"One of my favourite questions when teaching PR ethics is to try and establish whether or not an ethical PR practitioner can represent the Flat Earth Society (I would argue that there is no easy answer to this question)? Jim Hoggan tackles a rather more serious version of the debate in this interesting post, Clearing the air on climate change  (thanks, Trevor ).
"He begins his post by stating 'There is a line between PR and propaganda - or there should be.' It is an interesting post - but not one that actually locates this rather elusive line... "
Quite so: it is an elusive line. Wikipedia is a little bit helpful, defining propaganda  without actually differentiating it from public relations. But here's the line from Wiki that is most relevant: "... the message does not have to be untrue to qualify as propaganda, but it may omit so many pertinent truths that it becomes highly misleading."
This is certainly the case among the climate science deniers. Perversely, I have to acknowledge that these people can "mislead" without lying (although it would be too charitable by far to suggest there are no liars among them). Many of the participants in this debate are so ideologically driven or so wilfully blinded by their own self-interest that they may, in fact, believe in the narrow arguments they mount.
As to the ethical question, the Canadian Public Relations Society's Code of Professional Ethics  states: "A member shall practice the highest standards of honesty, accuracy, integrity and truth, and shall not knowingly disseminate false or misleading information.
Members shall not make extravagant claims or unfair comparisons, nor assume credit for ideas and words not their own. Members shall not engage in professional or personal conduct that will bring discredit to themselves, the Society or the practice of public relations."
The line "shall not knowingly disseminate false or misleading information" can excuse a lot of sins. But I'd argue any day that the people who are muddling the message on climate change are bringing "discredit to themselves, the Society (and) the practice of public relations."