By Kelsey Piper

A volcanic eruption in Indonesia. Supervolcanoes are among the natural threats that could kill humans.

Asteroids, gamma rays, and supervolcanoes are all exceptionally rare but could be catastrophic if they happened.

An asteroid killed the dinosaurs. Could that happen to us? What about a supervolcanic eruption blocking out the sun? Or a solar flare or nearby supernova event?

Anders Sandberg is a researcher at Oxford’s Future of Humanity Institute, where he writes about existential risks — dangers that threaten the continued survival of our species. Existential risks can be either man-made (like nuclear war, artificial intelligence, or bioengineering) or naturally occurring, like the asteroid that took down the dinosaurs.

In a 2018 paper, “Human Extinction From Natural Hazard Events,” Sandberg takes a look at the latter category.

So how likely are we to die of natural hazards, if we manage not to destroy ourselves with man-made ones? Cataloging all the things that could go wrong for Earth, Sandberg finds there are natural threats that are real and merit thinking about. But there is also a simple argument that we’re pretty safe: These events generally have a constant chance of occurring yet haven’t occurred in the past 65 million years. That means the chance they’ll sneak up on us in the next few centuries is very small.

How would disasters cause human extinction?

There are very few natural catastrophes that could annihilate all humans in one blow. Most of them would kill many or most humans while rendering Earth uninhabitable, and that may eventually kill the survivors of the initial catastrophe.

The difference between killing most humans and killing all humans might seem pretty irrelevant to many of us, who will be dead either way. Researchers into existential risks to humanity care about the difference a fair bit, though. If Earth-originating intelligent life is gone forever, that’s it for all the people who ever might have lived, all the things they ever might have done, and all the worlds they ever might have settled. In that respect, it’s a much bigger loss than 7 billion deaths.

There are lots of prospective disasters that would kill many humans, maybe even most of us. Killing all humans, though, is a taller order. We can learn how populations bounce back from near-extinction events by looking at other animals and by statistically modeling small human populations.

If there are at least a couple thousand humans in a survivor community, we’ll probably make it. If there are fewer, then due to genetic effects and bad luck, we probably won’t.

“Whether the population recovers or dies out depends on whether the survivors can form communities larger than the ecological minimum viable population (MVP). While small founder populations can be lucky and grow into large and stable populations, this is relatively unlikely. Populations smaller than the MVP are likely to become extinct due to further disasters or demographic, environmental, or genetic stochasticity,” Sandberg’s paper finds. “Simulations suggest that for humans with mortality and fertility as hunter-gatherer societies MVP is on the order of a few thousand individuals.”

That’s good news for the survival of humanity. Even disasters that killed billions of us could potentially be survivable for the species, as long as there are some areas where large survivor communities can flourish.

Supernovas and gamma ray bursts could make Earth uninhabitable

One risk Sandberg discusses is something straight out of comic books: supernovas or gamma ray bursts.

Supernovas happen when sufficiently large stars reach the end of their life. They release an unfathomable amount of energy and will likely kill most life on any nearby planets.

Gamma ray bursts are another high-energy astronomical event. Their cause isn’t as well understood, and they too release vast amounts of energy, enough to endanger nearby worlds. These risks, Sandberg argues,

could conceivably harm biospheres at astronomical distances. … Unless the explosion is close enough to cause heating, the risk comes from radiation penetrating the atmosphere, a resulting UV flash, possible cosmic ray showers, and formation of nitrous oxides that deplete the ozone layer, produce acid rain, and cause multiyear cooling climate effects. The effects, if intense enough, could plausibly cause a mass extinction.

Astronomers have observed these events in our galaxy — luckily, too far away to do us any harm.

Should this keep us up at night? While they’d be catastrophically bad if they happened, supernovas and gamma ray bursts are extremely rare. We also mostly know which astronomical objects cause them, and we can check our immediate vicinity to see if we have any neighbors that are candidates to go supernova or release a gamma ray burst soon.

Lucky for us, there aren’t: “There are at present no supernova or GRB candidates in the vicinity of the sun that will explode in the current epoch,” Sandberg finds. Our understanding of high-energy astronomical events like these is still developing, but it looks likely that we’re safe.

Asteroids have likely caused mass extinction on Earth before

Dinosaurs and many other species likely went extinct at the end of the Cretaceous period, due to an asteroid impact that rapidly changed the climate worldwide.

So could that happen to us? And would it kill us all if it did?

Researchers have modeled the effects on Earth of impacts from different objects. While there are some small effects from an object’s velocity and from the impact site, by far the most important factor in the damage done by an impact is the mass of the object. One study found that less than 1 percent of the human population would be killed by an impact from an object with a 1-mile diameter (1.6 km), while 100 percent of the human population would be killed by the indirect effects of an impact from an object 6 miles (10 km) across.

But Sandberg writes that these numbers are “at best an educated guess.” Since impacts we can observe are fairly rare, our models of their effects are incomplete.

NASA is responsible for mapping near-Earth objects (NEOs) that are larger than 1 km across. It has mapped more than 90 percent of them, …read more

Read more here: Here are the ways nature could wipe out humanity

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