An interesting letter to an editor might try to reference some of the science concerning climate change. We hear that it is being contested in the media, but we don't hear that the people challenging the idea of global warming are not usually climatologists. There is a false equivalence created in the media between experts and those who simply don't want to believe that climate change is a reality. So a good letter might try to present some of the evidence for climate change in a clear, cohesive way. 

Post #5 offers some excellent clarification and suggestions regarding approaches that could be used in constructing your letter to the editor. If this letter is supposed to reflect your real-life location and circumstances, you might address a local business that appears to be contributing to the furthering of climate change in some way and make suggestions for actions to change this situation.

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.

I agree---a letter on what about global warming?  There are still debates going on whether global warming in fact exists or not.  If it does exist, what created it?  How can it effectively be reduced, or even eliminated?  You must determine the direction your address will take.

I would think that the more likely content for such a letter to the editor would be about how to attack the problem.  There are many possible ways to try to reduce greenhouse gasses, each with its own pros and cons.  So I would suggest that you give your opinion as to what should be done to combat warming.

The two disparate elements here--a phenomenal event and a forum for addressing an opinion or suggestion to a public readership--sort of clash, because global warming is not receptive to private opinion; if the writer wanted to contest that there was or wasn't such a thing as global warming, a letter to an editor would not be a proper forum--if it is merely an opinion, it is not editorial; if the reader brings some new  scientific evidence to the stale argument, a letter to an editor is not a large enough audience.  A letter to an editor is a fairly specific sociological communication with a fairly specific audience, and the purpose is to give a personal--but informed--point of view to an ongoing debate. Perhaps a local consequence of global warming, such as precipitation changes that affect local farmproduction, could be the subject of a letter to the editor.