Overpopulation is one big problem. 7 billion soon.
Economy and ecology move in opposite direction.
World War 1 was fought because there were 'too many people' and
countries need more land. The world population back then was only
1.7billion. And that was considered as too much people.
Now, the world has exceed 6 billion people. It is disturbing to
think that nature no longer has any means to reset the equilibrium.
Even if something calamitic happens and manage to wipe out 90% of
humans, there will still be over 600millions left, which is still
too many to count.
You can wipe out 99%, and there will still be 60million people,
enough for the world to continue functioning.
At 99.9%,you have 6million left. We can still sustain a
civilisation. Look at the Romans. They had civilised Europe, with a
population of under 1m.
Humanity's worst disease, the Black Death, was only able to wipe
out 30-60% of the population. Our population has grown too vast for
nature to reset to equilibrium. Nothing short of an alien invasion
can effectively reduce our population to more sustainable
levels.
Perhaps, even aliens will be engulfed by 6 billion greedy,
hungry mouths.