How much did the oil industry know about the impact of fossil fuel emissions on the climate? When did they know it? And what did they do with that knowledge?  Those are the central questions in a series of court cases attempting to hold companies accountable for their role in climate change.  It's been called Big Oil's "Big Tobacco" moment, echoing the famous litigation that exposed how tobacco companies deliberately generated scientific uncertainty even though they knew that cigarettes cause cancer. "They are brought under very similar legal theory," said Ann Carlson, an environmental law professor at the UCLA School of Law. "That is that the companies are creating a public harm or public nuisance by misleading the public about whether their product actually causes harm." After years of pretrial wrangling, the first Big Oil case began in civil court in New York on Tuesday. Lawyers for Exxon Mobil face accusations from the state's attorney general that the company misled shareholders about how it was calculating the cost of future government policies to limit carbon emissions. Exxon Mobil has publicly denied the allegations. "The New York attorney general's allegations are false," Exxon Mobil spokesperson Steven Soper told CBC News in an email. "We tell investors through regular disclosures how the company accounts for risks associated with climate change. We are confident in the facts and look forward to seeing our company exonerated in court." Meanwhile, a similar investor case is being investigated by the attorney general of Massachusetts. Exxon was also under scrutiny this week in Washington as a congressional subcommittee examined the oil industry's efforts to suppress climate science. "Oil companies like Exxon knew the scientific reality 40 years about but waged a war of deception that cost us precious time in the fight to save our planet," said Jamie Raskin, the chairman of the subcommittee on civil rights and civil liberties. Two former Exxon scientists were questioned about what they knew about the effects of fossil fuels on climate change. "These Exxon scientists were excellent researchers," said Martin Hoffert, who was a paid scientific consultant for Exxon in the 1980s. "Our research was consistent with findings of the United Nation's Intergovernmental Panel on Climate Change on human impacts of fossil fuel burning." There are more than a dozen other lawsuits in the U.S. against the industry at various stages in the legal process. Those cases have been launched by municipal and state governments, including New York City, Baltimore, San Francisco and Rhode Island, all attempting to hold oil companies accountable for climate-related damages. The details of the cases differ, but they share a common thread.