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Here are the ways nature could wipe out humanity

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

Sinabung Volcano During Golden Sunset In Karo
A volcanic eruption in Indonesia. Supervolcanoes are among the natural threats that could kill humans.
Sutanta Aditya/Barcroft Images/Barcroft Media via Getty Images

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, which means we know their trajectories and can be confident that they aren’t on their way to impact Earth. The handful of NEOs that are larger than 10 km across — that is, big enough to kill us all if they struck Earth — are all in safe, stable orbits.

There’s another category of objects to worry about, though: long-periodic comets that swing through our solar system so infrequently that astronomers haven’t mapped them yet. To guess how likely that is, we have only rough estimates from looking at the comets we’ve observed; Sandberg concludes tentatively that the danger may be “on the order of one 2+ km impactor per 5 to 10 million years.”

That isn’t very often. Given how disastrous an asteroid or comet impact would be if it happened, though, and given that we might well be able to prevent it if we saw it coming far enough in advance, there’s a strong argument that it’s worth some effort and funding to improve our tracking of NEOs and our understanding of the situation.

Sandberg notes that “traditional statistical life valuations suggest that a $16–$32 billion annual investment in asteroid defense would be cost-effective, yet U.S. government spending on asteroid detection (with no mitigation) is around $4 million per year.” Impacts might be unlikely to kill us, but it’d still probably be sensible to invest further in making us more confident in that.

Of course, there are other risks that are much greater than the risks posed by asteroids, so if someone had an additional $1 billion to spend annually, it wouldn’t make sense to focus it on asteroids. But if we funded research into all hazards to our future in proportion to their plausibility, the existence of promising avenues for additional research, and the good we could do by mitigating them, we’d spend significantly more resources on asteroid detection than we currently do.

A supervolcano could cause rapid, devastating environmental changes

Humanity almost went extinct once before: It can be estimated from genetic analysis that the population once dipped to exceptionally low levels. Researchers think this might have been prompted by the supervolcanic eruption of Toba 70,000 to 75,000 years ago, though this is only one of many competing theories.

Whether or not a supervolcano has driven humans to the brink of extinction before, such massive volcanic eruptions — which release toxic gases and cool the planet by filling the upper atmosphere with sunlight-deflecting compounds — are suspected to have triggered several other mass extinctions in the fossil record. “Such events are rare,” the paper concludes, “with the geologic record suggesting return times of ten to a hundred million years.”

But would that kill humans today? That’s less clear. Massive changes to our climate — anthropogenic ones like we’re facing today, as well as ones caused by a supervolcano — would certainly kill many people. But humans today are spread more widely across the world than we were at the time of the near extinction. We can adapt to a wider range of climates. There are people working to make our food system resilient to shocks from a supervolcano, impact, or nuclear winter.

So in some respects, modernity has made us less vulnerable to shocks to our food supply or climate. In other respects, though, it’s made us more vulnerable. Many of our food, communications, and energy systems are now global, which means that shocks to the system will affect the whole world.

Sandberg argues that “many human systems such as food, energy, finance and communications are global, densely interconnected systems where failures can cascade rapidly,” which means that “the probability of global failures also has increased compared to more local, modular and redundant systems.”

Overall, it’s hard to say if we’re safer from extinction due to a supervolcano than we used to be. More preparation to maintain the food supply in a catastrophe might make us significantly safer.

Risks to humanity are under-studied and mitigation is underfunded

There’s a recurring theme here — most natural risks are pretty rare, but still worth studying and preparing for. Yet there’s not much in the way of mitigation efforts going on today, and there’s not enough research to answer key questions about risk management. “There are more academic papers on dung beetles than the fate of H. sapiens,” Sandberg writes. That’s a bizarre state of affairs. What’s going on?

Sandberg thinks it’s a combination of several factors. It’s not that most people don’t believe the world is in danger — the public actually tends to consider human extinction this century likelier than many of the experts think it is. But people still have a hard time taking seriously, or planning for, unlikely, “sci-fi,” and out-there events. In addition to that, there’s a free rider problem — most of us (probably) don’t want humanity to go extinct, but ideally, other people would pay for the work to protect us.

Ultimately, the biggest reason that natural risks to humanity are understudied may be one Sandberg’s paper doesn’t really discuss. Man-made risks are bigger than natural ones — most of the ways humanity is likely to drive itself extinct are caused by our own weapons and technologies, not by random acts of nature.

That means that mitigating risks requires more careful and thoughtful development of new technology, measures to avoid deployment of unsafe systems, and international coordination to enforce agreements that reduce risk. All of those are uphill battles. No wonder it’s more rewarding to study dung beetles.

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