A study at the University of Leuven in Belgium determined that clouds are doing more to melt the surface of Greenland's ice sheet than to prevent such. Why? They concluded that it's because of white surface of the ice. Why would the same not apply to sea ice? Now surely, where the ice has already melted and left mostly open water, the clouds would hinder insolation. However, what about when they cover a nice undisturbed area of white, high albedo ice? From what I have read lately, they no longer believe the clouds to be such a protection. Of course, NASA themselves are still learning when it comes to clouds. They have concluded that not all clouds are the same in regard to whether they effectively heat or cool a surface below them. When they do heat the surface its not by much; one of us humans would much prefer to be in the shade of a cloud and be just a little warmer than say maybe that of a tree, if we had to choose that or to be in the open sunlight. The ice however, as long as it absorbs enough energy starts to melt.
Also, as of late, researchers have learned that just as the climate changes clouds, clouds can change the climate, a feedback thing.
Not trying to say this is going to melt every crystal of ice in the Arctic by September, but it may have a greater impact than previously thought.