Scientists warn that carbon is relentlessly shrinking in Earth’s upper atmosphere: ScienceAlert

Rising levels of carbon dioxide in Earth’s atmosphere could exacerbate efforts to clean up our increasingly cluttered atmosphere of orbiting space junk.

According to two new studies, the greenhouse gas has contributed significantly to the shrinking of the upper atmosphere. This contraction has been hypothesized for decades. Now, for the first time, it has actually been noticed.

Some noticeable shrinkage is normal, and will bounce back; But scientists say the contribution made by carbon dioxide may be permanent.

This means that defunct satellites and other pieces of older technology in LEO are likely to stay in place longer due to reduced atmospheric resistance, cluttering the area and causing problems for newer satellites and space observations.

“One of the results is that the satellites will stay on for longer, which is great, because people want their satellites to stay up,” explains geospatial scientist Martin Melengak of NASA’s Langley Research Center.

“But the debris will also linger for longer and potentially increase the possibility that satellites and other valuable space objects will need to adjust their trajectory to avoid collisions.”

Descriptions of Earth’s atmosphere generally specify layers at specific altitudes, but the truth is that the volume of gases surrounding our world is not constant. It expands and contracts in response to various influences, perhaps the largest being the sun.

Now, the sun is not stationary either. It goes through cycles of activity, from high to low, and back again, roughly every 11 years. We are currently in the midst of the 25th such cycle since reckoning began, a cycle that began around December 2019. The previous cycle, #24, was unusually dim even at the peak of solar activity, and this enabled Mlynczak and colleagues to take the atmospheric deflation measurements.

Their attention focused on two layers, collectively known as the MLT: the mesosphere, which begins at an altitude of about 60 kilometers (37 miles); and the lower thermosphere, which starts at about 90 km.

Layers of Earth's atmosphere.
Layers of Earth’s atmosphere. (shoo_arts/iStock/Getty Images Plus)

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 nearly 20 years, from 2002 to 2021.

In some lower layers of the atmosphere, carbon dioxide creates a warming effect by absorbing and re-emitting infrared radiation in all directions, effectively trapping part of it.

However, some of the infrared radiation emitted by the carbon dioxide escapes into space, effectively carrying away heat and cooling the upper atmosphere. The higher the carbon dioxide level, the colder the atmosphere.

We already knew that this cooling causes the stratosphere to shrink. Now we can see that it’s doing the same thing to the mesosphere and the thermosphere above it as well. Using data from TIMED, Mlynczak and his team found that the MLT shrank by about 1,333 meters (4,373 feet). Roughly 342 meters of that is the result of radiative cooling caused by carbon dioxide.

“There has been a lot of interest to see if we can 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 contraction like this, on a global basis.”

Given that the thermosphere extends for several hundred kilometres, 342 meters might not seem like much. However, a paper published in September by physicist Ingrid Knusen of the UK’s British Antarctic Survey showed that cooling of the thermosphere could lead to a 33 percent decrease in atmospheric resistance by 2070.

Atmospheric drag is what helps satellites and rocket stages clear orbit after their missions are completed. Cnossen found that this reduction in clouds could extend the orbital life of defunct space junk by 30 percent by 2070.

As more and more satellites are launched into low Earth orbit, this will become an increasing problem, with no real mitigation measures in sight – either to reduce the number of satellites, or the amount of carbon dioxide.

“At every rise, there is a cooling and deflation that we attribute in part to the increase in carbon dioxide,” says Mlenjak. “As long as carbon dioxide is increasing at about the same rate, we can expect these rates of temperature change to also remain constant, at around half a degree Kelvin.” [of cooling] per contract.

Research published in Journal of Geophysical Research: Atmosphere.

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