Laurence Fink started off 2020 by dropping a big, green bombshell. In his annual letter to corporate executives, the BlackRock Inc. chief executive officer wrote that the world’s largest asset manager would push climate change to the center of its investment process. He also issued a warning, telling businesses to prepare for a “fundamental reshaping of finance.”

BlackRock, which manages about $6.5 trillion, has long been a target for environmentalists because of its vast holdings in companies most responsible for global warming. Fink’s call to arms was seen as a major shift for Wall Street, an unmistakable signal that the biggest pool of capital would be aimed squarely at the climate crisis. But there are obstacles—not least of which is the way BlackRock invests the bulk of its assets, that is, in index-linked funds. Fink’s plan has run up against CEOs and investors scrambling just to survive the economic fallout from the pandemic.