A few years ago, the hundreds of members of France’s Mulliez family, with a global retail empire worth more than $38 billion, decided they should take climate change more seriously — or rather, their investment portfolio should.

But where to start? Climate change and the fight against it could transform almost every sector of the economy as companies clamor for ways to cut emissions and even pull carbon dioxide from the air. "This space is very broad, and it’s complicated,” says Delphine Descamps, managing director at Creadev, the Mulliez family office, which has about €200 million ($236 million) to invest each year.

Then she met Regine Clement, the head of a small, secretive nonprofit called Creo Syndicate. An exclusive club of climate-focused investors, Creo’s mission is to speed up the flow of capital into investments that can slow global warming. The group focuses on the richest of the rich, working with about 200 families and investment outfits with a total of more than $800 billion under management. Prominent members include legendary investor Jeremy Grantham and Nat Simons, the son of Renaissance Technologies’ billionaire founder James Simons. Members must pay dues — a "very reasonable” flat fee, Clement says, that makes up about half the nonprofit’s revenue — and they must prove they’re serious by planning to make their first investment in climate and sustainability within six months. Members must also have assets of at least $100 million and get approved by the nonprofit’s board.