By 2050, Earth's population will be 9 billion. What's the most important change needed to live sustainably?

There was a great focus on population issues in the 1970s and '80s, but we lost the fight to stabilise Earth's population at a more sustainable level, and now we must face the reality that it will be far more difficult to live sustainably in the future. What we need are solutions that solve several... show more There was a great focus on population issues in the 1970s and '80s, but we lost the fight to stabilise Earth's population at a more sustainable level, and now we must face the reality that it will be far more difficult to live sustainably in the future.

What we need are solutions that solve several problems at once: e.g. how do we grow enough food sustainably to feed the planet, and solve climate change? How do we save the world's tropical forests, lift the poorest from their poverty and solve climate change?

This will be the century where human ingenuity is stretched to its utmost. What do you think is the single most important factor we need to change in order to sustain a burgeoning population?
Update: Yahoo! Canada Answers Staff note: This is the real Tim Flannery, climate change activist and author of An Explorer's Notebook (HarperCollins).
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