(Bloomberg) –TCI Fund Management Ltd., the firm run by billionaire hedge fund manager Christopher Hohn, warned companies in its portfolio to step up their climate-change action or risk divestment.

The London-based money manager, which oversees more than $30 billion of assets, sent letters to firms including Airbus SE, Charter Communications Inc. and Moody’s Corp. demanding better disclosure on greenhouse gas emissions, according to its website. The Financial Times reported on the letters earlier.

“We expect companies in which we invest to have a credible, publicly disclosed plan,” said the letters dated Nov. 30. “We will also evaluate divestment where a portfolio company refuses to disclose its emissions.”

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TCI’s warnings come as investors ramp up scrutiny of their holdings’ environmental impact. Hohn has personally and through the Children’s Investment Fund Foundation donated 200,000 pounds ($258,000) to Extinction Rebellion, which organized mass protests in central London in a push to urge politicians to reach zero emissions worldwide by 2025.

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The money manager is known as an activist investor, taking stakes in companies and agitating for change to boost their share prices. One recent campaign involved seeking to revamp the management at the London Stock Exchange Group Plc.