Would you believe the sun's magnetic field is a culprit?
According to a new study from the Danish National Space Center, cosmic rays created by the explosions of distant stars play an important role in cloud formation in the earth's lower atmosphere. Those clouds have a cooling effect on the planet. The sun's magnetic field, however, interferes with this process to some degree, and that field has doubled for some reason in the 20th century.
According to the Space Center's website:
The resulting reduction in cloudiness, especially of low-altitude clouds, may be a significant factor in the global warming Earth has undergone during the last century.
Grist for the mill. I'm sure it will only be a matter of time, however, before someone claims that the Danish National Space Center is secretly on the Exxon take.