Hawking made the remarks on a radio program to mark his 70th birthday, responding to questions submitted by listeners, The Daily Telegraph reported Friday.
"It is possible that the human race could become extinct but it is not inevitable. I think it is almost certain that a disaster, such as nuclear war or global warming, will befall the Earth within a thousand years," the Cambridge University cosmologist and theoretical physicist said.
"It is essential that we colonize space. I believe that we will eventually establish self-sustaining colonies on Mars, and other bodies in the solar system, although probably not within the next 100 years," Hawking said.
But if on its journey outward into space humanity should encounter alien races, the consequences could be disastrous, he warned.
"The discovery of intelligent life elsewhere in the universe would be the greatest scientific discovery ever. But it would be very risky to attempt to communicate with an alien civilization.
"If aliens decided to visit us, then the outcome might be similar to when Europeans arrived in the Americas," Hawking said. "That did not turn out well for the Native Americans."