This has been falling more than expected.
http://www.nasa.gov/topics/solarsystem/f...
http://www.ipcc.ch/ipccreports/tar/wg1/2...
From the NASA paper:
'Cahalan’s modeling, for example, suggests that the sun may underlie variations in stratospheric temperature more strongly than currently thought.'
'"We may have a lot more to learn about how solar variability works, and how the sun might influence our climate," Cahalan said.'
This is one of the expected results of an increased greenhouse effect. There's more infrared radiation being retained by the lower atmosphere, so less of it is reaching the stratosphere.
Primarily due to the ozone depletion. The ozone layer is in the stratosphere
http://www.wunderground.com/resources/cl...
Because of a reduction in extra ultra violet rays, which can vary as much as 6% over sunspot cycles.
