NEW YORK (Bloomberg) — Philanthropist David Rockefeller, Ronald Perelman, Larry Ellison, Sanford Weill, Ted Turner and Paul Allen joined a pledge started by Warren Buffett for billionaires to give more than half their wealth to charity.

The announcement brings to more than 30 the number of billionaires and wealthy executives who have publicly agreed to give most of their wealth to charity. Buffett has promised to donate more than 99 percent of his wealth. The greatest part of his fortune, estimated in March at $47 billion by Forbes Magazine, is being provided in annual installments to the Bill & Melinda Gates Foundation.

In June, Eli Broad and New York City Mayor Michael Bloomberg backed the initiative. Bloomberg is a part-time Bermuda resident. Broad, former chairman of insurer SunAmerica Inc. and founder of homebuilder KB Home, said he and his wife, Edythe, will give 75 percent of their wealth to philanthropic causes. Broad pocketed $3.4 billion when he sold his 19 percent stake in insurer SunAmerica Inc.

"Our family continues to be united in the belief that those who have benefitted the most from our nation's economic system have a special responsibility to give back to our society in meaningful ways," Rockefeller said in letter posted on the Giving Pledge website. "I hope others will accept this challenge — and opportunity — and will join us in this worthwhile endeavour."

EBay Inc. founder Pierre Omidyar said in his letter that he and his wife, Pam, had pledged in 2001 to give away "the vast majority of our wealth" during their lifetime.

"We have more money than our family will ever need," he said. "There's no need to hold onto it when it can be put to use today, to help solve some of the world's most intractable problems."