We don’t know how or why it happened–perhaps a particularly deadly, Homo sapien-specific virus. Or maybe it was a more drawn-out decline brought on by unfettered climate change, escalating conflict, and collapsing food supplies. Regardless of the circumstances, imagine that people have gone extinct. Suddenly the geologically brief moment of human dominance on Earth is over. So, what happens next?  We only have speculation. There’s no single correct answer, nor certainty. Yet people have considered versions of this apocalyptic thought experiment for centuries, if not millenia, says Carlton Basmajian, an associate professor of community planning at Iowa State University who studies cities. Ideas of human extinction and societal dissolution are rampant in religious texts and myths of ancient cultures.  In recent decades our environmental footprint has become more intense and well-understood and so, perhaps, has awareness of our own fragility. Through the threat of the atomic age, pandemics, and climate change, we have a clearer sense than ever before about how we could be eliminated or eliminate ourselves. “Perhaps people are even more cognizant now of the limits to human survival,” says Basmajian.  And, perhaps by considering Earth after us, we can get a realistic understanding of all the ways we’ve shaped and changed the planet, says Alan Weisman, an environmental journalist and author of multiple books including the 2007’s The World Without Us. For him, writing on the idea was a way to reach those who might otherwise be turned off by environmental literature, and get a broader swath of readers to consider the reality of our species-level legacy. “If we all just suddenly vanished, everything left would be the sum total of our environmental impact.”  Infrastructure degrades Without people around to keep things running, water and electricity would quickly stop flowing through pipes and wires, says Basajian. Gas and coal power plants require a steady diet of fuel and water pumps need both human operators and power. The subway and traffic tunnels underneath major cities would flood, says Weisman, without the functioning pumping systems that keep them dry now.  In humid environments, interior drywall would mold. Fallen trees would crush roofs in storms. Fires would go unextinguished. In seismically active zones, earthquakes would wear down and eventually topple structures. Vining plants would cover walls and push apart bricks and siding. And wooden structures, including the vast majority of residential buildings framed with wood beams, would rot. Even creosote-soaked telephone poles wouldn’t last more than 20 years, estimates Basmajian. “Anything wood, especially in a wet climate, is going to degrade pretty quickly,” he says. Newer construction buildings, built from the 1980’s onwards, are made from lighter wood and lower quality materials and would fall apart especially fast, he adds.  Mid-century steel and glass skyscrapers would last longer, Basmaijan imagines, but not forever–especially considering those flooded tunnels, which could easily collapse streets and lead to inundated foundations. The Empire State Building is anchored to bedrock, but if water were to seep into its lower levels from the subterranean train tracks beneath West 33rd Street, perhaps its supports would corrode and fall away.  Sturdy stone buildings would remain standing the longest, predict both Basmajian and Weisman. But over the course of a few centuries, most every municipality would fall into visible ruin–resembling the abandoned monuments and cities of the fallen Roman Empire, the Ancient Egyptians, or the Incas. Even now, there are parts of some American cities that have been effectively abandoned, which show how quickly these processes of decay take hold, Basmaijan notes. “Houses are caved in, streets are cracked, trees grow through structures,” all in a matter of a decade or two.  View of the ancient Inca city of Machu Picchu located in the Andes. Credit: Artur Widak/NurPhoto via Getty Images Of all surface-level human infrastructure, Basmaijan says roads would be the most persistent signs of civilization. Major highways through dry, non-seismic areas would be especially prone to lingering. Yes, they’d crack as the planet shifts beneath them, but “they’re so big and so over-engineered that they would persist for a long time,” he says.  If buildings, roads, and ruins were buried by sediment–perhaps in flood events or through land subsidence–they’d be more likely to last, says Jan Zalasiewicz, an author and emeritus professor of paleobiology at the University of Leicester in the U.K., who penned the 2008 book The Earth After Us.  Still though, we’re only talking on the order of thousands of years. With more time, less than a million years–“surface evidence of humans would be gone or difficult to recognize,” estimates Zalasiewicz. Some artifacts, like Bronze sculptures, ceramic pots and mugs, and gold ingots will hang on, buried by time, says Weisman. And still other signs will remain sub-surface. There will be so-called technofossils, made up of durable metals and plastics, Zalasiewicz explains, as well as fossil evidence of the mass extinction, climate change, and sea level rise that humans have precipitated.  The nuclear elephant in the room You might be wondering about nuclear power plants. Assuming they’re online at the time of the human extinction, many of the 440 active nuclear plants on Earth would eventually melt down without maintenance. Water would evaporate from their cooling systems, and the rising heat would lead to nuclear explosions. Would that be enough to vaporize or irreparably mutate everything on the planet, extinguishing life? Maybe, says Weisman, “that’s a wild card.”  