In reference to the environmental disaster scenario:
The short answer is "it depends". I think that good leaders with long term perspectives and a systems thinking orientation could persuade their people to work towards more universal solutions. In recent years I would say most of our leaders wouldn't be up to the task, but a few of them would be.
Another way to view this is that even a heartless systems thinker might conclude that it serves their interests best to find more universal solutions.
They can't really do that now when everything is pretty good.
If the global order breaks down, all of our "moral progress" will be lost, just as we've seen in Ukraine, or we see in the Sudan, but at a larger scale. Barbarism is as much a part of our nature as civilisation.
In reference to "uniting enough".
First off, I'll take a whack at an answer, but the quality (or lack thereof), of my answer is independent of the OP. Again, it's often the case that the first step in solving a problem is simply to make it explicit. Solving the problem often comes in later steps. Notice that this is mostly how science works.
With that said, I think an incremental step would be to look at each of the major religions and isolate the divisive messages from the rest. Muslims could still be Muslims, Christians could still be Christians and so on. But we could reform these religions and excise all the energy they direct towards denigrating "the others".
If you ask a Muslim (as I have many times), what are the core values or ideas in Islam, they almost always talk about ideas like family and charity. They almost never say "mistrust of non-Muslims". In fact they mostly deny their scripture calls for it. So why not reform the scripture?
It doesn't really answer the question, and I think what you suggest here is completely impractical as it assumes you can fix human irrationality by appealing to reason, but for the sake of discussion, let's assume it works.
Why would this unite us to any significant degree more than we are at present though? It might have a minor impact in a few places, but I can't see how it would be a game changer.
What degree of global division do you think is caused by religious scripture? What current problems are being prevented form being solved because of religious scripture?
Do you think religious differences stop Saudi Arabia, Russia, China and India finding common interests?
Personally, I think the biggest impediment to transactional cooperation is making moralistic demands of others rather than agreed mutual non-interference.
And if your alternative is something more than a proposal of transactional cooperation, it must be based on some form of shared ideology, not simply reforming religion. But what would that shared ideology be?
1 - I continue to see you point to examples of humans misbehaving. We have NO disagreement here. Humans behave badly, A LOT! So yes, I'm happy to grant you that humans misbehave.
But they also behave, A LOT ! It could be that where we disagree is about the ratio of misbehaving vs. behaving? It seems to me I think behaving is predominant and you think misbehaving is predominant?
2 - You also point out how often humans are irrational or biased or fear based or... Again, I agree. But again, I think the ratio of rational to irrational is far more on the good side than you seem to think?
As you haven't ever explained what degree of unity you think is possible, it is hard to respond.
I think there is no ratio between the things above, they can all manifest themselves to a greater or lesser degree based on a number of factors which constantly change.
Hence history goes in cycles rather than displaying progress (either linear or a fluctuating upward trend). The best case scenario is we can make slightly better decisions and reduce the frequency of the systemic shocks, but there is no real evidence humans collectively learn from experience or transcend their animalistic nature to consistently act rationally for a collective good rather than based on factional interests (and plenty of evidence against it).