I'm crafting a world in which the expansion of the human race has caused the release of so much pollutants released in current times such as carbon monoxide, nitrogen oxides and sulfur dioxide, etc. cause the world to be deemed uninhabitable by means of famine, lack of medical care, lack of food, the sheer heat, and basic things needed for survival. Along with those problems people begin mutating, developing cancers, plant life begins to die, things of that nature. This is because of the UV light that comes into the atmosphere because of the lack of ozone But before all hell broke loose, bunkers filled with a monopoly of necessary supplies had been made in the U.S. so that only 100,000 people decided as useful could enter within one of the ten bunkers all over the country. So the story takes place when the people are forced to come out of their bunkers by means of a pandemic that supposedly mutated from the harsh conditions. But what I want to know, is how long would they have to be there (it doesn't have to be the same generation) until the world was habitable again and they wouldn't die within weeks? And whether the world will recover fully isn't what I want to know, I want to know how long it would take to recover so that people could just barely survive on whats there.
closed as too broad by Aify, Vincent, L.Dutch♦, dot_Sp0T, sphennings Aug 19 '17 at 13:07
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At the very least you should tell which kind of "disaster" you have in mind.
Most of likely scenarios won't produce a works that's uninhabitable, "just" a world where it will be increasingly difficult to survive.
Having a world so polluted it will actually kill you actively (and not just refuse to feed you) would require, for example, a worldwide radiation level higher than that in Hiroshima right after the Bomb. Such a thing would require 100,000+ years to "recover" and would have destroyed most of life on Earth.
Another thing which would really make the world unfit for animal life is such a destruction of forests that oxygen levels would drop below what's needed for life. In that case it strongly depends on climate (desertification) how much it will be needed to recover, but a "mere" 1000 years could suffice.
The above has the added side effect that the whole fauna would be, with only small exceptions, wiped out, so your survivors have better have a Noah's Ark equivalent somewhere.
Other "pollutants" may behave very differently: an engineered virus targeting humans might kill everybody, but might be unable to survive mere weeks after the last man died out.
Plastic is slowly choking the seven seas, and will remain there almost indefinitely.
As said, the most realistic scenarios won't kill us directly, but will make it increasingly difficult to rise crops and to herd farm animals, perhaps "just" for lack of drinking water. We would thus face famine and diseases, which may end up destroying our worldwide commerce; if this happens there will be a huge problem because there is virtually no nation that can survive in isolation, so the spiral down will continue. The planet won't kill us directly, but it will refuse to feed us.
In this scenario "bunkers" would be useless, but you may want enclaves built around the "missing resources" (e.g.: fresh water) self-sufficient, at least from food point of view; such enclaves should be fortified to "resist" the hordes of people who would want to get in to "share", but that problem will soon solve itself.
How much such an enclave can resist without anything coming from outside is not clear, but it's well known that our civilization needs a minimal "critical mass" to sustain itself, otherwise we would revert to Dark Ages lifestyle (or worse).
As You see You need to decide which way to go, then (perhaps) we can help you.
Ok. You chose to kill us all with cosmic radiations.
Ozone depletion "culprit" is a certain class of chemical compounds (CFC, ChloroFluoroCarbons) we are currently trying to regulate.
I assume these, or something very similar, is the culprit in your case, only on a much higher level.
The ozone cycle is quite complex, but it is also quite fast, so recovery is (almost) immediate after you remove the "disturbing" factor.
This is good news.
Bad news is CFC compounds have an half-life (time necessary to halve the quantity present) of several decades (up to 100 years for Halon-1301). This means that, to get a reasonable cut of the presence (and thus recover of ozone layer, at least in the equatorial region, you'll have to wait between 100 and 200 years.
In general you can stipulate the problem is caused by some new compound (which is also very reasonable) that has the half-life you prefer.
Just keep in mind that polar regions will be more affected, followed by temperate zones and finally the equatorial zones; this is due to more light at equator producing more ozone and jet stream currents accumulating pollutants mainly around poles.
