In November 2015, world leaders descended on Paris to negotiate a historic agreement to limit global warming. But on the summit’s opening day, attention turned to a private citizen slouched awkwardly in a chair on stage: Bill Gates.

Gates, with his characteristic flop of sandy hair, grinned impishly as President Barack Obama called him a “visionary” and a “driving force” behind a pair of pledges that both saw as key to the larger agreement. First the world’s largest economies would double their spending on research into clean-energy technologies. Then investors like Gates would back the startups that emerged. The Microsoft cofounder had already enlisted two dozen other billionaires to join the effort.

For Gates, who’d become increasingly consumed by this unprecedented threat to human well-being, the moment marked his arrival as a leading figure in the fight against climate change.

“If we back over 50 different breakthrough approaches, we know that several of those will give us the solution that we need,” Gates said during his own speech. He would invest at least $2 billion of his own money in companies developing geothermal wells, new kinds of nuclear reactors and machines that pull carbon from the air, to name just a few.

With much less fanfare, however, Gates has put more than double that amount into a very dirty, old-school industry. At the time of the Paris summit, he was already the dominant shareholder of America’s No. 2 landfill operator, Republic Services. He has since increased his stake to 38% of the company, a position worth $29 billion.