Human sustainability isn't an issue,

environmental sustainability is.
Two different things.
I'm posting this as a separate thread because so many get posted here to advocate the idea that we have to get the population down, and then people come in saying it's inevitable that that will happen anyway, and in a few cases the OP even starts with this.
It won't. Sorry. There is ZERO evidence that human sustainability is at issue.
None. Rien. Nada. Zip.
There is plenty of evidence that biodiversity is getting zapped; I have no issue with that. I just have to look out at my now butterfly free garden (first year I've seen this, but I'm sure not the last) to know that.
But humans? Humans aren't just here, they are positively thriving.
Civilization has not run up against any limit at all yet. The evidence for this is absolutely overwhelming:

1 - World poverty is declining at a record rate. So, remember this when you hear someone saying the doom of civilization is coming: not only is poverty declining, not rising, it is declining at a record rate.
2 - There is no sign of any strain in commodity prices. One of the prime reasons for number one is here, in reason number two: the price of basic commodities is NOT outstripping the capacity of the global population to pay for them.



3 - Conflict, where it is occurring, is occurring among populations unfortunate enough to live in commodity supply regions. This is proof of number two, and is another good way of trying to figure out if human civilization is under strain: are the people living in the centers of civilization fighting more among themselves? The answer is a very definite no. Where conflict is occurring, it is occurring in the places that supply the commodities to the advanced centers. If civilization was under any real strain, the conflicts would be occurring among the advanced places where resources aren't as abundant (Japan is the most extreme example of such a place) while the people in the resource rich countries would be sitting back fat and happy on the money they were making off of those resources. Precisely the opposite is occurring.
This is a little hard to figure out how to measure. The method I came up with is to use two different things that are reasonably easy to objectively measure: the number of refugees created by violence and political repression, cross-referenced to the principal export of the nations these people are running from.
The evidence here is so overwhelming that I stopped trying to do any sort of sophisticated regression analysis because there was simply no reason to. The refugee data came from an organization called internal-displacement.org. The export data came from MIT's Economic Complexity project. There are 59 countries listed as having refugees because of violence and politics by the first organization. Out of the top 10 of these, the most miserable countries on the planet, MIT has export data for 8 of them. All 8 have as their principal export oil.
All 8. 100%.
Those top 10, all by themselves, account for 69% of all displaced people. So, to live in a place that supplies commodities is to have a far higher chance than the rest of the world of being miserable and homeless because of violence. Full bellies don't revolt, generally speaking. Poor people do, because they have nothing to lose. And the poorest and most miserable and luckless people on the planet are those who live in commodity supply regions.
That's because human sustainability is not at issue.

The facts and figures:

[code]
Countries ..............................Number of IDPs ..........Principal Export
Syria........................................ 6500000 ....................Oil
Colombia.................................. 5700000 ....................Oil
Nigeria..................................... 3300000 ....................Oil
Democratic Republic of the Congo 2634900 ....................Oil
Sudan...................................... 2426700 ....................Oil
Iraq......................................... 2300000 ....................Oil
Somalia................................... 1100000 ....................No data
South Sudan..............................1089200 ....................No data
Algeria...................................... 1000000 ....................Oil
[/code]
Total refugees in top 10: 23861600
Total refugees in all countries: 34627280
Percent of refugees in the 8 of those top 10 that are oil exporting countries: 69%

(By the way, don't try to argue that lots of the world is tied up in conflict and are refugees. Tokyo, at 37 million people, has a larger population than the sum total of all refugees in all the countries listed above.)

So, the fight, and it is a fight, is over how much damage we do. Civilizational collapse isn't going to be a saving white knight for the planet, because it's not going to happen, at least not in the lifetime of anyone reading this.
As there is no evidence that any resources are being currently strained by our current population, any projection that claims to scientifically predict where that limit is is flat fraudulent. A lie. Guru after scientific-sounding guru has placed the limit at various points, and human population has blown right past those points without a pause.
Will that continue? I have no idea.
Neither do you, or any guru you choose to cite.

Sources for number three:

Refugee figures: http://www.internal-displacement.org/global-figures
Exports: http://atlas.media.mit.edu/rankings/country/2011/ - (To find the principal export, click on the country, which will take you to a page displaying the exports and imports for that country.)