Humans will likely go extinct eventually, leaving behind a planet that has to adjust in their wake. While there is no true consensus as to what a human-free world will look like, there are a number of theories.
What will the Earth look like?
It is most likely that humans will be the cause of their own demise. "While there's a chance our species can survive the effects of climate change, it looks increasingly likely that we and many other living things will go extinct," said JV Chamary at the BBC. While warmer surface and ocean temperatures will affect biodiversity long into the future, without humans, an ice age could hit the planet. "Climate change may have postponed the next Ice Age, which could occur within 100,000 years," said the BBC.
The dregs of humanity, however, will likely stick around for a while. Humans "would leave mountains of waste that would persist for thousands of years, poisoning, trapping, cutting and harming other species in numerous ways," said The Spectator. The wooden houses and buildings left behind, though, would be "destroyed by termites, and creepers would grow through the cracks, causing the wood to rot and eventually collapse," said Science ABC. Concrete structures, on the other hand, would "deteriorate due to extreme weather conditions, leading to their eventual collapse."
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What would happen to other species?
It's no secret that humans have a significant impact on the entire planet, essentially living and operating on every corner of the world. As such, their disappearance would have major effects on many other animals. "Natural selection will drive some populations to split into distinct species" to "exploit vacant ecological niches," said the BBC. "This 'adaptive radiation' is why mammals diversified after dinosaurs died out." Farm animals would be "easy prey for bears, coyotes, wolves and panthers," said Carlton Basmajian, an associate professor of urban design at Iowa State University, at The Conversation. "The cats would go feral — that is, they would become wild, though many would be preyed upon by larger animals. Most dogs wouldn't survive, either."
Perhaps a new dominant species could take control. Some experts have suggested that the octopus could become the next big species after humans because of its potential for "filling an ecological niche in a post-human world," said Tim Coulson, an Oxford University biologist, at The European. Other primates, birds, fish and even insects are potential apex species as well. Much of how the world will look depends on the world that humans leave behind. If an ice age came to fruition, for example, larger animals would be evolutionarily favored.
How would the Earth recover?
While the Earth would go through turmoil in the immediate aftermath of human extinction, "in a thousand years, the world you remember would still be vaguely recognizable," said Basmajian. "Some things would remain; it would depend on the materials they were made of, the climate they're in and just plain luck." Chances are, the loss of humans will prove to be a positive for nature. For example, air pollution improved during the Covid-19 pandemic due to protocols during the lockdown. However, there are many still many variables, such as "when the mass extinction occurs, the power of technology at that time, the probability of wide-scale explosions and unforeseen climate changes, that impact what the outcome would be and how long it would take," said the Spectator.
Based on what is known about previous mass extinctions, "we often see biodiversity recovering after major mass extinctions within roughly 2-5 million years," said Erin Saupe, a palaeobiologist at Oxford University, to the BBC. But "recovery can take longer for the most severe environmental perturbations."
Ultimately, the extinction of humans "would reveal something about the way we treated the Earth," said Basmajian. "It would also show us that the world we have today can't survive without us and that we can't survive if we don't care for it. To keep it working, civilization, like anything else, requires constant upkeep."
