Rising levels of carbon dioxide in Earth’s atmosphere could intensify efforts to clean our increasingly cluttered envelope of space debris.
According to two new studies, the greenhouse gas was a major contributor to the contraction of the upper atmosphere. This contraction has been suspected for decades; now, for the first time, it has actually been observed.
Some of the observed shrinkage is normal and rebounds; but CO2’s contribution, scientists say, is likely permanent.
This means that decommissioned satellites and other bits of old technology in low-Earth orbit are likely to stay in place longer due to the reduction in atmospheric drag, cluttering the region and causing problems for newer satellites and space observations.
“One consequence of this is that satellites stay operational longer, which is great because people want their satellites to stay operational,” explains geospace scientist Martin Mlynczak of NASA’s Langley Research Center.
“But debris also stays up longer, likely increasing the likelihood that satellites and other valuable space objects will have to adjust their orbits to avoid collisions.”
Descriptions of Earth’s atmosphere generally place the layers at specific heights, but the truth is that the volume of gases surrounding our world is not static. It expands and contracts in response to various influences, the largest of which is probably the sun.
Well, the sun isn’t static either. It goes through activity cycles from high to low and back again about every 11 years. We are currently in the midst of the 25th such cycle since counting began, a cycle that began around December 2019. The previous cycle, number 24, was unusually dampened even at the peak of solar activity, and this is what allowed Mlynczak and his colleagues to take measurements of atmospheric contraction.
Their attention focused on two layers known collectively as the MLT: the mesosphere, which begins at about 60 kilometers (37 miles) altitude; and the lower thermosphere, which starts at about 90 kilometers.
Data from NASA’s TIMED satellite, an observatory that collects data on the upper atmosphere, provided them with pressure and temperature information for the MLT over 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.
However, up in the much, much thinner MLT, some of the infrared radiation emitted by CO2 escapes into space, effectively removing heat and cooling the upper atmosphere. The higher the CO2 level, the cooler the atmosphere.
We already knew that this cooling causes the stratosphere to contract. Now we can see that it does the same with the mesosphere and thermosphere above it as well. Using TIMED’s data, Mlynczak and his team found that the MLT contracted about 1,333 meters (4,373 feet). About 342 meters of this is the result of CO2-induced radiative cooling.
“There was a lot of interest to see if we could actually observe this cooling and shrinking effect on the atmosphere,” says Mlynczak.
“We finally present these observations in this paper. We are the first to show atmospheric shrinkage on a global scale in this way.”
Given that the thermosphere stretches for several hundred kilometers, 342 meters doesn’t 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 drag by 2070.
Atmospheric drag helps satellites and rocket stages de-orbit after the end of their mission. This reduction in drag could extend the orbital lifetime of defunct space debris by 30 percent by 2070, Cnossen found.
As more and more satellites are put into low-Earth orbit, this is becoming an increasing problem with no real mitigation measures on the horizon – either to reduce the number of satellites or the amount of CO2.
“At every altitude there is 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 these temperature changes to remain roughly constant as well, around half a degree Kelvin [of cooling] per decade.”
The research was published in Journal of Geophysical Research: Atmospheres.
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