Evolution, revolution and staying on top
The world as we know it is coming to an end.
This is axiomatically true as a corollary of change being the only constant. From a social and technological perspective things change and the world isn't what it was when you were a kid. This has been true for all of recorded history, and it applies to ecologies too. On a local scale some proto-semites trashed their garden of Eden by allowing human and goat numbers to grow unchecked.
This is not to say that they weren't pretty good at managing the environmental impact of people and goats; it's just that eventually there were far too many of both, and no amount of goat lore could save them.
It should also be noted that deserts are not biologically empty; they just aren't full of goats or people - and this is my first main point. The world goes on, it just ceases to be the world as we know it. If Kennedy and Kruschev had got overexcited and given us a glow-in-the-dark world, it wouldn't be empty. It would be positively bustling with cockroaches and rats and mould and grass and everything else that's adaptable and hardy. From their point of view such an event would have represented a marvellous opportunity, and increased mutation would only have accelerated their filling of vacant niches. It's only large, long-lived predators that puke everywhere and keel over.
But let's not dwell on nuclear apocalypse: it didn't happen and it probably won't. Instead, let us consider another type of apocalypse, one that certainly will happen: resource exhaustion. How can I be so certain? Basically because it already has happened, over and over. The primary difference now is globalisation, which means two things: (a) it will be global rather than localised, and (b) as a consequence of this there won't be immediate reseeding from large adjacent unaffected areas: neither rescue nor recolonisation will be forthcoming. The cavalry, gentlemen, is not coming.
It doesn't have to be a total wipeout. Individual resource consumption and pollution is not the problem. It's about total resource consumption and pollution, which is a direct function of population.
A population of a few million could sustain a medium technology civilisation, with all the really important stuff like refrigeration, hot water, potable water and medicine including reliable contraception.
This is the key to fast recovery from cataclysm: you aren't bootstrapping. You don't have to wonder how and where to mine or smelt iron. You know how to desalinate water, and with a small population spread wide you probably wouldn't need to - why tear your iron out of a mountain of dirt when there's a mountain of old cars? A population reduction like this will demand radical cultural changes (especially if we are to keep the population small) but in the initial turmoil there will be crucial, temporary advantages: a sudden relative abundance of the produce of a technological world. There will be twenty years before there is any need to manufacture heavy machinery.
This also represents a risk, because with no need to make, people will stop making - and they will forget how. And some technologies don't like neglect: if you turn off the power to an aluminium refinery and the aluminium solidifies in the vat, the plant is junked - time to build a new one. So a great deal of care will have to be taken to preserve certain technologies, or a decision made to abandon them. Fortunately, other technologies are more robust, and with a much reduced duty cycle, equipment will tend to last a lot longer.
A few tens of millions could more than sustain not just a medium technology, they could build a high technology. Reaching for the stars is a matter of economics. Get just one robot out to bring back an iceteroid and you have millions of tonnes of reaction mass and bonded oxygen. Once you have lots of reaction mass you can bring em back by the dozen and the economics of space colonisation becomes radically different.
All things in good time. Let us begin with what we definitely can do today.
In the short term, the most important things are that primary and secondary production continue. Demand will be reduced, and if operations is scaled back to simply fulfil requirements rather than greed then their own environmental and other resource demands are likewise mitigated.
Direct economic management is a fool's game, doomed from the start by the unpredictability of chaos systems and the decoupling of motivation. This should have been obvious, and if it wasn't, the Russians demonstrated it for us. If I sound like I can't make up my mind, it's because I'm trying to express the difference between a planned economy and a regulated economy.
Something else I'd like to see is the eradication of religion. Religion is by definition insane - you are asked to believe a bunch of absurdities in the absence of evidence on the basis that if you do then after you die you won't be dead. Which is just plain silly. Silliness on its own is merely annoying and sometimes entertaining, but when people start squabbling over the details of their collective silliness they steal resources from things that do matter. I don't really care if they want to torture each other but squandering resources I will not tolerate. Even if there were a watching maker of worlds, it is even ridiculous to suggest that it might be concerned with whether or how people kiss its metaphysical bum. I mean, really, if there is a god and we're made in its image then I rather suspect the reason it hasn't turned up for a while is that we're the embarassing relatives. We keep having hordes of illiterate ill-mannered children and beating each other up over how many pins can dance on the head of an angel. No, I didn't get that wrong - it's entirely hypothetical and the details do not matter.
The thing about resource exhaustion is that to avoid it you basically have to get rid of all your competitors before they consume the resources. Imagine you're in a life-raft with other people and limited supplies. Remarkably you know how long till landfall and you also know there isn't enough food and water. It is much smarter to get rid of your competitors immediately. You now have complex choices to make: do you ally with the strong to preserve them as resources or do you ally with the weak to dispatch the strong on the basis that it will be easier to get rid of the weak later? Ideally you would arrange for a credible accident to dispatch the weak so that you can retain the strong as resources without your being recognised as a threat.
Oil will run out in perhaps thirty years. But if the world's population dropped to perhaps ten million then even with per-capita consumption unabated the reserve would last 150+ years, which is plenty of time to redevelop technology and industry around renewable resources - especially since there's so much less of it to adapt.