Had Mr. Fink moved a decade ago to pull BlackRock’s funds out of companies that contribute to climate change, his clients would have been well served. In the past 10 years, through Friday, companies in the S&P 500 energy sector had gained just 2 percent in total. In the same period, the broader S&P 500 nearly tripled.

In an interview, Mr. Fink said the decision developed from conversations with “business leaders and how they’re thinking about it, talking to different scientists, reading different research.” Mr. Fink asked BlackRock to research the economic impacts of climate change; it found that they are already appearing in a meaningful way in the form of higher insurance premiums, for fires and floods, and expects cities to have to pay more for their bonds.

Wherever he goes, he said, he is bombarded with climate questions from investors, often to the exclusion of issues that until recently were once considered more important. “Climate change is almost invariably the top issue that clients around the world raise with BlackRock,” he wrote in his letter.

He wrote that he anticipated a major shift, much sooner than many might imagine, in the way money will be allocated.

“This dynamic will accelerate as the next generation takes the helm of government and business,” he wrote. “As trillions of dollars shift to millennials over the next few decades, as they become C.E.O.s and C.I.O.s, as they become the policymakers and heads of state, they will further reshape the world’s approach to