I'd say corruption. It's one of the main causes of any major problem. There is poverty because of corruption. Get rid of corruption, and you get rid of a lot of poverty and many other problems.
Global warming I don't think is a problem though. I think global warming is a load of hype. It's just too full of holes. I'm still for cleaner solutions as much as the next person is though, and there are other environmental problems.
I can say that they should eradicate corruption first among officials of the governernment. As long as there are people becoming greedy for power and money, they will not be able to address other concerns such as poverty and environmental issues