Fortunately, the topic of global warming has been written about at length, so finding sources won't be a problem. But, as you have discovered, it is a double-edged sword, as there can be too much information to handle easily. There are plenty of ways to whittle it down. You could focus on the evolution of opinion about the existence of global warming in the U.S., from "it's a bunch of tree-hugger nonsense" to "we have to do something about this right now!"; you could explore whether other countries, particularly those of the European Union, have had a better response to the issue than the U.S.; you could write about global warming's economic effect so far, if it has indeed been responsible for the disturbed weather patterns giving rise to the devastating tsunami or to hurricane Katrina, or the wildfires raging through southern California; or, you could write about the future: what steps will help? is it already too late? if not, when will it be, and what would the impact on the world as a whole actually be?
I hope these ideas can help get you started!