This is a very broad topic, which should help you find things to write about. First of all, you should probably decide if you think the phenomenon of global warming is real or not. Since you're writing about it, I'll assume you think it's actually happening (a viewpoint I happen to agree with). Then there's a big debate about whether it's happening due to natural events, or is man-made.
The politics of global warming is another rich topic - exploring the reasons why some politicians and companies want to deny any responsibility for it would be interesting.
Looking at the scientific evidence for global warming is a possible topic - there's a lot of evidence, but it's all indirect and subject to some interpretation. This is also true of the predictions of what the earth and its creatures will be like if the planet gets two degrees warmer, or five, or ten.
You may want to wrap up with a discussion about some possible actions that individuals and/or governments should be pursuing to help head off global warming.