Carbon is shrinking Earth’s upper atmosphere unabated, scientists warn: ScienceAlert
Rising levels of carbon dioxide in Earth’s atmosphere could worsen efforts to clean up our increasingly cluttered mantle of orbiting space junk.
According to two new studies, greenhouse gases have contributed significantly to the contraction of the upper atmosphere. This contraction has been assumed for decades; now, for the first time, it was actually noticed.
Some of the shrinkage observed is normal and will return; but CO2’s contribution, scientists say, is likely permanent.
This means that aging satellites and other pieces of old technology in low Earth orbit are likely to remain in place longer due to reduced atmospheric drag, cluttering the region and causing problems for newer satellites and space observations.
“One of the consequences is that satellites will stay on longer, which is great, because people want their satellites to stay on,” explains geospace scientist Martin Mlynczak NASA’s Langley Research Center.
“But the debris will also stay up longer and will likely increase the likelihood that satellites and other valuable space objects will have to adjust their trajectories to avoid collisions.”
Descriptions of the Earth’s atmosphere generally place the layers at certain 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, one of which is probably the Sun.
Now, the Sun is not static either. It passes activity cycles, from high, to low, and back again, about every 11 years. We’re in the middle of it right now 25. such a cycle since counting began, a cycle that began around December 2019. The previous cycle, number 24, was unusually dim even at the peak of solar activity, and that’s what allowed Mlynczak and his colleagues to measure atmospheric contraction.
Their attention was focused on two layers, collectively known as the MLT: the mesosphere, which begins at about 60 kilometers (37 miles) above sea level; and the lower thermosphere, which begins at about 90 kilometers.
Data from NASA 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, they effectively trap 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, the colder the atmosphere.
We already knew this cooling causes the stratosphere to contract. Now we can see that it does the same with the mesosphere and the thermosphere above it. Using data from TIMED, Mlynczak and his team found that the MLT has shrunk by about 1,333 meters (4,373 feet). About 342 meters of that is the result of radiative cooling caused by CO2.
“There was a lot of interest in seeing if we could actually observe this effect of cooling and shrinking the atmosphere,” Mlynczak says.
“We finally present those observations in this paper. We are the first to show this reduction in the atmosphere, on a global basis.”
Given that the thermosphere extends over several hundred kilometers, that 342 meters may not seem like much. However, a paper published in September physicist Ingrid Cnossen from the British Antarctic Survey in Great Britain has shown that thermospheric cooling could result in a 33 percent reduction in atmospheric drag by 2070.
Atmospheric drag is what helps satellites and rocket stages exit orbit after the mission is complete. This reduction in drag could extend the orbital lifetime of space debris by 30 percent by 2070, Cnossen found.
As more and more satellites are launched into low Earth orbit, this will become an increasing problem, with no real mitigation measures in sight – be it 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 the increase in carbon dioxide,” Mlynczak says. “As long as carbon dioxide increases at about the same rate, we can expect these rates of temperature change to remain roughly constant, at about half a degree Kelvin [of cooling] per decade.”
The research was published in Journal of Geophysical Research: Atmospheres.
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