Concerns about climate change often focus on Earth’s lower atmosphere, where most of the weather we experience occurs. However, climate change also affects the upper atmosphere. Understanding climate trends in the upper atmosphere could aid many applications, such as planning satellite missions and interpreting their data, managing space debris, and assessing the risk of disruptive space weather.

Research by Cnossen provides new insights into trends and drivers of upper atmosphere climate change, highlighting the important roles of both carbon dioxide and Earth’s magnetic field.

Although the lower atmosphere has been warming, the upper atmosphere—above 100 kilometers in altitude—has been cooling in recent decades. Previous research suggests that this cooling trend is driven by a combination of greenhouse gas emissions, shifts in Earth’s magnetic field, and long-term variations in solar and geomagnetic activity associated with the solar cycle.

To further understand these drivers, Cnossen used the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X) to simulate changes in atmospheric temperature and density from Earth’s surface to an altitude of 500 kilometers between 1950 and 2015. The analysis included careful accounting of the effects of the solar cycle—typically a major challenge in upper atmosphere research. After factoring in these effects, the simulation confirmed earlier suggestions that rising carbon dioxide levels are the main driver of long-term cooling in the portion of the upper atmosphere known as the thermosphere. However, long-term shifts in Earth’s magnetic field also appear to play a significant role in thermospheric climate change toward the North and South Poles.

The analysis also addressed the ionosphere, the charged portion of the upper atmosphere. The simulation suggested that long-term changes in ionospheric density are driven by both carbon dioxide and Earth’s magnetic field. Magnetic field effects on the ionosphere’s climate are particularly pronounced above a region that stretches roughly from northeastern South America, across the Atlantic, to western Africa.

These findings could help inform future research into upper atmosphere climate change and its long-term implications. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2020JA028623, 2020)

—Sarah Stanley, Science Writer