Will humans become extinct by the end of the century?

What is the chance of the human race surviving the 21st century? There are many dangers – climate change for example, or nuclear war, or a pandemic, or planet Earth being hit by a giant asteroid. Around the world a number of research centres have sprung up to investigate and mitigate what’s called existential risk. How precarious is our civilisation and what can be done to stop a global catastrophe? David Edmonds talks to four expert witnesses to try and find the answer. (Apocalyptic landscape. Credit: Santoelia/ Getty images)