I'm a cosmologist – my professional interests focus far away from the Earth. This might seem an incongruous viewpoint from which to address practical terrestrial issues. But I also believe that our civilisation may be threatened by 21st-century technology. A cosmic perspective sharpens this concern; it strengthens the imperative to cherish what Carl Sagan called our "pale blue dot" in the cosmos.
The stupendous timespans of the evolutionary past are now part of common culture. But many still perceive humanity as some kind of culmination; cosmologists, in contrast, are mindful that still vaster timespans lie ahead. The unfolding of intelligence and complexity could still be near its cosmic beginnings: in far-future aeons even more marvellous biodiversity could emerge. From this perspective, the present century seems the most crucial for Earth's history – it is a century when human choices and actions could ensure the perpetual future of life (which may lie not just on the Earth, but far beyond it); in contrast, through malign intent, or through misadventure, the next few generations could jeopardise life's potential, foreclosing its human and post-human future.
Viewed from deep space, our entire habitat of land, oceans and clouds is revealed as a thin, delicate glaze – its beauty and vulnerability contrasting with the stark and sterile moonscape on which the astronauts left their footprints. During nearly all its 4.5-billion-year history, Earth's appearance has altered very gradually. The only abrupt worldwide changes were triggered by major asteroid impacts or volcanic super-eruptions. Apart from those brief traumas, nothing happened suddenly – the continental land masses drifted; the ice cover waxed and waned; successions of new species emerged, evolved and became extinct.
But in just a tiny sliver of the Earth's history – the last one-millionth part, a few thousand years – the patterns of vegetation altered much faster than before. This signalled the start of agriculture – the imprint on the terrain of a population of humans, empowered by tools. The pace of change accelerated as human populations rose. Quite different transformations were then manifest; and these were even more abrupt. Within 50 years – little more than one-hundredth-of-a-millionth of the Earth's age – the amount of carbon dioxide in the atmosphere, which over most of Earth's history had been slowly falling, began to rise anomalously fast. The planet became an intense emitter of radio waves (the total output from all television, cellphone and radar transmissions.) And something else happened, unprecedented in Earth's history: metallic objects – albeit very small ones, a few tonnes at most – left the planet's surface and escaped the biosphere completely. Some were propelled into orbits around the Earth; some journeyed to the Moon and planets; a few even followed a trajectory that would take them deep into interstellar space, leaving the solar system for ever.