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Billionaire Tom Steyer recalls a dinner at the U.S. Treasury in Washington with two senior department officials and six money managers. It was August 2012, and the meal was part of an effort by the agency to keep up with what the financial community was worrying about. The diners discussed China’s slowdown, Federal Reserve policy and other trends affecting the U.S. economy.

Steyer says they were overlooking the biggest game changer of all. He told the group the country would have to overhaul its energy policy to address greenhouse gas emissions, Bloomberg Markets magazine will report in its November issue. His fellow guests were skeptical.

“It’s like I was saying that what’s going to make a difference in the economy is unicorns,” says Steyer, 56, the founder of Farallon Capital Management LLC, a San Francisco hedge-fund firm with about $20 billion in assets. He declines to name the other people present because the meeting was off the record but says they control a lot of money. “I thought to myself: These guys need to be made aware of the risks here.”

So in December, Steyer ended his 26-year career as a hedge-fund manager and set out to make an economic case for addressing climate change. He wasn’t the only person from the financial world to have this idea: Henry Paulson, Treasury secretary from 2006 to 2009 and a longtime conservationist, and Michael Bloomberg, the outgoing mayor of New York, which had suffered the costliest hurricane damage in its history, were also plotting how to reframe the issue.