Physicist Stephen Hawking says pollution, greed and stupidity are the greatest threats to Earth.

Hawking told Larry King Now on Saturday that he's worried by overcrowding.

"We certainly have not become less greedy or less stupid," Hawking said. "Six years ago I was worrying about pollution and overcrowding. They have gotten worse since then."

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Hawking and King also discussed artificial intelligence. Hawking says governments seem to be engaged in "an AI arms race."

"A rogue AI could be difficult to stop," the physicist said. "We need to ensure that AI is designed ethically with safeguards in place."

(Read also: Today's 10 most influential scientists and their discoveries)

He also asked King whether his eight marriages are "a triumph of hope over experience."

"You make a good point, Stephen," King said. "I think the answer is yes."

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