To we humans, the Earth and the nurturing Sun it orbits are constant and immutable. You can go to bed at night safe in the knowledge that the Sun will rise in the morning and the planet we call home will be just as it has always been. Unfortunately this assumption is desperately flawed.
Humans are like the toddler who can’t conceive of a time that he didn’t exist – for the entire duration of our existence (and that of our entire species) very little has changed on planet Earth, so it’s natural to assume that it will always be so. What we don’t take into account is just how insignificantly short our tenure on planet Earth has been.
Modern humans have walked the planet for 200,000 years, which seems like a long time until you realise that life on Earth first started some 3.8billion years earlier – that’s 3,800,000,000 years. Compared to vast pantheon of life that preceded us, humans have really only just turned up to the party. In all that time when we weren’t around to bear witness, the Earth has changed very drastically indeed.
But surely if life has persisted for almost four billion years, it’s not unreasonable to assume the same will be true for the next four billion years? Well, that assumption is not only unreasonable, it is just plain wrong and it may be that, when it comes to life on Earth, the party is almost over.
Above: The last Earthlings: Just as they were the first to colonise our planet, the last life on Earth will be bacteria. But even bacteria is unlikely to survive beyond the next billion years as Earth becomes as inhospitable as Venus
The trouble is, our Sun is not nearly as constant as we like to believe. For the last 4.6billion years, it has been burning through hydrogen at the rate of 600million tonnes every second and, for all of that time, it has been getting steadily hotter – and its not going to stop. This steady increase is going to have a catastrophic impact on our lovely little blue world.
So far we have been lucky – our planet has always occupied an orbit that is just the right distance from the Sun for life to thrive but, with the Sun getting brighter, we will soon find that orbit uncomfortably close.
Estimates of when it will happen vary, but within the next billion years (but possibly as soon as 370million years) the Sun will have increased in strength so much that carbon dioxide levels in the atmosphere will fall and plant life will die out. Animal life will quickly follow.
As temperatures increase, the oceans will boil away into the atmosphere – wrapping the planet in a thick insulating blanket of gases. This thick, moist atmosphere will trap even more heat and, before we know it, the Earth will be transformed into searingly hot, Venus-like world. At this point, some bacteria may still be clinging to life in some corners of the planet, but they are just postponing their ultimate fate.
Over the next two billion years, heat from the Sun will be so intense that the surface of the Earth will become hot enough to melt rock. Thanks to natural geological cycles and spurred on by the loss of the oceans, the Earth’s tectonic activity will shut down and the core will solidify. Without the swirling molten dynamo to drive it, the Earth’s magnetic field will collapse – leaving the atmosphere to the mercy of space.
Bombarded by a constant stream of solar radiation, in as little as 10,000 years the atmosphere will be sand-blasted from the planet and the last vestiges of the gases that you and I once breathed will be lost to space.
Earth is now dead. All life and all of evidence that it ever existed have been wiped from the planet.
With no one to marvel at the sight, over the next few billion years, the Sun will grow ever larger in Earth’s black sky until it fills the heavens.
Above: When the Earth is finally swallowed by the Sun in about seven billion years, life’s brief tenancy will be just a distant memory
So we could already be 92 per cent of the way through the Earth’s habitable period – but that’s not as bad as it sounds. That last eight per cent still represents at least another 370 million years, which is really quite a long time. The 370 million years that preceded man’s arrival on Earth saw the first trees conquer the land, the evolution of the first amphibians, the rise (and fall) of the dinosaurs and the appearance of the first ape.
There is another ray of hope in the fact that the same process that will bump the Earth out the habitable zone, will see Mars move in to occupying it. Perhaps we could move there?
That is, of course, assuming humanity hasn’t already been destroyed by cataclysmic (self-induced) climate change, nuclear armageddon or by a race of giant flesh-eating space-badgers.