In Brief Answers to Big Questions, Hawking discusses a great number of challenges to the planet and its inhabitants, including the most dangerous threats to our sustained suitability. He describes several global catastrophic risks that could damage mankind on a worldwide scale. He discusses an asteroid impact similar to the proposed impact that wiped out the dinosaurs. His primary worry in this regard seems to be that there is no real way of stopping such an impact.
Hawking also discusses the challenge of combating climate change, warning that the release of too much CO2 released into the atmosphere would make our atmosphere unsuitable for human life, and that the amount of CO2 being released is already at dangerous levels.
Nuclear war seems to also be of concern. Hawking seems to have accepted that at some point, a nuclear holocaust will devastate if not completely destroy humanity. He also uses an entire section of the book to ponder on whether technology will save or doom us, weighing the pros and cons of artificial intelligence and human augmentation.