Earth's thermosphere: photo from UCAR.
Though I didn't see the story picked up on the usual climate sites, several mainstream outlets headlined it, usually with words like "mysterious" and "scientists baffled." But the CS Monitor's headline was seriously attention-getting: "Earth's upper atmosphere collapses. Nobody knows why."
It's the thermosphere, which apparently has its ups and downs, but this was the biggest contraction in 43 years, with none of the usual suspects as reasons:
The collapse occurred during a period of relative solar inactivity – called a solar minimum from 2008 to 2009. These minimums are known to cool and contract the thermosphere, however, the recent collapse was two to three times greater than low solar activity could explain. "Something is going on that we do not understand," Emmert said.
And yes, a key global heating factor seems to be involved, but this may be one of those synergistic effects no one imagined. Of course it may not, but...
Emmert suggests carbon dioxide (CO2) in the thermosphere might play a role in explaining the atmospheric collapse.
This gas acts as a coolant, shedding heat via infrared radiation. It is widely-known that CO2 levels have been increasing in Earth's atmosphere. Extra CO2 in the thermosphere could have magnified the cooling action of solar minimum.
"But the numbers don't quite add up," Emmert said. "Even when we take CO2 into account using our best understanding of how it operates as a coolant, we cannot fully explain the thermosphere's collapse."
One of the functions of the thermosphere is screening out harmful UV rays. So this is a mystery worth looking into.