Journal of Geophysical Research: Atmospheres (2022). DOI: 10.1029/2022JD036767″ width=”800″ height=”530″/> Daily global mean temperature (55°N to 55°S) (K) at five different pressure levels from January 2002 to December 2021. Over 7,200 days of data are plotted for each pressure level. Credit: Journal of Geophysical Research: Atmospheres (2022). DOI: 10.1029/2022JD036767
Daily global mean temperature (55°N to 55°S) (K) at five different pressure levels from January 2002 to December 2021. Over 7,200 days of data are plotted for each pressure level. Credit: Journal of Geophysical Research: Atmospheres (2022). DOI: 10.1029/2022JD036767
Near the Earth’s surface, increasing concentrations of carbon dioxide in the atmosphere are causing temperatures to rise. But from about 60 kilometers (37 miles), in the outermost layers of the atmosphere called the mesosphere and lower thermosphere (MLT), carbon dioxide actually cools the atmosphere, causing it to shrink and contract. This process of cooling and contraction has been assumed for more than three decades. Now, new research reveals the first evidence that on a global scale, the shrinking of the upper atmosphere has begun.
A new study uses satellite-derived pressure and temperature data to show that the MLT contracted by more than 1.3 kilometers (0.8 miles) between 2002 and 2019. About 340 meters (1,115 feet) of this shrinkage are due to carbon dioxide, and this contraction is likely permanent, according to the researchers.
The rest of the contraction is due to a drop in solar activity during this time. The study was published in the Journal of Geophysical Research: Atmosphereswhich publishes research that advances understanding of the Earth’s atmosphere and its interaction with other components of the Earth system.
A cooling and shrinking MLT will lead to increased longevity of space debris at higher altitudes, including the upper thermosphere, posing risks to the International Space Station and other space objects in low Earth orbit. Tens of thousands of known pieces of space junk, from naturally occurring meteoroids to man-made technological junk, are currently orbiting Earth.
Over time, most debris sinks and falls out of orbit. Models in an article published earlier in Geophysical Research Letters the projected cooling in the thermosphere would result in about a 33% decrease in drag and a 30% longer lifespan for space debris by 2070.
“One of the consequences is that the satellites will stay in place longer, which is great because people want their satellites to stay in place. But the debris will also stay in place longer and will probably increase the likelihood that the satellites and other valuable space objects will have to adjust their trajectory to avoid collisions,” said Martin Mlynczak, lead author of the Journal of Geophysical Research: Atmospheres study and geospatial scientist at NASA’s Langley Research Center. Longer-lasting debris could increase space insurance costs and be a major consideration in future space legislation and policy decisions, he added.
The thermosphere is the uppermost layer of the atmosphere before 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 is composed primarily of oxygen and nitrogen. Most of the incoming UV radiation from the Sun is absorbed by oxygen, heating the thermosphere and causing it to expand. The heating varies from one solar cycle to another and plays an important role in regulating the temperature of the thermosphere and its shrinking or swelling.
The cooling causes a contraction in the upper atmosphere
Low in the atmosphere, carbon dioxide absorbs energy and radiates it downward, heating the atmosphere. But in the mesosphere and lower thermosphere, where the atmosphere is millions of times thinner, carbon dioxide molecules absorb incoming energy and emit infrared radiation into space, helping to cool the upper atmosphere. Higher concentrations of carbon dioxide in the MLT then reflect more energy back into space. This radiative cooling, together with fluctuations in solar activity, drives the contraction.
Carbon dioxide concentrations in the mesosphere and thermosphere have increased in parallel with concentrations on the Earth’s surface. Scientists predicted in the 1980s that cooling and contraction would occur, but the new study is the first to demonstrate global observations of contraction.
“There has been a lot of interest in seeing if we can actually observe this cooling and shrinking effect on the atmosphere,” Mlynczak said. “We are finally presenting these observations in this paper. We are the first to show atmospheric shrinkage like this, on a global scale.”
As the thermosphere cools, it contracts and results in a 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 additional carbon dioxide.
New JGR: Atmospheres The study used temperature and pressure data from NASA’s TIMED satellite (in year 21 of what was originally a two-year mission) to look for predicted cooling and contraction patterns. The researchers found that the uppermost elevation of the mesosphere and lower thermosphere cooled by up to 1.7 degrees Celsius (35 degrees Fahrenheit) and contracted by more than a kilometer between 2002 and 2019.
The last solar cycle was weak, allowing researchers to separate the effects of carbon dioxide and solar radiation on atmospheric temperatures. At the higher elevations of the MLT, the weaker solar cycle over the past 20 years is responsible for most of the observed cooling, in addition to cooling due to increased carbon dioxide.
“At each altitude there is cooling and contraction that we attribute in part to increased carbon dioxide,” Mlynczak said. “As long as carbon dioxide increases at about the same rate, we can expect these rates of temperature change to also remain about constant, within about half a degree Kelvin. [of cooling] per decade.
Martin G. Mlynczak et al, Cooling and contraction of the mesosphere and lower thermosphere 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 in the 21st century, Geophysical Research Letters (2022). DOI: 10.1029/2022GL100693
Provided by American Geophysical Union
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