The public debates in which Singer received most criticism have been about second-hand smoke and global warming. He questioned the link between second-hand smoke and lung cancer, and was an outspoken opponent of the mainstream scientific view on climate change; he argued there is no evidence that increases in carbon dioxide produced by human beings is causing global warming and that the temperature of the earth has always varied. A CBC "Fifth Estate" documentary in 2006 linked these two debates, naming Singer as a scientist who has acted as a consultant to industry in both areas, either directly or through a public relations firm. Naomi Oreskes and Erik Conway named Singer in their book, "Merchants of Doubt", as one of three contrarian physicists—along with Fred Seitz and Bill Nierenberg—who regularly injected themselves into the public debate about contentious scientific issues, positioning themselves as skeptics, their views gaining traction because the media gives them equal time out of a sense of fairness.