-A little more than 200 years ago,

the world experienced something that's on the edge,

I would say, of an existential threat

and that was the explosion of the volcano Tambora.

It exploded in 1815 and it was horrible.

The year after that was called the year without a summer

in Europe and much of America.

It reduced global temperatures

and that led to global levels of starvation.

Tambora is one of the reasons why we see

supervolcanoes as the single,

biggest natural threat to the future of the human race.

The astrophysicist, Neil deGrasse Tyson once said

that, "The universe wants to kill you,"

but the great thing is that we are not just dinosaurs

sort of wandering around the planet.

We have the scientific means to comprehend the risks

we face, and we actually have the ability to fight back.

(loud rumbling)

I'm Bryan Walsh, the author of

"End Times: A Brief Guide to the End of the World".

An existential risk is a global threat so great

that it could either cause human beings

to become extinct or reduce the human population

so much that there would really be no

human future that we know.

The true horror of an existential threat

is not my death, it's not your death,

the death of all 7.7 billion people

who exist now.

It's the nullification of the future.

The future matters to us more than we can really understand.

Humans are really terrible at evaluating these kinds

of existential risks, in part because of their rarity.

We may be afraid of terrorism or mass shootings.

Even though those events are actually

on a day by day base, very unlikely.

Because they pop out at us, we tend to overstate

the danger they pose to us.

Now, the reverse of that is often we see with existential

risks where, because they have never happened,

we tend to assume they must not be possible,

that's a mistake.

Five times over the course of the planet's history,

we've seen what are called major extinction events.

That's where you see something big happen

that might knock out three-quarters of the life

on this planet.

Obviously, the one people are most familiar with is

that asteroid that collided with the Earth 66 million

years ago that led to the extinction of the dinosaurs.

But four out of the five mass extinction events,

we believe really have some kind of volcanic cause.

Existential risk experts really believe that Toba,

was the closest human beings have ever come

to extinction.

This was an actual supervolcano that exploded

about 74 to 75,000 years ago.

That's a long time ago, but there were human beings

around.

It was an 8 on the Volcanic Explosivity Index,

which is 1 out of 8.

It's kind of a Richter scale for volcanoes.

You might remember something like Mount St. Helen's,

where that was so terrible it was actually

on the cover of Time magazine.

That was about 1% as strong as Toba was.

By some climate models, it would have reduced global

temperatures by 15 degrees fahrenheit or as much as 30.

The aftermath of big, supervolcanoes like Toba would be

cold, dark and very hungry.

When they blow, they blow all that debris,

including a lot of sulfur into the atmosphere.

It can actually reduce the amount of sunlight that can

come into the planet.

Volcanic ash is made up of little, almost hook like

rocks that will, actually, tear human tissue.

It also can contaminate crops, it can contaminate

water supplies.

It's very dense and can actually cave in roofs.

We still find deposits of ash from the Toba

super eruption, thousands of miles away

from the volcano.

You can see it in India, you can see it in parts

of Africa.

You can actually trace it back to Toba.

By some estimates, it may have reduced human population

to as little as 4,000 people.

That shows how unbelievably powerful,

how unbelievably dangerous how existential a supervolcano

of that kind of scale can be.

And, guess what?

That can happen again.

For instance, the famous Yellowstone Park.

All those geysers you see are an effect of an incredibly

powerful underground volcanic system

that one day could become strong enough

to actually create something on the scale

of a super eruption that would bring a halt

really to human life as we know it.

Awareness is really our best weapon against

supervolcanoes.

We can't stop them but what we can do is try to organize

society in a way that will actually be able

to endure that kind of event.

That means thinking through and preparing

for what's going to happen afterwards, for day two.

We could put food in a reserve.

We could actually begin to research methods

of food production that don't depend on the sun.

We could create something like a global monitoring

system so we have a much better sense

as to when the might explode.

Ultimately, what we're doing here is trying

to preserve the future.

And we can do that but only if we take this seriously.

We really have no excuse.

If we go extinct now, almost certainly it's because

we did not try hard enough to keep ourselves safe.

- [Director] If you liked this video with Bryan Walsh,

check out Freethink's new series

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