Because of rising levels of carbon dioxide in the atmosphere, temperatures near the surface of the Earth are going up. The mesosphere and lower thermosphere (MLT), the uppermost layers of the atmosphere, are around 37 miles in altitude and where carbon dioxide cools the atmosphere, causing it to condense and contract.
For more than three decades, this cooling and contracting process has been postulated. Now, fresh research provides the first indication that upper atmospheric shrinking has started on a global scale.
The MLT has shrunk by over 0.8 miles between 2002 and 2019, a recent study that uses satellite-derived pressure and temperature data reports. According to the researchers, the carbon dioxide-related portion of this contraction — about 340 meters (1,115 feet) — is probably permanent. At that time, solar activity was decreasing, which accounts for the remaining contraction.
Mesosphere And Lower Thermosphere Dwindling
Daily global average (55°N to 55°S) temperature (K) at five different pressure levels from January 2002 through December 2021. More than 7,200 days of data are plotted for each pressure level.
Credit: Journal of Geophysical Research: Atmospheres (2022). DOI: 10.1029/2022JD036767
A cooling and shrinking mesosphere and lower thermosphere will increase the longevity of space debris at higher altitudes, including the upper thermosphere. This puts the International Space Station and other low-Earth orbit space objects at risk. Tens of thousands of pieces of known space debris, ranging from natural meteoroids to man-made technological junk, are currently orbiting the Earth.
Most debris descends and falls out of orbit over time. Cooling in the thermosphere, according to models published earlier in Geophysical Research Letters, would result in a 33% decrease in drag and 30% longer lifetimes for space debris by 2070.
“One consequence is satellites will stay up longer, which is great, because people want their satellites to stay up. But debris will also stay up longer and likely increase the probability that satellites and other valuable space objects will need to adjust their path to avoid collisions,”
said lead author Martin Mlynczak. He also pointed out that debris that lasts longer could raise the cost of space insurance and be a major factor in future decisions about space law and policy.
Atmospheric Heating And Pressure
The thermosphere is the uppermost part of the atmosphere, just before the exosphere, or what most people would consider “space,” However, it typically ranges from altitudes of about 50 to 60 miles to between 300 and 600 miles. It is determined by atmospheric pressure.
The thermosphere is composed mostly of nitrogen and oxygen. This is different from the atmosphere near the Earth’s surface. The majority of the UV radiation from the Sun is absorbed by oxygen, which causes the thermosphere to heat up and expand.
The heating varies from one solar cycle to the next and is crucial in determining the temperature of the thermosphere as well as its expansion or contraction.
At low altitudes, carbon dioxide warms the atmosphere by absorbing energy and sending it down. However, in the mesosphere and lower thermosphere, where the atmosphere is millions of times thinner, carbon dioxide molecules absorb incoming energy and emit infrared radiation back into space, assisting in the cooling of the upper atmosphere.
Upper Atmosphere Cooling
Consequently, higher concentrations of carbon dioxide in the MLT return more energy to space. In conjunction with variations in solar activity, this radiative cooling causes contraction.
The concentrations of carbon dioxide in the mesosphere and thermosphere have increased in tandem with those at the Earth’s surface. In the 1980s, scientists predicted that cooling and contracting would occur, but the new study is the first to provide global evidence of the contraction.
There is considerable interest in determining whether or not scientists could actually observe this atmospheric cooling and contraction.
“We finally present those observations in this paper. We’re the first to show the shrinking of the atmosphere like this, on a global basis.”
said Mlynczak.
As the thermosphere cools, it contracts and consequently becomes less dense. As a result, a satellite at a given altitude in the thermosphere now encounters less dense air and, consequently, less drag than before the addition of carbon dioxide.
TIMED Satellite Mission
NASA Diagram of TIMED mission. Credit: NASA
NASA’s Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission is dedicated to studying the effects of solar and human energetics and dynamics on the least explored and understood region of Earth’s atmosphere – the Mesosphere and Lower Thermosphere/Ionosphere.
The new study used temperature and pressure data from the TIMED satellite, which is in its 21st year of what was planned to be only a two-year mission. The study’s aim was to search for the predicted cooling and contracting patterns.
Between 2002 and 2019, the highest altitude of the mesosphere and lower thermosphere has shrunk by more than a kilometer and cooled by as much as 1.7 degrees Celsius (35 degrees Fahrenheit).
Researchers were able to separate the effects of carbon dioxide and solar radiation on atmospheric temperature because the most recent solar cycle was weak. The weaker solar cycle over the past 20 years is responsible for the majority of the observed cooling at the MLT’s highest altitudes, in addition to the cooling caused by rising carbon dioxide levels.
“At every altitude, there is a cooling and a contraction that we attribute in part to increasing carbon dioxide,”
said Mlynczak.
The researchers think that these rates of temperature change will stay about the same, at about 0.5 degrees Kelvin per decade, as long as carbon dioxide keeps rising at about the same rate.
References:
Mlynczak, M. G., Hunt, L. A., Garcia, R. R., Harvey, V. L., Marshall, B. T., Yue, J., et al. (2022). Cooling and contraction of the mesosphere and lower thermosphere from 2002 to 2021. Journal of Geophysical Research: Atmospheres, 127, e2022JD036767
Cnossen, I. (2022). A realistic projection of climate change in the upper atmosphere into the 21st century. Geophysical Research Letters, 49, e2022GL100693.