Technological advances improve life but they can also unleash vast destructive power. Martin Rees warns that the next 100 years could be a make-or-break time for humanity
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 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 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 misadventure, the next few generations could jeopardise life's potential, foreclosing its human and posthuman 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. Over its 4.5 billion-year history, Earth's appearance would have altered very gradually. The only abrupt worldwide changes were triggered by major asteroid impacts or volcanic super-eruptions. Apart from these 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 - 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 (from television, cellphone and radar transmissions).
And something else unprecedented happened: metallic objects - albeit 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 forever.
A race of scientifically advanced extraterrestrials watching our solar system could confidently predict Earth's final doom in another 6 billion years, when the Sun, in its death throes, swells up into a "red giant" and vapourises anything remaining on our planet's surface. But could they have predicted this unprecedented spasm less than halfway through the Earth's life span - these human-induced alterations occurring with runaway speed?
If they continued to keep watch, what might these hypothetical aliens witness in the next 100 years? Will a final squeal be followed by silence? Or will the planet stabilise? And will some of the small metallic objects launched from the Earth spawn new oases of life elsewhere in the solar system, eventually extending their influences - via genetically engineered life or exotic machines - far away, creating an expanding "green sphere" that eventually pervades the galaxy?
The answer will depend on how humans handle new technologies. The 20th century brought us "the bomb", and nuclear threats will never leave us. But 21st-century hazards could threaten humans and the global environment still more. Populations could be wiped out by "engineered" airborne viruses; human character may be changed by techniques far more targeted and effective than the nostrums and drugs familiar today.
Conceivably, ordinary citizens could command the destructive capacity that in the 20th century was the frightening prerogative of the handful of people who held the reins of power in the nuclear superpowers. If there were millions of independent fingers on the button of a doomsday machine, then one person's act of irrationality, or even one person's error, could do us all in.
That extreme situation is perhaps so unstable that it can never be reached, just as a very high house of cards can never be built. Long before technology reaches doomsday potential, millions of people will be empowered to trigger events on the scale of the worst present-day terrorist outrages.
An organised terror network would not be required: just a fanatic or weirdo with the mindset of those who design computer viruses.
The ensuing disruption - its psychic and economic impact amplified by the media, and by the awareness that repeated instances are unpreventable - may become so pervasive that society corrodes and regresses.
By mid-century, nations may have drastically realigned; people may live very differently, their minds and personalities modified by medication, chip implants and so forth. But one thing is unlikely to change: there will still be disaffected or embittered loners and dissident groups. Just one such loner could be one too many in an interconnected world vulnerable to new "bio" or "cyber" threats.
The future of the cosmos is potentially infinite. But will this eternity be filled with ever more complex and subtle forms of life, or will it be as empty as the Earth's first sterile seas? That choice will be made this century.
Sir Martin Rees is astronomer royal and professor of astronomy at the University of Cambridge. His book Our Final Century? is published by William Heinemann on May 1 (?17.99). He will speak on the matter at the Cheltenham Festival of Science on June 4.