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We don't often think of contraception as a sustainability issue, yet it is. While it's controversial as to just how many people the planet can support, the point is moot when the vast majority of new births are in undeveloped countries, which generally most emphatically cannot support such increased population.
Sustainability in the face of an exponentially growing population requires serious thinking about food production, how to enhance and influence infrastructure development in third world countries, and about contraception practices.
Even if the world can support the extra billions of people soon to come, without a focused effort on sustainability the majority of those will be living lives "poor, nasty, brutish, and short" (Thomas Hobbes). Sustainable practices hope to keep this from happening.