The rise in carbon dioxide levels in Earth's atmosphere may necessitate a doubling of efforts to clean our increasingly cluttered space environment filled with debris. According to two new studies, greenhouse gases have significantly contributed to the shrinkage of the upper atmosphere, a phenomenon hypothesized by scientists for decades and now observed for the first time. While some of the observed shrinkage is natural and will rebound, scientists say that the contribution from carbon dioxide may be permanent.
This means that defunct satellites and other pieces of outdated technology in low Earth orbit are likely to remain in place longer due to the weakened resistance of the atmosphere, leading to increased congestion and causing problems for newer satellites and space monitoring operations. Geospatial scientist Martin Melinjak from NASA's Langley Research Center explains, "One result will be that satellites will stay in orbit longer, which is great, as scientists want their satellites to remain up. But debris will also last longer, likely increasing the chance that satellites and other valuable space objects will need to adjust their paths to avoid collisions."
Descriptions of Earth's atmosphere generally delineate layers at specific altitudes, but the reality is that the volume of gases surrounding our world is not static; it expands and contracts in response to various influences, perhaps the most significant of which is the sun. The sun itself is not static either, undergoing cycles of activity, from high to low and back again, roughly every 11 years. We are currently in the midst of such the twenty-fifth cycle since record-keeping began, which started in December 2019. The previous cycle, number 24, was unusually weak even at solar maximum, which enabled Melinjak and colleagues to take measurements of atmospheric shrinkage.
They focused on two layers known collectively as MLT: the mesosphere, which begins at an altitude of about 60 kilometers, and the lower thermosphere, which starts around 90 kilometers. Data from NASA's TIMED satellite, an observatory that collects information about the upper atmosphere, provided measurements of pressure and temperature for MLT over nearly 20 years, from 2002 to 2021. In some of the lower layers of the atmosphere, carbon dioxide creates a warming effect by absorbing and re-emitting infrared radiation in all directions, effectively trapping some of it.
However, some of the infrared radiation emitted by carbon dioxide escapes into space, effectively carrying heat away and cooling the upper atmosphere. The higher the carbon dioxide levels, the cooler the atmosphere becomes. The team stated, "We already knew that this cooling causes a contraction of the stratosphere. Now we can see that it does the same for the mesosphere and the thermosphere above it." Using data from TIMED, Melinjak and his team found that MLT shrank by approximately 1,333 meters, with about 342 meters attributed to radiative cooling from carbon dioxide.
Melinjak remarked, "There has been a lot of interest in whether we could actually observe this cooling and contraction effect on the atmosphere. We are finally providing these observations in this paper. We are the first to show global atmospheric shrinkage of this kind." Considering that the thermosphere extends several hundred kilometers, 342 meters may not seem substantial. However, a research paper published last September by physicist Ingrid Knosson from the British Antarctic Survey indicated that cooling in the thermosphere could lead to a 33% decrease in atmospheric drag by 2070. Atmospheric drag helps satellites and rocket stages enter orbit after their missions are complete, and Knosson found that this decrease in drag could extend the orbital lifetime of defunct space debris by 30% by 2070.
As more satellites are launched into low Earth orbit, this issue will worsen, with no true mitigation measures on the horizon—either to reduce the number of satellites or to lower carbon dioxide levels.
