I've been going through our latest batch of Electric Spec submissions, and I've noticed a number of science fiction stories themed around global warming. The message about the horrible impact of global warming is important, but selling us a story based on that theme is a long shot. Why? First of all, it has been done, and perhaps overdone. The concept of global warming is hardly cutting-edge science. Second, the idea that in the future, nature will be paved over, or the living environment on Earth will be nearly unbearable, or that nobody will have heard of a tree isn't that original. A story with such a "revelation" will not grab me.
Still want to submit a story about global warming? Fine--just make sure you put a twist on it nobody else will think of.