That sources this study:

http://nationalacademies.org/hmd/~/media/Files/Report%20File...

That study states that "there is moderate evidence that there is no statistical evidence for ... lung cancer".

Now most studies in this area make mention of the fact that even for heavy marijuana smokers, they never smoke as much as a pack a day cigarette smoker. Which is to say, that even though there is more tar in the smoke of marijuana per gram, people don't smoke as much because they don't feel a need to.

That does not mean it is safe, but there is no evidence that it's as bad a cigarettes.