This is the explanation the Heartland Institute gives for their billboards on their website:
Because what these murderers and madmen have said differs very little from what spokespersons for the United Nations, journalists for the “mainstream” media, and liberal politicians say about global warming. They are so similar, in fact, that a Web site has a quiz that asks if you can tell the difference between what Ted Kaczynski, the Unabomber, wrote in his “Manifesto” and what Al Gore wrote in his book, Earth in the Balance.
The point is that believing in global warming is not “mainstream,” smart, or sophisticated. In fact, it is just the opposite of those things. Still believing in man-made global warming – after all the scientific discoveries and revelations that point against this theory – is more than a little nutty. In fact, some really crazy people use it to justify immoral and frightening behavior.
Of course, not all global warming alarmists are murderers or tyrants. But the Climategate scandal and the more recent Fakegate scandal revealed that the leaders of the global warming movement are willing to break the law and the rules of ethics to shut down scientific debate and implement their left-wing agendas.
Scientific, political, and public support for the theory of man-made global warming is collapsing. Most scientists and 60 percent of the general public (in the U.S.) do not believe man-made global warming is a problem. (Keep reading for proof of these statements.) The people who still believe in man-made global warming are mostly on the radical fringe of society. This is why the most prominent advocates of global warming aren’t scientists. They are murderers, tyrants, and madmen.