Heartland Pulls Billboard on Global Warming Do you?" the billboard read in large maroon letters. Hours later, the digital billboard was gone. It seems that the ad campaign, sponsored by the conservative Heartland Institute, had bombed. "We know that our billboard angered and disappointed many of Heartland's friends and supporters, but we hope they understand what we were trying to do with this experiment," the institute said late Friday afternoon said in a statement. "We do not apologize for running the ad, and we will continue to experiment with ways to communicate the "realist" message on the climate." In opening the campaign, Heartland had said that Mr. Kaczynski would not be the only persona gazing down on Chicago's commuters. Among his brethren, it said, would be Charles Manson, Fidel Castro, Osama bin Laden and James J. Lee, Heartland said. The Heartland Institute, a libertarian think tank based in Chicago that describes its chief mission as promoting free-market solutions to social and economic problems. The institute chose to feature "some of the world's most notorious killers" on the billboards "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," Heartland said at its Web site. The institute's site did acknowledge that "not all global warming alarmists are murderers and tyrants." In announcing that it was suspending the campaign, Heartland said that "this provocative billboard was always intended to be an experiment." "And after just 24 hours the results are in: It got people's attention," it said. The billboard was "an attempt to turn the tables on the climate alarmists by using their own tactics but with the opposite message," it said. At its Web site, the Heartland Institute is meanwhile directing readers to a quiz in which they can guess whether various quotations are from Mr. Kaczynski's notorious Unabomber manifesto or rather from Al Gore's book "Earth in the Balance." The billboard campaign was rolled out in advance of the Heartland Institute's seventh International Conference on Climate Change, which opens on May 21 in Chicago.