated by the model.

Climate Changes in the Upper Atmosphere: Contributions by the Changing Greenhouse Gas Concentrations and Earth's Magnetic Field From the 1960s to 2010s

“…Previous studies found that CO 2 plays a dominant role in the cooling trend in the thermosphere, with very minor contributions from the other three GHGs (∼95% vs. ∼5%) (e.g., Akmaev et al, 2006;Qian et al, 2013). Cnossen (2020) employed multilinear regression analysis to a transient WACCM-X simulation from 1950 to 2015 to estimate and attribute climate change in the T-I. Spatial patterns of trends in hmF2 and NmF2 indicated a superposition of CO 2 and geomagnetic field effects, with the latter dominating trends in the region of ∼50-20°N, ∼60°W to 20°E.…”

“…Cnossen (2020) employed multilinear regression analysis to a transient WACCM‐X simulation from 1950 to 2015 to estimate and attribute climate change in the T‐I. Spatial patterns of trends in hmF2 and NmF2 indicated a superposition of CO 2 and geomagnetic field effects, with the latter dominating trends in the region of ∼50–20°N, ∼60°W to 20°E.…”

Review of Long-Term Trends in the Equatorial Ionosphere Due the Geomagnetic Field Secular Variations and Its Relevance to Space Weather

“…Cnossen [10] used the WACCM-X to consider realistic variations in solar and geomagnetic activity, the Earth's magnetic field, and trace gas emissions, including CO2, for the period 1950-2015. Trends due to Earth's magnetic field secular variation were obtained through a multiple linear regression fitting to the modeled output values where solar activity and geomagnetic activity were considered through F10.7 and Kp indices.…”

“…Even though anthropogenic forcing seems to be the main trend driver until now, there are also other ionospheric long-term change forcings of natural origin. Among them is the secular variation of the Earth's magnetic field, which affects not only the electron density, but ionospheric conductivity, currents flowing in the ionosphere and magnetosphere, and radio wave propagation as well [8][9][10][11][12][13]. The ionosphere, as a part of the space weather environment, plays a crucial role through the modulation of the global electrodynamic circuit, its coupling to the magnetosphere and as a key medium for communication, sounding, and navigation.…”

Response of atmospheric carbon dioxide to the secular variation of weakening geomagnetic field in whole atmosphere simulations

“…CO 2 in the model is specified as a boundary condition in the lower thermosphere and is uniformly distributed horizontally, although without consideration of the spatial and temporal variability of CO 2 in the upper atmosphere. Considering the CO 2 variability, Cnossen (2020) analyzed the long‐term database of WACCM‐X using a modified regression method to examine the impact of CO 2 and geomagnetic field changes on upper atmosphere climate change. Based on a whole atmosphere model, GAIA, the CO 2 doubling effect in the upper atmosphere and the dependence of geomagnetic activity upon the CO 2 ‐driven trend have been investigated recently (Liu HX et al, 2020, 2021).…”