Rising levels of carbon dioxide in Earth’s atmosphere could exacerbate efforts to clean up our increasingly messy shell of orbiting space junk.
The greenhouse gas has contributed significantly to the contraction of the upper atmosphere, according to two new studies. This contraction has been hypothesized for decades; now it is actually being observed for the first time.
Some of the observed shrinkage is normal and will bounce back; but CO2’s contribution, scientists say, is probably permanent.
This means that defunct satellites and other bits of old technology in low Earth orbit are likely to remain in place longer due to the reduction in atmospheric drag, cluttering the region and creating problems for newer satellites and space observations.
“A result is that satellites stay in the air longer, which is great because people want their satellites to stay in the air,” explains space geoscientist Martin Mlynczak of NASA’s Langley Research Center.
“But debris will also stick around longer and likely increase the likelihood that satellites and other valuable space objects will have to adjust their path to avoid collisions.”
Descriptions of Earth’s atmosphere generally place the layers at specific heights, but the truth is that the volume of gases that surround our world is not static. It expands and contracts in response to various influences, the largest of which is probably the sun.
Now the sun is not static either. It goes through cycles of activity, from high to low and back again, about every 11 years. We are currently in the 25th of this cycle since the start of the calculation, a cycle that started around December 2019. The previous cycle, number 24, was unusually moderate even at the peak of solar activity, suggesting that Mlynczak and his colleagues to make measurements of atmospheric contraction.
Their attention focused on two layers, known collectively as the MLT: the mesosphere, which begins at about 60 kilometers (37 mi) altitude; and the lower thermosphere, which begins at about 90 kilometers.
Data from NASA’s TIMED satellite, an observatory that collects data on the upper atmosphere, gave them pressure and temperature information for the MLT for a nearly 20-year period, 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.
At the top of the much, much thinner MLT, however, some of the infrared radiation emitted by CO2 escapes into space, effectively dissipating heat and cooling the upper atmosphere. The higher the CO2, the cooler the atmosphere.
We already knew that this cooling causes the stratosphere to contract. Now we can see it doing the same with the mesosphere and the thermosphere above it. Using the data from TIMED, Mlynczak and his team found that the MLT contracted about 1,333 meters (4,373 feet). About 342 meters of that is the result of CO2-induced radiative cooling.
“There’s been a lot of interest in seeing if we can actually observe this cooling and shrinking effect on the atmosphere,” says Mlynczak.
“We finally present those observations in this paper. We are the first to show the shrinking of the atmosphere in this way worldwide.”
Since the thermosphere extends for several hundred miles, that 1,150 feet 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 percent reduction in atmospheric resistance by 2070.
Atmospheric drag helps satellites and rocket stages move out of orbit after their missions have ended. This drag reduction could extend the life of defunct space junk by 30 percent by 2070, Cnossen found.
As more and more satellites are launched into low Earth orbit, this is becoming a bigger problem, with no real mitigation measures in sight – either to reduce the number of satellites or the amount of CO2.
“At any altitude, there is a cooling and contraction that we attribute in part to increasing carbon dioxide,” says Mlynczak. “As long as carbon dioxide is increasing at roughly the same rate, we can expect this rate of temperature change to also remain roughly constant, at about half a degree Kelvin. [of cooling] per decade.”
The research has been published in Journal of Geophysical Research: Atmospheres.