As climate change becomes a focus of the US election, energy companies stand accused of trying to downplay their contribution to global warming. In June, Minnesota's Attorney General sued ExxonMobil, among others, for launching a "campaign of deception" which deliberately tried to undermine the science supporting global warming. So what's behind these claims? And what links them to how the tobacco industry tried to dismiss the harms of smoking decades earlier? To understand what's happening today, we need to go back nearly 40 years. Marty Hoffert leaned closer to his computer screen. He couldn't quite believe what he was seeing. It was 1981, and he was working in an area of science considered niche. "We were just a group of geeks with some great computers," he says now, recalling that moment. But his findings were alarming. "I created a model that showed the Earth would be warming very significantly. And the warming would introduce climatic changes that would be unprecedented in human history. That blew my mind."