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[Great Order]

Let's assume we don't do anything like build underground bunkers or make any special efforts to survive:

If the ozone layer is stripped away in an instant, how long could humanity survive before succumbing to the effects of irradiation? Ozone layer will fix itself eventually obviously, but I'd imagine that'd take longer than the human race would have under this scenario.

[Frumple]

Like... a few months, something like that? The bigger problem than the direct effects of irradiation on human biology is that it'd kill basically every plant on the planet, collapsing most of the biosphere. Starvation would set in pretty rapidly. Eating the last of the animals before they starved, too, would extend things for a bit, but most of the biosphere, us included, would probably be dead within the year. We wouldn't last very long even if the sunlight itself didn't kill us off.

[Il Palazzo]

Like... a few months, something like that?

[citation needed]

[Frumple]

I mean, google's as close for you as it is for me

From a lookabout, though, there's not much argument it would be lethal for basically everything. Smaller plants die within a pretty short period (they can't photosynthesize without ozone cutting back on the solar radiation, they starve), larger ones last longer but not that long. Once those are gone, the ecosystem's gone shortly after and we last as long as it takes to starve.

Stored food might stretch things out a bit for a limited population, but, like, most of the species isn't going to make it when more or less every perishable foodsource perishes. You got a few weeks for the edible plants to die, a few more weeks for everything that ate them to die. Majority of most species (human included) is dead within a few months, depending on how the social stuff shakes out, the rest of it may follow shortly after. Few months to a year, there's just not much left anywhere.

[Great Order]

Ocean life should be fineish, water's a pretty good radiation absorber. Terrestrial life's definitely buggered, as is any shallow/surface water life.

Give it another few million years and I guess life might start crawling out of the sea again.

[Il Palazzo]

Citation needed for a few months. Not for lack of ozone is bad.
How do you know it's not a few hours/days/weeks or years/decades? Instant sterilisation or increased mortality over generations. And if something closer to the latter, then how certain can you be of it being bad enough for population collapse? Across all species or just some?
Things like that.

[McTraveller]

Is "ozone layer being stripped away in an instant" the sequel to Geostorm or whichever weather disaster movie it was that had temperatures drop so rapidly it would flash-freeze the ocean? (Which isn't even possible if you exposed to the ocean to absolute zero - heat transfer is just not that fast...)  Maybe it was The Day after Tomorrow? I can't remember which terrible movie it was and I'm not even going to imdb it.

[Frumple]

Ocean life should be fineish, water's a pretty good radiation absorber. Terrestrial life's definitely buggered, as is any shallow/surface water life.

Give it another few million years and I guess life might start crawling out of the sea again.

Eh... ocean life is nearly as heavily dependent on photosynthesis as the rest of us, and it's not trivial for the seabound sources of that to just move deeper or whatever. There'd probably be more things that survive in sustainable numbers down there (maybe, depending on what losing basically all of the world's photosynthesizers does to the atmosphere and subsequently the chemical makeup of the oceans), but it'd still be a tremendous mass die off.

Stuff might crawl back out eventually, but yeah, it'd be a while.

Citation needed for a few months. Not for lack of ozone is bad.
How do you know it's not a few hours/days/weeks or years/decades? Instant sterilisation or increased mortality over generations. And if something closer to the latter, then how certain can you be of it being bad enough for population collapse? Across all species or just some?
Things like that.

I mean, I'm guessing from eyeballing descriptions of what'd happen? Plants can survive a few weeks without functioning photosynthesis (the real killer for the ozone layer suddenly going poof is that stops working across the entire planet; most of the other dangers appear to be slower even if they're just as deadly in the long run), most animals can survive days to a few weeks without food -- between those you have a month or so before deaths to starvation become more or less necessarily wide spread. You have a month or three baked in just by basic biology, maybe more for a subset of some populations with stored food + shelter, or specialized adaptations for longer periods of privation.

After that it's not the direct exposure to solar radiation that's the issue (that appears to take a while to kill or is more avoidable), it's the fact just about the entirety of the planet's photosynthesizers starve to death. That's... going to cause widespread population collapse across the entire biosphere, land or sea. There's extremely few things on this planet that aren't fundamentally reliant on those for their continued existence. It wouldn't be all species (there's bacteria that'd probably continue surviving for a good long while even if we outright lost atmo, iirc), but it'd be most. It'll take more than a few days just 'cause there doesn't appear to be anything about the levels of solar radiation involved that kills within a few days (especially if stuff has the sense to break direct contact with the deadly space lasers, ha), but it's not going to be years.

[Great Order]

Thing is you gotta remember that stuff was photosynthesising *before* there was an ozone layer. Sure, it's going to suck for a lot of things, but stuff like kelp living deeper than a few meters will be A-OK. Half a meter of water absorbs 40% of UV-B apparently.

