Rising levels of carbon dioxide in Earth’s atmosphere could exacerbate efforts to clean our increasingly cluttered shell of orbiting space debris.
Greenhouse gas has contributed significantly to the contraction of the upper atmosphere, according to two new studies. This contraction has been assumed for decades; now, for the first time, this has actually been observed.
Some of the observed shrinkage is normal and will rebound; but the contribution made by CO2 is, according to the scientists, probably permanent.
This means that obsolete satellites and other pieces of old technology in low Earth orbit will likely stay in place longer due to reduced atmospheric drag, cluttering the region and causing problems for new satellites and space observations. .
“One of the consequences is that satellites will stay up longer, which is great because people want their satellites to stay up,” says geospatial scientist Martin Mlynczak of NASA’s Langley Research Center.
“But the debris will also stay in place longer and likely increase the likelihood that satellites and other valuable space objects will need to adjust their trajectory to avoid collisions.”
Descriptions of Earth’s atmosphere usually define layers at specific altitudes, but the truth is that the volume of gas surrounding our world is not static. It expands and contracts in response to various influences, the most important of which is probably the Sun.
Now the Sun is also not static. It goes through cycles of activity, up and down, and vice versa, approximately every 11 years. We are currently in the middle of the 25th such cycle since the beginning of the calculation, a cycle which began around December 2019. The previous cycle, number 24, was exceptionally moderate even at the height of solar activity, and it is what allowed Mlynczak and his colleagues to take measurements of atmospheric contraction.
Their attention focused on two layers, collectively known as MLT: the mesosphere, which begins at about 60 kilometers (37 miles) above sea level; and the lower thermosphere, which begins about 90 kilometers away.
Data from NASA’s TIMED satellite, an observatory collecting data on the upper atmosphere, provided them with pressure and temperature information for the MLT over a period of nearly 20 years, from 2002 to 2021.
In some lower layers of the atmosphere, CO2 creates a warming effect by absorbing and re-emitting infrared radiation in all directions, effectively trapping some of it.
However, in the much, much thinner MLT, some of the infrared radiation emitted by CO2 escapes into space, effectively carrying away heat and cooling the upper atmosphere. The higher the CO2, the colder the atmosphere.
We already knew that this cooling causes the stratosphere to contract. Now we can see that this does the same for the mesosphere and the thermosphere above. Using data from TIMED, Mlynczak and his team found that the MLT had contracted about 1,333 meters (4,373 feet). About 342 meters of this is the result of CO2-induced radiative cooling.
“There has been a lot of interest in seeing if we can actually observe this cooling and shrinking effect on the atmosphere,” Mlynczak said.
“We are finally presenting these observations in this paper. We are the first to show atmospheric shrinkage like this, on a global scale.”
Given that the thermosphere spans several hundred kilometres, this 342 meters may not seem like much. However, a paper published in September by physicist Ingrid Cnossen of the British Antarctic Survey in the UK showed that thermospheric cooling could lead to a 33% reduction in atmospheric drag by 2070.
Atmospheric drag is what helps satellites and rocket stages de-orbit after their missions end. This reduction in drag could extend the orbital lifetime of extinct space junk by 30% by 2070, Cnossen found.
As more and more satellites are launched into low Earth orbit, this will become a growing problem, with no real mitigation measures in sight – either to reduce the number of satellites or the amount of CO2.
“At each altitude there is cooling and contraction that we attribute in part to increased carbon dioxide,” says Mlynczak. “As long as carbon dioxide increases at about the same rate, we can expect these rates of temperature change to also remain about constant, within about half a degree Kelvin. [of cooling] per decade.
The research has been published in Journal of Geophysical Research: Atmospheres.
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