I'll let its own press release for its upcoming conference explain, as there's simply no need to finesse it further: 
 
 Billboards in Chicago paid for by The Heartland Institute point out that some of the world's most notorious criminals say they "still believe in global warming" – and ask viewers if they do, too…The billboard series features Ted Kaczynski, the infamous Unabomber; Charles Manson, a mass murderer; and Fidel Castro, a tyrant.
– A conservative think tank has pulled a controversial billboard that compared those who believe in global warming to deranged killers. The Heartland Institute was taking heavy flak for the digital sign in Chicago that featured a photo of Unabomber Ted Kaczynski and the question, "I still believe in Global Warming. Do you?" reports the Washington Post's Plum Line blog. The institute explained on its website that the "experiment" was designed to be deliberately provocative. It had originally planned to follow up with images of Osama bin Laden and Charles Manson, among others. An earlier post from the site explained that "the most prominent advocates of global warming" are "murderers, tyrants, and madmen." It added, helpfully, that "not all global warming skeptics are murderers or tyrants," notes the Guardian, which first spotted the billboard. Andrew Sullivan at the Daily Beast thinks it's everything that's wrong with the right—"a refusal to acknowledge scientific reality; and a brutalist style of public propaganda that focuses entirely on guilt by the most extreme association." GOP Rep. James Sensenbrenner, a prominent skeptic about global warming, vowed to pull out of an institute-sponsored conference later this month if the ad campaign continued; it ended soon after.