What if humans disappeared?

From damming rivers to changing the chemistry of the atmosphere, humans have had a powerful impact on our planet. But what would happen if we suddenly disappeared? The first few weeks would be chaotic. Within hours, power plants would run out of fuel and shut down. As lights go out and electric fences lose their sting, over one and a half billion cows, nearly a billion pigs and more than 20 billion chickens will break out of their enclosures, desperate for food. Without any humans to feed them, most of these livestock will starve or become food for over half a billion dogs and a roughly equal number of cats who now have to fend for themselves. Of course, many of our fancy breeds are ill-suited for life in the wild, and will probably be outcompeted by hardier mutts, not to mention wolves, coyotes and wildcats. Other animals that depend on humans, from rats to cockroaches, will suffer drastic population declines. Some, like body lice and head lice, will go extinct. In the cities, many of our famous boulevards are now rivers. Without electric pumps to keep them dry, underground subway tunnels will quickly fill with water. Other streets will be overtaken with weeds and vines, followed by larger plants and trees. But before that happens, most cities will be wiped out by fire. Modern houses, especially in the suburbs, are still largely made of timber. With no firefighters around, a single lightning strike is all it takes to start a fire that could burn whole subdivisions to the ground. In the countryside, many wood structures will be destroyed within a few decades, if not by fire, then by termites and other decomposers. After 100 years most wood structures will be gone, and anything made of steel - from apartment buildings to cars and even bridges - aren’t far behind. Steel is mostly iron, and without constant application of paints and coatings, it will quickly react with oxygen in the atmosphere and return to its native form of iron oxide, or rust. By this time humans have been gone for a few hundred years, and most species of animals around the world - at least, the ones we haven’t driven to extinction - will have bounced back to the levels they were at before we evolved. But their distribution will remain forever altered. Camels now roam Australia, while in North America dozens of species of songbirds imported from Europe will continue to thrive. It’s even possible that in some parts of the world, escaped zoo animals could form new wild populations, leading to the prospect of lions on the Great Plains, or hippos in South American rivers. Forever proliferating through space will be the electromagnetic radiation we created from our radios, satellites and phones. But if anything will outlast us on Earth, it may be our trash. The chemical bonds that hold together plastics or vulcanized rubber are immune to most of the digestive enzymes used by bacteria to break down natural polymers and unlike metals, plastics don’t rust or corrode. These microplastics escape into waterways or drift along in the ocean and are eventually deposited in sediments. Hundreds of millions of years from now, alien geologists from outer space may be surprised to find sedimentary rocks full of tiny carbon-based particles that were once part of rubber tires or plastic bags. Of course, whether something survives depends greatly on the conditions. Everything will last much longer in deserts, where there is no moisture to speed up rusting or support decomposing organisms. And while the carbon cycle will return CO2 levels to equilibrium after a few thousand years, local deposits of long-lived organic chemicals or radioactive material could persist for a very, very long time. It’s hard to know what alien paleontologists of the future will make of us; how they will explain our love of plastic or the fact that within a geological eyeblink we exploded out of Africa to colonize virtually every inhabitable space on earth. But they’ll definitely wonder why, if we were so successful for so long, we disappeared so quickly.

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