But maybe not. The worst meltdown ever to occur was the Chernobyl explosion, which released about 400 times as much radioactive material as the bomb that the U.S. dropped on Hiroshima. Radioactive fallout from Chernobyl contaminated about 58,000 square miles of land. The most acutely impacted zone, known as the Chernobyl Exclusion Zone, is about 1,000 square miles.  A view of a damaged building as Ukrainian soldiers visit the 30-kilometer exclusion zone surrounding the Chernobyl nuclear power plant and the town of Pripyat where thousands of people who once worked at the nuclear power plant live, in Chernobyl, Ukraine on March 17, 2024. Credit: Gian Marco Benedetto/Anadolu via Getty Images Yet Earth is 197 million square miles big. “The damage here will be relatively minor and local,” says Zalasiewicz. There are caves, deep oceans, and large expanses of land and water very far away from any nuclear plant. It’s probable that lots of life would survive 440 Chernobyls. After all, atomic weapons tests following WWII released about 100 to 1,000 times as much radioactivity compared to Chernobyl, according to estimates from the International Atomic Energy Agency, and we’re still here.“We’ve had meltdowns. They char an area, but not the entire planet. It’s a big disastrous fire and a wave of radiation, but it doesn’t go on forever,” says Basmajian. And even in the most irradiated parts of Chernobyl, nature has made a big comeback. In many ways, the Zone is now something of a wildlife refuge, with lingering radiation protecting plants and animals from people. “When humans fled, nature flooded in,” says Weisman.   Life finds a way A short list of species would be doomed to die with us, including co-evolved pests like head lice and bed bugs, as well as human-specific microbes, says Weisman. Then, there are the domestic breeds of animals people have created, which would quickly be outcompeted by their wild peers, cows and dogs wouldn’t last long on their own, he says. Housecats, in contrast, would likely be fine–continuing to live on as feral mesopredators–eating birds and rodents and being eaten by coyotes, wolves, bears, and bigger cats.  A handful of endangered species are currently supported through human breeding programs and hands-on interventions. The fate of animals like giant pandas, vaquita porpoises, and Panamanian golden frogs would depend on the conditions that exist for these animals in our absence, says Weisman. Most of their difficulties, like habitat loss or poaching, are human-caused. Yet critically small populations and established animal diseases wouldn’t necessarily magically resolve once we disappear.  For everything else, life would continue on. Humans have shaped evolution in innumerable ways. Some of our influence would linger on in our absence. Climate change will remain a factor for millennia. Though temperatures will stop increasing after a few decades without more human emissions, the global average won’t fall back to what it was before the industrial revolution for thousands of years. Carbon feedback loops that have already been triggered, including wetland methane release and permafrost thaw could extend this timeline, says Weisman.  On a local level, heavy metal and chemical pollution from unmaintained factories and infrastructure will exert pressures on the organisms trying to survive nearby, along with the radiation from those aforementioned meltdowns. Though with enough time, all of this contamination will be diluted until the effects are negligible.  Despite these difficulties, and despite the mass extinction we’ve already caused, we won’t be leaving behind a barren Earth. In the worst past extinction, the Permian-Triassic, 80-90% of all marine species and 70% of all terrestrial vertebrates died off. Then, life rebounded, as it always seems to do. “After every extinction event, this place is devastated, but life is so resilient it comes back,” says Weisman.  It’s impossible to know what exact species might succeed us to dominate the planet, if anything–perhaps things settle to a different sort of equilibrium. But “as in past extinctions, the survivors are usually the generalists–small, tough, adaptable,” says Zalasiewicz. Rats may no longer have human structures and food to rely on, but maybe they’d find new resources to take advantage of in their large social colonies, he posits. The universal long-haul In our absence, Earth would continue on. Our planet does not ‘need’ us. Life would last at least until our sun grows too hot to support it. Even if some organisms manage to survive boiled oceans, when our star begins its final death knell and expands into a red giant, billions of years from now, “all the inner-ring planets will probably turn into cinders,” says Weisman.But while rocks retain their layers in this heating process, our impacts on the planet will remain visible. “As long as the Earth persists and has recognizable strata, the signs of humans will persist,” says Zalasiewicz.  And maybe, deep in space, some signs will last even longer. The voyager spacecrafts that have left our solar system were built to persist and carry evidence (in the form of a golden record) of human civilization to possible distant alien worlds. Barring a collision or a black hole, they’ll keep going. “If I had to place a bet–and it’s a safe bet because no one will ever be able to collect on it–I would say those are going to be the longest lived human artifacts out there,” says Weisman. This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us. The post Without humans, what would happen to Earth? appeared first on Popular Science.