There are lots of problems such as war and poverty that have always been with us and very well always might be. But global warming is a climate change that humans have never seen and probably can not live through. It threatens the whole species and probably many others as well.
No, global warming is not the world's greatest problem today, because it is not a large problem that is actually happening. As I write this, there is a boat stuck in the ice in Antarctica. I'm sure they wouldn't mind a little global warming right now. Scientists are far from in agreement as to the extent of the problem of global warming.
Global warming is, most would agree, a definite threat, but it's one that will visit our children or our children's children. That makes it difficult for people to take action on it now, because they aren't seeing the major effects from it, just the small ones. That being said, there are many other problems in the world today that we can see, from genocide to war to slavery. Sadly, there are many problems facing the world right now, all of which need to be addressed. Some would argue human rights violations are worse than global warming.