Thursday, July 22, 2021

NASA satellites see upper atmosphere cooling and contraction due to climate change

The sky isn't falling, but scientists have found that parts of the upper atmosphere are gradually contracting in response to increased man-made greenhouse gas emissions.

Combined data from three NASA satellites have produced a long-term record that reveals that the mesosphere, the layer of the atmosphere 30 to 50 miles above the surface, is cooling and contracting. Scientists have long predicted this effect of man-made climate change, but it has been difficult to observe trends over time.

"You need several decades to control these trends and isolate what's happening due to greenhouse gas emissions, changes in the solar cycle and other effects," said Scott Bailey, atmospheric scientist at Virginia Tech in Blacksburg, and study leader , published in the Journal of Atmospheric and Solar-Terrestrial Physics. "We had to gather data from three satellites."


Together, the satellites provided about 30 years of observations, indicating that the summer mesosphere above the Earth's poles is cooling by four to five degrees Fahrenheit and contracting 500 to 650 feet per decade. Without changes in human carbon dioxide emissions, researchers expect these rates to continue.


Because the mesosphere is much thinner than the part of the atmosphere we live in, the impacts of rising greenhouse gases such as carbon dioxide differ from the warming we experience at the surface. A researcher compared where we live, the troposphere, to a thick quilt.


“Near the Earth's surface, the atmosphere is dense,” said James Russell, study co-author and an atmospheric scientist at Hampton University in Virginia. "Carbon dioxide traps heat, just like a quilt traps body heat and keeps you warm." In the lower atmosphere, there are plenty of molecules nearby, and they easily trap and transfer Earth's heat to each other, keeping the heat similar to a quilt.


This means that little of the Earth's heat reaches the highest and thinnest mesosphere. There, the molecules are few and far between. Since carbon dioxide also efficiently emits heat, any heat captured by carbon dioxide escapes into space before finding another molecule to absorb it. As a result, an increase in greenhouse gases such as carbon dioxide means that more heat is lost to space - and the upper atmosphere cools. When the air cools, it contracts, the way a balloon shrinks if you put it in the freezer.


This cooling and contraction was not a surprise. For years, "models have shown this effect," said Brentha Thurairajah, an atmospheric scientist at Virginia Tech who contributed to the study. "It would have been more awkward if our data analysis didn't show that."


Although previous studies have observed this cooling, none have used a data record of this length or have shown the contraction of the upper atmosphere. The researchers say these new results increase their confidence in our ability to model the complicated changes in the upper atmosphere.


The team analyzed how temperature and pressure changed over 29 years, using all three datasets, which covered the summer skies from the north and south poles. They examined the stretch of sky 30 to 60 miles above the surface. At most altitudes, the mesosphere cooled as carbon dioxide increased. This effect meant that the height of any atmospheric pressure dropped as the air cooled. In other words, the mesosphere was contracting.

Middle Earth Atmosphere


While what happens in the mesosphere has no direct impact on humans, the region is important. The upper limit of the mesosphere, about 50 miles above Earth, is where the coldest atmospheric temperatures are found. It is also where the neutral atmosphere begins to transition into the tenuous, electrically charged gases of the ionosphere.


Even higher, 150 miles above the surface, atmospheric gases cause satellite drag, the friction that pulls satellites out of orbit. Satellite drag also helps clean up space junk. When the mesosphere contracts, the rest of the upper atmosphere sinks with it. As the atmosphere contracts, the satellite's resistance can decrease - interfering less with the operation of the satellites, but also leaving more space junk in low Earth orbit.


The mesosphere is also known for its bright blue ice clouds. They are called noctilucent clouds or mesospheric polar clouds, so called because they live in the mesosphere and tend to cluster around the north and south poles. Clouds form in summer, when the mesosphere has all three ingredients to make clouds: water vapor, freezing temperatures, and meteor dust that burns in this part of the atmosphere. Noctilucent clouds were detected in northern Canada on May 20, kicking off the northern hemisphere noctilucent cloud season.


As clouds are sensitive to temperature and water vapor, they are a useful sign of change in the mesosphere. “We understand the physics of these clouds,” said Bailey. Over the past few decades, clouds have caught scientists' attention because they are behaving strangely. They are getting brighter, moving away from the poles and appearing earlier than usual. And, there seem to be more of them than in previous years.


“The only way to expect them to change that way is if the temperature is getting colder and the water vapor is increasing,” Russell said. Colder temperatures and abundant water vapor are both related to climate change in the upper atmosphere.


Russell currently serves as principal investigator for AIM, short for Aeronomy of Ice in the Mesosphere, the newest of the three that contributed data to the study. Russell has served as a leader on all three NASA missions: AIM, the SABER on TIMED instrument (Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics) and the HALOE instrument on the UARS (Upper Atmospherics Research Satellite) since retired.


TIMED and AIM were launched in 2001 and 2007, respectively, and both are still in operation. The UARS mission lasted from 1991 to 2005. “I always kept in mind that we would be able to put them together in a study of long-term change,” Russell said. The study, he said, demonstrates the importance of long-term space observations around the world.


In the future, researchers expect stronger views of noctilucent clouds that move further away from the poles. Because this analysis focused on the poles in summer, Bailey said he plans to look at these effects over longer periods and — following the clouds — study a wider stretch of atmosphere.


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These AIM images span June 6-18, 2021, when the Northern Hemisphere noctilucent cloud season was well underway. The colors - from dark blue to light blue and bright white - indicate the albedo of clouds, which refers to how much light a surface reflects compared to the total sunlight falling on it. Things that have a high albedo are shiny and reflect a lot of light. Things that don't reflect much light have a low albedo and are dark. 

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