If the cloud cover is less the planet is warmer and visa versa, an relative increase in cloud cover causes the planet to cool. (realclimate.org)
A small increase in cloud cover during summer daytimes can have an effect at least as strong as the doubling of CO2, and in the opposite direction. (realclimate.org)
A possible 2 % increase in cloud cover following a 1 % increase in mean surface temp has been written about already, but I lack the references to the published instances; I mention them to avoid seeming to claim that the ideas I wrote in the paragraph above originated with me.. (judithcurry.com)