Population correction?
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This is an interesting slideshow piece. Worth considering, although it offers very little in detail or solution
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I've often cited the fact that we had 2 billion less than 100 years ago, now we're at 8 billion. That's a big jump. See chart. Luckily we should slow down and reverse eventually. The problem similar to what Jolly said, is the booming populations will be in places like Nigeria (3rd biggest in world) in a decade or two where the infrastructure is not set up for families to have 6 kids each when in "first world countries" we are having like 0.8 kids.
Idiocracy is a stupid movie with a realistic plot. (not related to this chart, btw)
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Energy. I figured out fossil fuel so we multiplied faster than was naturally sustainable using only near real-time power from the Sun. After the fossil fuel cache is spent, we either exploit nuclear or revert to whatever the Sun can naturally support on Earth.