An increase in CO2 could be one reason why a layer of
Earth's upper atmosphere went through its biggest contraction in 43 years.
THE GIST
·
Earth's
thermosphere went through its biggest contraction in 43 years.
·
Researchers
expected to see a contraction due to a solar minimum, but not this significant.
·
One explanation
may be an increase in carbon dioxide in the atmosphere.
More than half of the upper atmosphere's shrinkage cannot be explained. Click to enlarge this image.
NASA
NASA
RELATED TOPICS
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Earth
Scientists are mulling over why part of the Earth's atmosphere recently
suffered its biggest collapse since records began, and is only now
starting to rebound.
The collapse occurred in a region known as the thermosphere, a rarefied
layer of the planet's upper atmosphere between 90 and 600 kilometers (56 to 373
miles) above the surface, which shields us from the sun's far and extreme ultra
violet (EUV) radiation.
A report in Geophysical Research Letters by
a team led by John Emmert from the United States Naval Research Laboratory has
found that the thermosphere went through its biggest contraction in 43 years.
The thermosphere usually expands and contracts in line with the sun's
11-year solar cycle. During solar maximum when solar activity increases, it
causes the thermosphere to heat up -- reaching temperatures of 1100°C -- and
expand like a marshmallow in a camp fire. The opposite happens during solar
minimum.
Currently, the sun is experiencing its longest solar minimum on record,
with little sunspot activity and few solar flares or coronal mass ejections.
To see what effect solar minimum is having on the thermosphere, Emmert and
colleagues monitored the impact of atmospheric drag on satellites in low-Earth
orbit (LEO). These satellites fly through the thermosphere, so the thicker the
thermosphere the more drag it puts on spacecraft.
The researchers expected to see a contraction in line with solar minimum,
but the level of collapse was up to three times greater than solar activity
alone can explain.
They believe an increase in carbon dioxide in the atmosphere may explain
the contraction. CO2 has a cooling effect in the thermosphere, which would then
amplify the impact of the extended solar minimum.
But the researchers found low levels of EUV radiation only account for
about 30 percent of the collapse, while the increase in CO2 levels account for
another 10 percent at most.
That still leaves some 60 percent, which can't be explained by current
modelling.
Furthermore the current anomaly appears to have commenced in 2005, well
before the current solar minimum began.
Emmert and colleagues think there may be an as yet unidentified
climatological tipping point involving both energy and chemical feedbacks.
Phil Wilkinson of the Ionospheric Prediction Service with the Australian
Bureau of Meteorology says it highlights something is going on that science
doesn't understand.
"They are suggesting that the whole composition and chemistry of the
thermosphere might have changed and they way we come out of this solar minimum
will tell us how it's changed," said Willkinson.
"Or it could be that minor constituents in the thermosphere play a far
more important role than we thought and we're only realizing that now. If
that's the case then the thermosphere will eventually return to normal
conditions."
Wilkinson says the only real impact of the thermosphere collapse is in
space, where less atmospheric drag will keep spacecraft in orbit longer.
"That's good news if you want to keep your satellite flying, bad news
if you're trying to de-orbit space junk," he said.
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