This is an easy one. If the world gets too hot, we've proven that we can survive it because the "Cradles of Civilization" were in hot, dry areas. If the world gets cold, then we've already proven that we can withstand ice ages, and that's without any "high" technology. If the world gets overpopulated, it will, out of necessity, balance itself out. I don't think we need to worry about surviving the next 100 years. The real question is how to we keep the next 100 years from becoming the next Dark Age.