I was interested not so much in the forcing effect of clouds themselves so much as the change in albedo which might result from a change in the overall extent of global cloud cover. (realclimate.org)
Global cloud cover varies but is typically 60 - 70 % of the earth's surface area (or so it seems). (realclimate.org)
He suggests that some unknown mechanism has caused global cloud cover to decrease over the past century. (skepticalscience.com)