And I'm specifically after the radiation effects. We'll assume aliens are airdropping food and clean water for humanity in this grand experiment.

[dragdeler]

I think the water will be cleaner than before that won't be the issue, but if it's what takes for space communism let me concoct some fckw in the bathtub (hehehe gotta be quick is all).

Thing would be kind of ok then? I mean we'd all be living like these albino kids. There is worse.

[Frumple]

And I'm specifically after the radiation effects. We'll assume aliens are airdropping food and clean water for humanity in this grand experiment.

Oh. If you're assuming magic/aliens keeping the knockon effects from being an issue and it just being the radiation that's... probably actually survivable? Unfiltered solar radiation is pretty damn dangerous, but it's not like your skin instantly melts off your bones or somethin'. Folks could theoretically work with that if they're not also dealing with starvation and the issues related to pervasive biosphere collapse, heh. Lot of cancer issues and we'd have to be a lot more careful about moving around, but that's probably not a guaranteed death sentence.

... i mean, between the ozone layer getting magicked away and it being exposed that aliens really do exist, realistically we probably tear ourselves apart pretty rapidly one way or another, but it'd be us that kills us more than the sudden space lasers :V

Thing is you gotta remember that stuff was photosynthesising *before* there was an ozone layer. Sure, it's going to suck for a lot of things, but stuff like kelp living deeper than a few meters will be A-OK. Half a meter of water absorbs 40% of UV-B apparently.

That said, the ozone layer's currently blocking like 97+% of uv radiation, and the water absorption's already baked in. Kelp (and more importantly, phytoplankton, which are one of the most important parts of our entire bloody biosphere due to just oxygen production, never mind everything else they're involved in) would probably have as bad a day as everything else when the amount they're getting increases that much, heh.

They're adapted to what they're getting now, y'know? Not much reacts well to sudden changes that massive in their environment, and water absorption or not that's still going to be a massive shock.

[King Zultan]

Is "ozone layer being stripped away in an instant" the sequel to Geostorm or whichever weather disaster movie it was that had temperatures drop so rapidly it would flash-freeze the ocean? (Which isn't even possible if you exposed to the ocean to absolute zero - heat transfer is just not that fast...)  Maybe it was The Day after Tomorrow? I can't remember which terrible movie it was and I'm not even going to imdb it.

It's The Day After Tomorrow that everything flash freezes, which could also be stopped by burning a few books and nothing else in a fireplace.

[anewaname]

I don't think photosynthesis was "first", I think electrolysis was first. And I'd speculate that a week without the ozone layer would cause the death of most things on the planet within a month. Mere speculation, but read the links and quotes from them, this is meant to further your speculation... and I am considering the "ozone gone" and "ozone returned" events are mcguffin (not gradual) and very shocking to the biosystem.

Non-photosynthesis oxygen production:

Spoiler (click to show/hide)

From National Science Foundation, "Now a research team led by geoscientists at the University of California at Riverside (UCR) corroborates recent evidence that oxygen production began in Earth's oceans at least 100 million years before the GOE." (The GOE is...)

Also, recently there has been chatter that polymetallic nodules create surplus oxygen far below photosynthesis depths; these are the rocks that mining companies wants to have a go at. There are also oceanic geothermal vents that have unique biomes where I suspect electrolysis creates oxygen in minute quantities due to the constant roiling of thermal transfers and matter-state changes and tiny lifeforms.

So, how much oxygen production would be lost if there was no ozone? Consider that while photosynthesis happens below 40m, most of it happens above 10m depth. I'd speculate, above 10m depth, probably 95%+ of the photosynthesis happens and anything UVB didn't kill in 4 days would have significant DNA damage meaning it would die without reproducing. What happens if just 50% of the global plant-biomass is dead (maybe most trees are still struggling but many smaller plants are dead)? Is methane and co2 released, combining with the lack of oxygen production? What does that do to the air ratios and the temperatures? What else dies then? Wouldn't there be global "hypoxia events", killing deep-water marine life as well as causing health issues for everything on the surface that cannot regulate their air ratio? We only see the effects of current oceanic "hypoxia events" when thousands of fish wash up on some beach. What happens when the events are bigger and more common, where larger marine animals cannot get away? Ever had a wildfire that was 300 miles away screw up your air for a week and what would it be like if instead of particulate matter, that week was just low-oxygen?
 
This was also a good read.

[King Zultan]

I know that the ozone will heal itself once damaged, but if it is completely removed will it be able to come back or will it stay gone?

[Great Order]

It'll fix itself, it's not a self-sustaining process. UV breaks up oxygen molecules and sometimes they'll come back together as ozone rather than oxygen. Ozone keeps breaking up too, both because it's less stable and because it's being broken up by the UV it absorbs, so eventually you meet an equilibrium.