If the recent clear sky “brightening” trend were due to cleaner air and fewer aerosols alone, it should be accompanied by an increase in direct downwelling shortwave radiation, one part of solar radiation reaching the surface directly from the Sun. That didn’t happen, Long reported during the American Geophysical Union fall meeting in San Francisco.
Instead, Long and his colleagues found that at the continental United States sites they analyzed, direct downwelling shortwave radiation remained roughly steady between 1995 and 2007, under cloud-free skies. Rather, it was the diffuse shortwave radiation that increased. That simply couldn’t happen if fewer aerosols alone were the reason behind the brightening. If anything, fewer aerosols should mean less diffuse shortwave radiation, because particles in the atmosphere can bounce light around and back to space.
So the scientists dug deeper, and in a provocative new analysis, not yet published, Long suggests that a high-altitude “ice haze,” created by water and other emissions from aircraft, is responsible. “I’m talking about a sub-visual contrail-generated haze of ice, which we do not classify as a cloud but gives blue sky more of a whitish tint.” Long said.
The finding—if verified—could mean that we are in essence already conducting a geoengineering experiment on the atmosphere, adding ice particles that change the way solar radiation reaches Earth’s surface. Understanding the overall impact of those changes on warming or cooling at the surface will take more research, Long said. 
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