I am worried about the greenhouse effect. Prof Hawking, who is confined to a wheelchair and speaks with the aid of a computer, has motor neurone disease, a degenerative disease of the brain. He said space travel would not solve the problems of over-population but, without colonising other planets, humans were presented with the possibility of becoming extinct. Earth is the most suitable planet in the solar system. But unless the human race spreads into space, I doubt it will survive the next thousand years.
Some accident can wipe out life on a single planet.
Prof Hawking also said the challenge for physics was to produce an explanation of everything - a complete unified theory. It is like a jigsaw. We think we have the edges of complete unified theory but there is a lot to be filled in, in the middle. This is the challenge for the 21st century.