First of all, let's get the terminology straight - no one in the sciences uses the term "global warming" any more, it's correctly referred to as "climate change". This immediately addresses the contention alluded to in #4 that it's not happening; pretty much everyone agrees that the climate IS changing. However, there's still a lot of disagreement about the cause of it. Is it natural? Or is it being caused by human activity? Before you can write a letter to the editor, you need to decide which of those two ideas you want to support.
If you are going to argue that climate change is natural, then a logical thing to write about would be support for programs that help people to be prepared for difficult weather. If you are of the opinion that climate change is being either caused or accelerated by human activity, then your letter might be about programs that push people and industries to lower their carbon dioxide emissions.