In 1998, Edward Teller proposed Stratospheric Radiation Management (SRM)" in his article "A Sunscreen For Planet Earth:
http://www.evolutionquebec.com/site/archives/teller.htm
He looked at the result of the Mt. Pinatubo eruption in 1992 and saw that it cooled the planet by ejecting 20 million tons of sulfur dioxide into the atmosphere, and proposed that humans could do the same to cool the planet.
Chemtrail conspiracy theorists have been claiming that persistent contrails are evidence of SRM.
If SRM were taking place, it would be evident in atmospheric readings of the transmission of solar radiation through the atmosphere, some of it would be blocked and transmission would decrease as it had during past eruptions.
If SRM were taking place, transmission would decrease.
Here is a graph of such transmission:
Note the effect of Pinatubo, and also note that no such change has occurred. Indeed, the transmission levels are no different than ordinary background levels since 1955. This disproves, probably more than anything else, the claim that SRM has been taking place.