Questions about Your Essay's Supporting Evidence: Going "Green"

The author does a very good job of incorporating facts and evidence throughout the essay. Almost every single paragraph contained at least one citation and several had more than one. All of his arguments have sufficient evidence to back his thesis. He uses a variety of different sources and types of points to strengthen his claims. For example, in the fifth paragraph he uses statistics from a survey as well as an example of a specific school. Reviewing his sources, they all seem to be reputable and ought to be without bias. They are dated between 2006 and 2008, so they are contemporary sources for the piece. All six of his sources are directly relevant to his topic, in reading their titles they all are about environmentalism or the green movement in some capacity. All but one of his sources seems to be in support of his argument.  Overall his remaining sources all seem to share pretty similar opinions.  I do not feel that one oppositional source is sufficient to cover the variety of opinions on this matter. I feel like he could have strengthened his refutation if he had maybe one additional source to specifically dissect and respond to.