Geomagnetic storms are large-scale phenomena that significantly alter the Earth's upper atmosphere - the ionosphere and thermosphere. These changes can have serious impacts on technological systems, including GPS positioning and satellite orbits in low Earth orbit (200–2,000 km). Current understanding suggests that increasing concentrations of greenhouse gases, particularly carbon dioxide (CO2), are leading to a decrease in the neutral density of the thermosphere. This phenomenon is primarily due to the fact that CO2 acts as a radiative cooler at high altitudes, leading to a decrease in temperatures in the mesosphere and thermosphere and a subsequent contraction of the upper atmosphere.The ionosphere is also affected by changes in CO2. The present study aims to understand how changes in the state of the upper atmosphere caused by increases in CO2 alter its response to geomagnetic storms.

The Community Earth System Model (CESM) with the Whole Atmosphere and Thermosphere-Ionosphere Extension (WACCM-X) was used to examine these impacts. This comprehensive model includes interactions between different components of the Earth system, from the surface to the upper thermosphere (approximately 500–700 km). For the study, simulations of a geomagnetic superstorm similar to the one in May 2024 were performed for four scenarios with different surface CO2 concentrations: 403, 500, 652 and 918 ppmv, corresponding to the years 2016, 2040, 2061 and 2084. These years were chosen intentionally, as they were close to solar minimum and had similar levels of solar flux, allowing for a better isolation of the impact of CO2. The goal of these simulations is to provide a first assessment of how changes in greenhouse gas concentrations may affect the response of the near-space environment to a strong geomagnetic storm.

Thermosphere response to storms with increasing CO2: CESM(WACCM-X) simulations revealed important findings regarding the neutral density of the thermosphere at an altitude of 350 km. Rising CO2 levels weaken the absolute neutral density thermosphere response to storm surge. In absolute terms, the change in neutral density during a storm is largest at present and gradually decreases with increasing CO2 concentrations. This reduced response is partly due to the pre-existing background decrease in neutral density due to the increase in CO2. Conversely, the relative response (percentage change) of neutral density increases with increasing CO2 levelsThis phenomenon is consistent with previous results and is partly due to a smaller change in the absolute response compared to the significant decrease in the neutral density background. The physical mechanisms behind these changes are primarily Joule heating and cooling with nitric oxide (NO). At higher CO2 concentrations, the storm-induced Joule heating change weakens above about 140 km. Similarly, the storm-induced NO cooling change weakens. Since the decrease in Joule heating is larger than the decrease in NO cooling, this leads to a smaller absolute change in the neutral density of the thermosphere with increasing CO2 concentrations. The rate of thermospheric recovery after a storm also changes, and the post-storm cooling weakens.

Ionosphere response to storms with increasing CO2: The ionosphere's response to geomagnetic storms is influenced by greenhouse gas concentrations, although the changes are more complex and show considerable variability depending on altitude, local time, and region. The study showed that In absolute terms, the ionospheric response, measured as the change in total electron content (ΔTEC), weakens with increasing CO2 levels.This means that both positive and negative storm effects are diminishing. By 2084, with CO2 increasing from about 400 to 900 ppmv, these differences from 2016 could be significant, reaching or exceeding 50 % of total storm TEC change. However, relative changes in ΔTEC are more complex and show areas of both enhanced and weakened storm response.. For example, the relative increase in TEC at polar latitudes, which is caused by enhanced ion production due to auroral particle precipitation, is larger with increasing CO2 concentrations. Similarly, the relative increase in TEC at low latitudes and in the mid-latitudes of the Southern Hemisphere is larger. The influence of CO2 on the ionospheric response is related to changes in the thermospheric composition (O/N2 ratio) and meridional winds, which are the main factors influencing the ionospheric response to geomagnetic storms. The weakened storm-induced Joule heating contributes to the weakening of the O/N2 decrease at mid- to high latitudes, which may explain the smaller absolute storm-induced TEC response in the mid-latitudes of the Northern Hemisphere. Changes in meridional winds are also affected by increasing CO2 levels, which contributes to regional trends in TEC.

Study brings key findings: the absolute response of the ionosphere and thermosphere to a geomagnetic storm weakens with increasing CO2 concentrationsThe absolute change in thermospheric neutral density decreases by 20 %–25 %, and the ionospheric response by up to 50 % as CO2 increases from about 400 to 900 ppmv. Nevertheless, the relative response of the thermosphere is greater at higher CO2 concentrations, partly due to a significant decrease in the neutral density background. The impact on the ionosphere is more complex, with regional variations in relative TEC changes. These findings are important for understanding and predicting the behavior of the Earth's upper atmosphere in a changing climate. However, limitations of the study must be taken into account, such as the uncertainty in the model's reproduction of the upper atmosphere's response to CO2 changes and the neglect of the influence of geomagnetic field changes. JRi

The study is published in the journal Geophysical Research Letters .