An increase in CO2 could be one reason why a layer of Earth's upper atmosphere went through its biggest contraction in 43 years.
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.