NEW YORK (CNN) - CNN founder Ted Turner is stepping down as vice chairman of AOL Time Warner, the parent company of CNN and CNN/Money, in order to spend more time on philanthropic endeavors, AOL's chairman announced Wednesday.

Turner's resignation as vice chairman is effective in May.

AOL Time Warner Vice Chairman Ted Turner

In handing in his notice, Turner, 64, said he wants to spend more time to pursue the charity and "socially responsible business efforts" he has become involved with in recent years.

"Over the past five years, it has become even clearer to me how much personal satisfaction I derive from these activities. Therefore, I would like to now devote even more time, effort and resources to them," he said in his resignation letter.

In 1997, Turner pledged $1 billion to a foundation to support the United Nations. He is an outspoken environmentalist and has become the largest private landowner in the United States.

His decision follows a series of shakeups at the world's largest media company, formed by the merger of Internet service America Online and the Time Warner news and entertainment conglomerate in 2001. AOL Chairman Steve Case announced earlier this month that he will resign that post in May.

CEO Richard Parsons said the company is grateful for Turner's "vision and genius."

"Ted's leadership role in the formation of global media in general and AOL Time Warner in particular is uniquely significant," Parsons said in a written statement. "His values of journalistic independence and public service are a permanent part of who we are and what we do."

Turner sold his cable networks, including CNN, to Time Warner in 1996. He became the company's vice chairman, a title he retained after AOL and Time Warner merged in 2001.

Turner was one of the most enthusiastic boosters of that deal. But the company has experienced a severe drop in its market value since the merger was announced at the peak of the Internet stock boom. AOL Time Warner announced a $45.5 billion charge Wednesday to reflect the shrinking value of various business units, mostly America Online.