Near the earth’s surface, increasing concentrations of carbon dioxide in the atmosphere are causing temperatures to rise. But above about 60 kilometers (37 miles) altitude, carbon dioxide in the uppermost layers of the atmosphere, the mesosphere and lower thermosphere (MLT), actually cools the atmosphere, causing it to shrink and contract. This cooling and contraction process has been suspected for over three decades. Now new research reveals the first evidence that the global shrinkage of the upper atmosphere has begun.
A new study uses satellite-derived pressure and temperature data to show that the MLT shrank by over 1.3 kilometers (0.8 miles) between 2002 and 2019. About 340 meters (1,115 feet) of that shrinkage is due to carbon dioxide, and that contraction is likely permanent, the researchers said.
The remainder of the contraction is due to a drop in solar activity during this time. The study was published in Journal of Geophysical Research: Atmosphereswhich publishes research that advances the understanding of the Earth’s atmosphere and its interaction with other components of the Earth system.
Cooling and shrinking MLT will lead to longer lifetimes of space debris at higher elevations, including the upper thermosphere, posing a risk to the International Space Station and other space objects in low Earth orbit. Tens of thousands of known pieces of space junk, ranging from natural meteoroids to man-made technological junk, are currently orbiting the earth.
Over time, most debris sinks and falls out of orbit. Models in a previously published article in Geophysical Research Letters Projected cooling in the thermosphere would lead to about a 33% reduction in drag and a 30% increase in the lifespan of space debris by 2070.
“One consequence is that satellites stay in the air longer, which is great because people want their satellites to stay in the air. But debris will also stay aloft longer, likely increasing the likelihood that satellites and other valuable space objects will have to adjust their path to avoid collisions,” said Martin Mlynczak, the lead author of the Journal of Geophysical Research: Atmospheres Graduate and geospace scientist at NASA’s Langley Research Center. Prolonged debris could increase space insurance costs and be an important consideration in future space law and policy decisions, he added.
The thermosphere is the highest layer of the atmosphere in front of what many people probably think of as “space” or the exosphere. It is defined by atmospheric pressure but generally ranges from altitudes of about 80 to 90 kilometers (50 to 60 miles) to between 500 and 1,000 kilometers (300 to 600 miles).
Unlike the atmosphere near the earth’s surface, the thermosphere consists mainly of oxygen and nitrogen. Most of the incoming UV radiation from the sun is absorbed by oxygen, causing the thermosphere to heat up and expand. Warming varies from one solar cycle to the next and plays an important role in setting the temperature of the thermosphere and shrinking or swelling.
Cooling causes contraction in the high atmosphere
At low altitudes in the atmosphere, carbon dioxide absorbs energy and releases it downward, warming the atmosphere. But in the mesosphere and lower thermosphere, where the atmosphere is a million times thinner, carbon dioxide molecules absorb incident energy and emit infrared radiation back into space, helping to cool the upper atmosphere. Higher concentrations of carbon dioxide in the MLT then send more energy back into space. This radiative cooling, coupled with fluctuations in solar activity, drives the contraction.
Carbon dioxide concentrations in the mesosphere and thermosphere have been increasing in step with concentrations at the Earth’s surface. Scientists predicted cooling and contraction in the 1980s, but the new study is the first to demonstrate global observations of the contraction.
“There’s been a lot of interest to see if we can actually observe this cooling and shrinking effect on the atmosphere,” Mlynczak said. “We finally present these observations in this paper. We are the first to show atmospheric shrinkage on a global scale in this way.”
As the thermosphere cools, it contracts, resulting in lower density. Because of this, a satellite at a given altitude in the thermosphere now experiences relatively less dense air, and therefore less drag, than before the addition of extra carbon dioxide.
The new JGR: Atmospheres The study used temperature and pressure data from NASA’s TIMED satellite (in the 21st year of what was originally a two-year mission) to look for the predicted cooling and contraction patterns. The researchers found that between 2002 and 2019, the highest altitude of the mesosphere and lower thermosphere cooled by up to 1.7 degrees Celsius (35 degrees Fahrenheit) and shrank by over a kilometer.
The recent solar cycle was weak, allowing researchers to separate the effects of carbon dioxide and solar radiation on atmospheric temperatures. At the highest elevations of the MLT, the weakening solar cycle over the past 20 years accounts for most of the observed cooling and contributes to cooling from increasing carbon dioxide.
“At every altitude there is a cooling and a contraction that we attribute in part to increasing carbon dioxide,” Mlynczak said. “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.”
Martin G. Mlynczak et al, Mesosphere and lower thermosphere cooling and contraction from 2002 to 2021, Journal of Geophysical Research: Atmospheres (2022). DOI: 10.1029/2022JD036767
I. Cnossen, A realistic projection of climate change in the upper atmosphere into the 21st century, Geophysical Research Letters (2022). DOI: 10.1029/2022GL100693
Provided by the American Geophysical Union
Quote: Carbon Dioxide Shrinks Upper Atmosphere, Extending Life of Space Debris (2022 November 18) Retrieved November 18, 2022 from https://phys.org/news/2022-11-carbon-dioxide-uppermost-atmosphere-prolonging .html
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