He has theorized in the past that the chance of a disaster on Earth adds up over time, so that it's a "near certainty" in the next 1,000 or 10,000 years, but the human race will survive if it expands into outer space.
http://www.time.com/4767595/stephen-hawking-100-years-new-planet/

Professor Stephen Hawking thinks the human species will have to populate a new planet within 100 years if it is to survive. With climate change, overdue asteroid strikes, epidemics and population growth, our own planet is increasingly precarious.
http://www.bbc.co.uk/mediacentre/mediapacks/tomorrows-world/television

Maybe he's a little bit overdramatizing... I don't know.