Jun 30,
2021
NASA Satellites See Upper Atmosphere Cooling and
Contracting 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 rising
human-made greenhouse gas emissions.
Combined
data from three NASA satellites have produced a long-term record that reveals
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
human-driven climate change, but it has been difficult to observe the trends
over time.
“You need
several decades to get a handle on these trends and isolate what’s happening
due to greenhouse gas emissions, solar cycle changes, and other effects,” said
Scott Bailey, an atmospheric scientist at Virginia Tech in Blacksburg, and lead
of the study, published in the Journal of Atmospheric and Solar-Terrestrial
Physics. “We had to put together three satellites’ worth of data.”
Together,
the satellites provided about 30 years of observations, indicating that the
summer mesosphere over Earth’s poles is cooling four to five degrees Fahrenheit
and contracting 500 to 650 feet per decade. Without changes in human carbon
dioxide emissions, the researchers expect these rates to continue.
Since the
mesosphere is much thinner than the part of the atmosphere we live in, the
impacts of increasing greenhouse gases, such as carbon dioxide, differ from the
warming we experience at the surface. One researcher compared where we live,
the troposphere, to a thick quilt.
“Down near
Earth’s surface, the atmosphere is thick,” said James Russell, a study
co-author and atmospheric scientist at Hampton University in Virginia. “Carbon
dioxide traps heat just like a quilt traps your body heat and keeps you warm.”
In the lower atmosphere, there are plenty of molecules in close proximity, and
they easily trap and transfer Earth's heat between each other, maintaining that
quilt-like warmth.
That means
little of Earth's heat makes it to the higher, thinner mesosphere. There,
molecules are few and far between. Since carbon dioxide also efficiently emits
heat, any heat captured by carbon dioxide sooner escapes to space than it finds
another molecule to absorb it. As a result, an increase in greenhouses gases
like carbon dioxide means more heat is lost to space — and the upper atmosphere
cools. When air cools, it contracts, the same way a balloon shrinks if you put
it in the freezer.
This
cooling and contracting didn’t come as a surprise. For years, “models have been
showing this effect,” said Brentha Thurairajah, a Virginia Tech atmospheric
scientist who contributed to the study. “It would have been weirder if our
analysis of the data didn’t show this.”
While
previous studies have observed this cooling, none have used a data record of
this length or shown the upper atmosphere contracting. The researchers say
these new results boost their confidence in our ability to model the upper
atmosphere’s complicated changes.
The team
analyzed how temperature and pressure changed over 29 years, using all three
data sets, which covered the summer skies of 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. That effect meant
the height of any given atmospheric pressure fell as the air cooled. In other
words, the mesosphere was contracting.
Earth’s
Middle Atmosphere
Though what
happens in the mesosphere does not directly impact humans, the region is an
important one. The upper boundary of the mesosphere, about 50 miles above
Earth, is where the coolest atmospheric temperatures are found. It’s also where
the neutral atmosphere begins transitioning to the tenuous, electrically
charged gases of the ionosphere.
Even higher
up, 150 miles above the surface, atmospheric gases cause satellite drag, the
friction that tugs satellites out of orbit. Satellite drag also helps clear
space junk. When the mesosphere contracts, the rest of the upper atmosphere
above sinks with it. As the atmosphere contracts, satellite drag may wane —
interfering less with operating satellites, but also leaving more space junk in
low-Earth orbit.
The
mesosphere is also known for its brilliant blue ice clouds. They’re called
noctilucent or polar mesospheric clouds, so named because they live in the
mesosphere and tend to huddle around the North and South Poles. The clouds form
in summer, when the mesosphere has all three ingredients to produce the clouds:
water vapor, very cold temperatures, and dust from meteors that burn up in this
part of the atmosphere. Noctilucent clouds were spotted over northern Canada on
May 20, kicking off the start of the Northern Hemisphere’s noctilucent cloud
season.
Because the
clouds are sensitive to temperature and water vapor, they’re a useful signal of
change in the mesosphere. “We understand the physics of these clouds,” Bailey
said. In recent decades, the clouds have drawn scientists’ attention because
they’re behaving oddly. They’re getting brighter, drifting farther from the
poles, and appearing earlier than usual. And, there seem to be more of them
than in years past.
“The only
way you would expect them to change this way is if the temperature is getting
colder and water vapor is increasing,” Russell said. Colder temperatures and
abundant water vapor are both linked with climate change in the upper
atmosphere.
Currently,
Russell serves as principal investigator for AIM, short for Aeronomy of Ice in
the Mesosphere, the newest satellite of the three that contributed data to the
study. Russell has served as a leader on all three NASA missions: AIM, the
instrument SABER on TIMED (Thermosphere, Ionosphere, Mesosphere Energetics and
Dynamics), and the instrument HALOE on the since-retired UARS (Upper
Atmospherics Research Satellite).
TIMED and
AIM launched in 2001 and 2007, respectively, and both are still operating. The
UARS mission ran from 1991 to 2005. “I always had in my mind that we would be
able to put them together in a long-term change study,” Russell said. The
study, he said, demonstrates the importance of long-term, space-based
observations across the globe.
In the
future, the researchers expect more striking displays of noctilucent clouds
that stray farther from the poles. Because this analysis focused on the poles
at summertime, Bailey said he plans to examine these effects over longer
periods of time and — following the clouds — study a wider stretch of the
atmosphere.
By Lina
Tran
NASA’s
Goddard Space Flight Center, Greenbelt, Md.
Last
Updated: Jun 30, 2021
Editor:
Lina Tran
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