ABSTRACT
About 4.5 billion years have elapsed since the Sun condensed from a cosmic cloud. The proto-Sun was encircled by a swirling disk of gas. A memorable early photograph taken from space depicted 'Earthrise', as viewed from a spacecraft orbiting the Moon. The evolution of Earth's biosphere can be traced back several billion years: the future of physical universe is reckoned to be more extended still, perhaps even infinite. But despite these expanded horizons, both past and future, one time-scale has contracted: many of us are less confident that our civilisation will survive the next century than were our forebears, who devotedly added bricks to cathedrals that would not be finished in their lifetime. What happens on Earth, in this century, could conceivably make the difference between a near-eternity filled with ever more complex and subtle forms of life, and one filled with nothing but the elements.
