I've often wondered what the effect would be on the Earth if the human race winked out of existance overnight: I always figured the nuclear power stations would be where the trouble lay. Turns out, in this fascinating New Scientist piece, that multiple worldwide meltdowns wouldn't be such a big deal after all.
In fact, apart from long-term traps like groundwater and the ocean beds, it'd only take a few decades before the worst of the damage we're doing to the environment would have repaired itself.
Imagine that all the people on Earth – all 6.5 billion of us and counting – could be spirited away tomorrow, transported to a re-education camp in a far-off galaxy. (Let's not invoke the mother of all plagues to wipe us out, if only to avoid complications from all the corpses). Left once more to its own devices, Nature would begin to reclaim the planet, as fields and pastures reverted to prairies and forest, the air and water cleansed themselves of pollutants, and roads and cities crumbled back to dust.
“The sad truth is, once the humans get out of the picture, the outlook starts to get a lot better,” says John Orrock, a conservation biologist at the National Center for Ecological Analysis and Synthesis in Santa Barbara, California. But would the footprint of humanity ever fade away completely, or have we so altered the Earth that even a million years from now a visitor would know that an industrial society once ruled the planet?
A lot of its conclusions on how long it will takes cities and the more obvious tell-tale signs of humanity to disappear are based on what happened around Chernobyl in the years following the disaster – buildings are crumbling thanks to invasive plants, but wildlife (after a few years dominated by rats and feral dogs) is booming:
“I really expected to see a nuclear desert there,” says [environmental biologist Ronald] Chesser. “I was quite surprised. When you enter into the exclusion zone, it's a very thriving ecosystem.” [snip] Wild boar are 10 to 15 times as common within the Chernobyl exclusion zone as outside it, and big predators are making a spectacular comeback. “I've never seen a wolf in the Ukraine outside the exclusion zone. I've seen many of them inside,” says Chesser.
It's not all rosy – some species are past the point of no return already, some damaged ecosystems have already struck a new balance that squeezes out native wildlife, and nothing's stopping global warming for a while yet – but for the most part things just keep on getting better.
All things considered, it will only take a few tens of thousands of years at most before almost every trace of our present dominance has vanished completely. Alien visitors coming to Earth 100,000 years hence will find no obvious signs that an advanced civilisation ever lived here.
[snip]
Ocean sediment cores will show a brief period during which massive amounts of heavy metals such as mercury were deposited, a relic of our fleeting industrial society. The same sediment band will also show a concentration of radioactive isotopes left by reactor meltdowns after our disappearance. The atmosphere will bear traces of a few gases that don't occur in nature, especially perfluorocarbons such as CF4, which have a half-life of tens of thousands of years. Finally a brief, century-long pulse of radio waves will forever radiate out across the galaxy and beyond, proof – for anything that cares and is able to listen – that we once had something to say and a way to say it.
But these will be flimsy souvenirs, almost pathetic reminders of a civilisation that once thought itself the pinnacle of achievement. Within a few million years, erosion and possibly another ice age or two will have obliterated most of even these faint traces. If another intelligent species ever evolves on the Earth – and that is by no means certain, given how long life flourished before we came along – it may well have no inkling that we were ever here save for a few peculiar fossils and ossified relics. The humbling – and perversely comforting – reality is that the Earth will forget us remarkably quickly.