evidence: An overwhelming majority of scientists agree — global warming is happening and human activity is the primary cause. Yet several prominent global warming skeptic organizations are actively working to sow doubt about the facts of global warming. [...] These kinds of claims are nothing new for CEI. Back in 1991, CEI was claiming that "The greatest challenge we face is not warming, but cooling." [32] More recently, CEI produced an ad calling for higher levels of carbon dioxide. [33] One CEI scholar even publicly compared a prominent climate scientist to convicted child molester Jerry Sandusky. [34] [...] Popular outcry forced the Heartland Institute to pull down a controversial billboard that compared supporters of global warming facts to Unabomber Ted Kaczynski [38], bringing an early end to a planned campaign first announced in an essay by Heartland President Joseph Bast, which claimed "… the most prominent advocates of global warming aren’t scientists. They are murderers, tyrants, and madmen." [39]