On Christmas Eve 1968, the Apollo 8 astronaut William Anders took a photograph of the view outside the window as his spaceship orbited the moon. The now iconic Earthrise image shows our half-moon blue planet under a decoration of clouds rising from the blackness of space over the lunar surface.

The picture encapsulated Earth’s precariousness in the cosmos and, for many, contained a message of humility and stewardship for our home.

We’ve had Earthrise and images like it from the Apollo missions for half a century now. But suppose some aliens had been viewing our planet for its entire 4.5bn-year history. What would they have seen?

Over nearly all that immense time, changes would have been very gradual: continents drifted; the ice cover waxed and waned; successive species emerged, evolved and became extinct during a succession of geological eras.

But visible change has accelerated rapidly in the past few thousand years – a tiny sliver of the Earth’s history. Now geologists have decided those changes have been so profound, so global and so permanent that our catalogue of the Earth’s history needs to change accordingly. Since the last ice age, around 11,000 years ago, human civilisation has flourished in the climatically benign Holocene. Now they believe that epoch has come to an end and we have entered a new human-influenced age, the Anthropocene.

The Anthropocene epoch: scientists declare dawn of human-influenced age Read more

The changes that our aliens could observe from space are not hard to spot. In just the last few thousand years, the patterns of vegetation altered much faster than before. These human-induced changes signalled the start of agriculture.

And human activity manifested itself in other ways that will leave traces in the geological record. Constructs of concrete and metal sprawled across the continents; domesticated vertebrates numerically overwhelmed wild ones; the carbon dioxide in the atmosphere rose anomalously fast; traces appeared of plutonium and other “un-natural” substances.

The imaginary aliens watching our world would have noticed something else unprecedented in geological history. Rockets launched from the planet’s surface escaped the biosphere completely. Some were propelled into orbits around the Earth; some journeyed to the moon and planets.

What do these trends portend? Should we be optimistic or anxious? It’s surprising how little we can confidently predict – indeed, we can’t predict as far ahead as our forebears could. Our medieval ancestors thought the Earth was only a few thousand years old, and might only last another thousand. But they didn’t expect their children’s lives to be very different from theirs. They built cathedrals that wouldn’t be finished in their lifetime.

Our time horizons, both past and future, now stretch billions of years, not just thousands. The sun will keep shining for about another 6bn years. But ironically we can’t forecast terrestrial trends with as much confidence as our ancestors could. Their lives and environment changed slowly from generation to generation. For us, technological change is so fast that scenarios quickly enter the realm of wild conjecture and science fiction.

But some things we can predict, at least a few decades ahead. By mid-century, the world will be more crowded, and our collective footprint will be heavier. World population is now 7.2 billion and is forecast to rise to around 9 billion by 2050. Experts predict continuing urbanisation – and huge growth of megacities such as Lagos, São Paulo and Delhi. Population trends later this century depend largely on what happens in Africa, where some UN predictions foresee a further doubling between 2050 and 2100.

Moreover, if humanity’s collective impact on nature pushes too hard against what Johan Rockstrom calls “planetary boundaries”, the resultant “ecological shock” could irreversibly degrade our biosphere. And if global warming reaches a tipping point that triggers melting of Greenland’s ice, coastlines a millennium hence would be drastically different. Extinction rates are rising. We’ve only identified about two million of the (estimated) 10 million living species: we’re destroying the book of life before we’ve read it. To quote the great ecologist EO Wilson, “mass extinction is the sin that future generations will least forgive us for”.

The darkest prognosis for the next millennium is that bio, cyber or environmental catastrophes could foreclose humanity’s immense potential, leaving a depleted biosphere. Darwinian selection would resume, perhaps leading, in some far-future geological era, to the re-emergence of intelligent beings. If this happens, or if there are aliens out there who actually visit and study the Earth, then, digging through the geological record (and applying archaeological techniques as well) they would uncover traces of a distinctive transient epoch, and ponder the all-too-brief flourishing of a species that failed in its stewardship of “spaceship Earth”.

But there is an optimistic option.

Human societies could navigate these threats, achieve a sustainable future, and inaugurate eras of post-human evolution even more marvellous than what’s led to us. The dawn of the Anthropocene epoch would then mark a one-off transformation from a natural world to one where humans jumpstart the transition to electronic (and potentially immortal) entities, that transcend our limitations and eventually spread their influence far beyond the Earth.

Even in a cosmic time-perspective, therefore, the 21st century is special. It marks our collective realisation that the Anthropocene has begun – and it’s a century when human actions will determine how long that epoch lasts.