Billionaire activist Tom Steyer will have officially divested all of his holdings from fossil-fuel firms by the end of June, marking a major step in his high-profile transformation from hedge-fund manager to a powerful force in America’s fight against climate change.

Steyer’s ongoing evolution is two years in the making, according to a Washington Post profile on Monday. It began in the summer of 2012 following a hike in the Adirondacks with Bill McKibben, a prominent environmentalist and founder of the movement. Steyer, 56, who owned Farallon Capital Management at the time, was conflicted by his financial involvement with carbon-intensive energy firms. At the base of the mountains, he committed to unloading those “ecologically unsound” investments.

In the years since, Steyer has become one of the most vocal opponents of the proposed Keystone XL pipeline and is poised to become a major big-money donor in upcoming elections. In 2013, he founded a super PAC called NextGen Climate, which could spend up to $100 million this year to help elect senators and governors committed to tackling climate change. The goal is to make the climate a pivotal issue in the 2016 presidential race.

According to the Post profile, when Steyer gave up his ownership of Farallon, he directed that his personal investment holdings be divested only from tar sands and coal—two energy sources that Steyer identified as “the fossil fuels that are having a specific impact on climate,” Heather Wong, a spokeswoman for Steyer, told the Post. In late 2013, he extended the ban to include natural gas and all oil investments “because he felt it was simply the right thing to do,” Wong said.

Steyer’s disentangling from fossil fuels has been costly. He reportedly “left millions on the table” when he relinquished his Farallon management shares and abandoned enormous tax advantages and a share of additional profits, the Post said. But Steyer said that the move has helped to clear his conscience.

“I came to realize I could no longer in good conscience remain in a business that by definition was invested in every sector of the economy, including the energy sector,” he said in an April statement.