BlackRock, the largest asset manager in the world, threw its weight around recently. Chairman and CEO Larry Fink wrote a letter to CEOs in January explaining BlackRock’s further divestment from the fossil fuel industry. He said BlackRock would hold companies and directors accountable when they don’t address sustainability and climate change.

In the letter, he says BlackRock will exit investments that present a high sustainability risk, such as thermal coal producers, which will lead to sales of $500 million worth of stock, according to The Wall Street Journal. The company will launch new investment products that screen for fossil fuels.

“ … We will be increasingly disposed to vote against management and board directors when companies are not making sufficient progress on sustainability-related disclosures and the business practices and plans underlying them,” Fink wrote.

This is not the first time Fink has been written such a salvo. Last year, his letter to CEOs about purpose-driven organizations creating long-term shareholder value received widespread attention as well. Because of its size and influence, BlackRock can pressure corporate America to change; in particular, BlackRock can target the publicly traded companies who rely on BlackRock’s support.

The tide is indeed turning. Research firm Morningstar notes that U.S. funds focused on sustainability experienced $20 billion of net flows last year, nearly four times the year prior.

As Alan Murray, the president and CEO of the publisher Fortune, points out, most of BlackRock’s funds are index funds that buy the companies that are in indexes — including coal, oil and gas companies. So this change may not impact as many funds as observers may think. By its sheer size, BlackRock will remain one of the largest investors in fossil fuels.

Environmentalists who have been hounding Larry Fink and BlackRock for years are wary. The executive director of the Sierra Club, Michael Brune, points out that one of BlackRock’s environmental, social and governance (ESG) funds owns stock in oil and gas companies.

He repeats the message of youth climate activist Greta Thunberg, who said: “The biggest danger is not inaction. The real danger is when politicians and CEOs are making it look like real action is happening, when in fact almost nothing is being done, apart from clever accounting and creative PR.”