This means that, in order to have a global effect you'll be postulating devastating levels near the poles (that will take much longer to "decay").
A lot depends on the nature of the pollutants. Some stick around a lot longer than others, some cause more issues than others. Chernobyl is a good case study in this. 30 years after the meltdown it's now a thriving wildlife reserve and people are living within the fallout area. It's still not considered 'safe', there are side effects to living there, but it is definitely possible to do so.
The larger question is, how long does it take the remaining people on the surface to either stop polluting the planet or die out. That could take hundreds of years.
One thing which people don't seem to take into account is the Earth is a planet, and has massive amounts of both buffering capacity and inertia.
Take climate change. In the European Warm Period, the climate warmed to the extent that Vikings settled Iceland (Tilly), Greenland and even the coast of Labrador (Vineland). French vinters complained about the competition from Scottish wines and the population actually boomed (look up very old parish records). The climate changed gradually enough that people adapted to their good fortune, until around 1400, the climate went in the other direction and Earth entered a "Little Ice Age". Rivers froze solid, crops failed and plagues stalked the land. OF course, people didn't die out because of that, and the Spanish, French and English (among others) were sending explorers to the Americas in this deadly climate in the 1500's.
What is far more likely is that our intricate and interconnected systems could receive some sort of shock which creates a cascade failure as interconnected systems take each other down. A failure of a large part of the power grid would shut down a lot of the industrial infrastructure, and transportation (especially bulk and long distance transportation) infrastructure will rapidly break down without fuel, signals (think of railways and airports) and eventually maintenance. The interconnectedness of all things can be seen by a few examples: when the US decreed a large fraction of the corn crop would be converted to ethanol to meet government regulations concerning the addition of biofuel, it caused food riots across the world as the corn market tightened, prices rose and materials made from corn began to be in short supply. In the opposite direction, the growth of US shale oil due to Fracking has significantly dropped global oil prices. This is a blessing for some, but for Petro states which depend of the price of crude oil for their earnings, this is a disaster.
Suitably bunkered down people could conceivably survive for years or even decades, depending on the level of preparation and the size of their shelters (a very massive system of bunkers and caves would be needed to house thousands of people for decades), but would need things like nuclear reactors and greenhouses to really effectively survive. The sheer storage space for millions of tons of caned goods, water and fuel would be huge, and difficult to hide while building and stocking the bunkers.
So in short, a world ending disaster would require some large shock which disrupts the interconnected systems that support our modern civilization, something like an asteroid strike, Carrington Event or nuclear level terrorism. The Earth itself will likely recover in a matter of years or decades (although it will be unrecognizable to the survivors used to an industrial civilization), and with suitable preparation and tools being stored away, they could likely start rebuilding at a lower level almost right away. By lower level I'm thinking 1700's level of technology....
The maximum possible ozone depletion would not make the Earth's surface "uninhabitable" (whatever that means). Hence, zero time would be required for the morons who fled into caves because of UV to emerge.
It is the dose that makes the poison. Without exact concentrations of exact compounds, I couldn't even begin to answer this question (assuming that you change the very poor idea that UV is such a problem to one of a lethal (poisonous) concentration of some pollutant). The amount of these gases you'd need would be mind boggling, but perhaps you should consider either (or both) an asteroid strike or several supervolcanoes. Other than CO2 or SO2, I can't imagine a pollutant that could be that much of a problem. Even radioactivity from the detonations of all of the nukes in the world. See nuclear winter. I think it is instructive to consider what we know we don't know. There are a fairly large number of potential serious problems due to our CO2 pollution. While none (that I'm aware of) would (as a worst case) result in an extinction level event, there are a few which could reduce global population by one or two powers of ten (unlikely, but can't be ruled out completely). We're far more likely to introduce a pathogen which exterminates us than something unintentional or some side-effect (like CO2), imho.