Credulity (or credulousness, if you like) is the hallmark of the lefty envirodingbats. They'll believe virtually anything, if said notion fits their agenda. There's neither a shortage of it nor a challenge to it, at least in the Left.
Perhaps you were thinking of 'credibility'? Lord knows, the Left are thoroughly credibility-challenged.
Credibility of whom? This is no challenge to credibility except as affirmation of lack by GWers.