What are the biggest threats to humanity?

Human extinction may be the stuff of nightmares but there are many ways in which it could happen. Popular culture tends to focus on only the most spectacular possibilities: think of the hurtling asteroid of the film Armageddon or the alien invasion of Independence Day. While a dramatic end to humanity is possible, focusing on such scenarios may mean ignoring the most serious threats we face in today's world. And it could be that we are able to do something about these. In 1815 an eruption of Mount Tambora, in Indonesia, killed more than 70,000 people, while hurling volcanic ash into the upper atmosphere.