To me the biggest problem at the moment is world conflict. Maybe if countries spent less money on building up their armies then they would have more to spare for some of the other causes.
Although it would no doubt be optimistic to assume that budget cuts to the military would result in that money being used for more constructive purposes, external or otherwise... most likely it would go to some other self-serving enterprise that would do equally little to improve areas that need it.
Aside from what I've already stated, I think the biggest issue in the 21st century is that we are using up natural resources far too much and far too often. I agree with world peace, too, but if it came down to solving it, it's as far-fetched as ending poverty.
That probably. AIDS and shit just affects human beings, while global warming affects the entire world. If the climate changes drastically it's going to kill certain animals which will affect other animals, icecaps will melt etc. May not affect this generation drastically but it will future generations.
However I haven't properly read up about it so it might not be pressing at all.