If you were to say that Yellowstone erupting leads to World War III, well, I wouldn't argue that it's implausible. But, similarly, world wars are not extinction-level events. Nuclear weapons can't cause extinction: nuclear winter is basically volcanic winter with a much less effective stratospheric pump (a VEI6 eruption, such as Krakatoa, pushes at least 1km³ of rock into the air; a VEI8 eruption, like Yellowstone, is 100× that). There's a hard cap on the proportion of your population you can mobilize to front-line troops, and while it is possible for one side's losses to exceed this fraction, on a global scale, only 60% or so of the population can be killed in a war, a far cry from the 99.9999% you'd need to kill to effect extinction.

It's certainly possible for an astonishingly large proportion of the population to perish, but extinction requires extermination to be nearly complete.

I think your expectations are overly optimistic. Perhaps it wouldn't be 99.9999%, but in the event of global nuclear war, supply chains for virtually everything (including food) will come to a complete halt. People in cities will be utterly screwed when fuel and food run out.

People in farmland might not be that much better off. Masses of starving, potentially-armed people searching for food will not necessarily be the most respectful of people's property rights. We can expect animal wildlife populations to plummet as they're killed for food, and agricultural output to drop precipitously as supply chains for fuel, fertilizer, and non-terminator-gene seeds go to zero, not to mention as it becomes exceedingly difficult to protect large patches of farmland from raids.

We have exhausted enough of the easily-accessible resources close to the Earth's surface that all it takes is a sufficient dip in technology, and we enter a rut which we can never get back out of.

> only 60% or so of the population can be killed in a war

Why? I'd assume that biological warfare could do more, or destroying water/power supplies.

That's reasonable but even in a world war, you need only put people on nuclear submarines to protect humanity from extinction. Martian colonies are not necessary.

I think at the point where you're talking about saving fewer than hundreds of thousands of people out of billions (<= 0.01%), it's functionally an extinction-level event for humanity.

Extinction = 0.

Species can recover even from only a few mating pairs.

It would be catastrophic though.

Only in the most pointlessly absolute of interpretations.

Ten might as well be zero. 50 people (25 pairs) is necessary with complete control of breeding in order to have sustainable genetic diversity. Without complete control, hundreds are necessary in any given pocket of civilization.

Even if there were tens of millions of humans remaining, there is virtually zero chance of meaningful recovery since all of the low-hanging fruit of industrial-age energy resources have been mined.

Depends on your horizon, but if you look at how fast humanity progressed in 2,000 or even 500 years, I think a recovery from a million+ people in a few thousand years is very realistic, especially if knowledge is adequately recovered through books.

Only up until the technical level of the industrial revolution. Getting past that would require easy access to an easy to use energy source, i.e. fossil fuels. None of which are left. Today we dig kilometers under the seabed to access energy. A society just entering the industrial revolution will have not the knowledge, tools, or materials to do that.

Just try to invent some runaway positive-feedback loop of destruction that could make Earth less inhabitable than Mars. I bet you can't. I can't.

You can add the entire nuclear arsenal, super volcanos, meteors, gamma ray burst at will.

Alien invasion. Gray goo. The Book of Revelations.

> The Book of Revelations.

If God were going to destroy all of humanity in a Final Judgement, why we wouldn't He include the pocket of humans on Mars as well?

The Bible makes no mention of humans living on other planets. Mars is also named after a Greek god. It would be a good bet you'd be out of His jurisdiction.