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What would the world look like if humans disappeared?

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If humans were to die out tomorrow, how long would it take for nature to take over and overgrow most traces of our existence? After like 10,000 years, would you have to undertake an archaeological dig to find evidence of us, or would parts of major cities still be standing and distinguishable? —Jim Huff

WHY NOT take it a step further: what if humans never existed at all? As the Republican primary race drags on, I can’t say it’s not an alluring proposition, and—helpfully—one that was broached this year by researchers at Denmark’s Aarhus University. They came to the fairly obvious conclusion that, sans Homo sapiens, the rest of the world's fauna would be a hell of a lot better off—so much so that most continents would resemble Africa in the diversity of their mammal populations. In a human-free world, the authors imagine, not only wolves and bears but elephants and rhinos would right now be roaming northern Europe.

Alas, we have to work with the facts we’ve got, namely: (1) we exist on earth, and (2) someday we might not—whether by disease or nuclear winter, or because we’ve ditched this rock for one that’s not yet totally hosed. For the sake of your question, though, let’s imagine we simply vanished—a kind of nondenominational rapture.

As it happens, such a scenario was entertained by the journalist Alan Weisman in his 2007 book The World Without Us. Weisman’s conceit was apparently seductive enough that it inspired not one but two documentary franchises: the History Channel series Life After People and National Geographic Channel’s Aftermath: Population Zero. Granted, that latter title carries a real whiff of basic-cable cheese, but Weisman’s no slouch. Working from interviews with botanists, structural engineers, art conservators, et al., he credibly predicts what might happen in cities and less-populated areas, as well as at sites whose abandonment would lead to notably dramatic results—think oil refineries and nuclear reactors.

A particularly vivid passage gives the play-by-play in New York City. How quickly would urban infrastructure go to shit in a rapture scenario? Very, very quickly. “After we’re gone, nature’s revenge for our smug, mechanized superiority arrives waterborne,” Weisman writes. In New York’s case it comes from below: with no one to operate the pumps that keep water out of the subway tunnels, the system finds itself inundated in “no more than a couple of days.” (Superstorm Sandy gave us a taste of what this might look like.) As the water rises toward ground level, it eats away at the soil; within 20 years the streets collapse, becoming rivers. Pipes burst, gas lines ignite—your standard post-apocalyptic hellscape. Within 50 years, their foundations scoured out by water, skyscrapers start to falter and crumble. It’s another few centuries before trees really recolonize the place. (Interestingly, the animals that don’t make it are ones that adapted too well to human dominance, including several species fabled for their supposed indestructibility: cockroaches, which can’t handle northern winters without heating, and rats, which can’t replace the caloric value of a zillion tons of garbage.)

But you’re thinking on a bigger scale than this, Jim. Here are the headlines:

• Debris in high earth orbit stays there for more than a century.

• Suspension bridges collapse within 300 years; other, heftier designs might hold up for a millennium.

• In cities like New York, the most durable structures will be stone walls, like those of St. Paul’s Church; Weisman sees them lasting “thousands of years.”

• Meanwhile, the estimated erosion rate at Mount Rushmore is just one inch per 10,000 years. From this, Weisman extrapolates that we can expect parts of it to remain recognizable for about 7.2 million years.

In 10,000 years, then, a visitor surveying the earth’s surface will find it largely reforested, with stone ruins here and there indicating the former presence of human life. How long till those are gone too? Here’s where Weisman and another scientist who’s written on the subject—astrophysicist Mayank Vahia, of India’s Tata Institute of Fundamental Research—demur. Vahia suggests that stone and metal building materials will hang on “for tens of thousands of years,” while Weisman figures whatever’s still standing in 20,000 or so years will be erased by another ice age.

What’s left then? PVC plastics and glass remain under the ice, ground to a powder. Wiring and plumbing, which show up as subterranean metal deposits. Heavy metals and nuclear materials like uranium and plutonium residues, whose half-lives only begin at 24,000 years. You’ve heard of the Anthropocene, I presume—the name geologists have proposed giving to our current geological epoch, so profoundly affected by humans. Epochs are demarcated by identifiable shifts in the earth’s strata; the aforementioned is all the stuff alien archaeologists will find as evidence of us, millions of years in the future, just as today’s geologists find evidence of past glaciation. Of course, the likelihood of a coming ice age looks even dimmer now than it did back when Weisman wrote his book: we’re not doing such a hot job keeping the atmosphere cool. But that’s an existential problem for another day.