Martin Rees: We need to cherish our 'pale blue dot' in the cosmos

From a speech by the Astronomer Royal to the Foundation for Science and Technology's Christmas reception

For about 40 years, we've been familiar with the view of Earth from Space. Suppose some aliens had been watching our planet for its entire history, what would they have seen? Over nearly 4.5 billion years, Earth's appearance would have altered very gradually. The continental land masses drifted; the ice cover waxed and waned; successive species emerged, evolved and became extinct.

Yet in just a tiny sliver of the Earth's history - the most recent one millionth part, just a few thousand years - the aliens would have seen that the patterns of vegetation altered much faster than before. The start of agriculture would have set that in train. Then would have come the imprint on the terrain of humans, powered by tools.

But then there were quite different and even more abrupt transformations. Within 50 years, or little more than one hundredth of a millionth of the Earth's age, the carbon dioxide in the atmosphere began to rise anomalously fast. The planet became an intense emitter of radio waves. At about the same time metallic objects 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.

The aliens could predict that the biosphere would face doom in 6 billion years, when the predictions show the Sun will die. But could they have predicted this unprecedented spike less than halfway through the Earth's life, signalling alterations induced by human beings occupying less than a millionth of the elapsed lifetime of the Earth?

What might the [aliens] witness in the next 100 years? Will there be a final spasm followed by silence? Or will the planet stabilise? This enlarged perspective gives an extra motive to cherish our "pale blue dot" in the cosmos.