I'd suggest you do some preliminary research before writing out a thesis.
This is the key, for sure. Jon gave great advice, here... You need to give yourself a crash course in the issue by reading about 5 articles written in the past few years. There are tons of articles available if you google "global warming" and "debate."
While you read, take notes about the interesting arguments people make. Write a paragraphs about every argument you read. Soon, the paper will be taking form. When you have written 10 paragraphs, you will know what your own argument is, because you will be becoming an expert about global warming.
so, do some reading and take some notes, and you will discover your argument!
:-)