I'm not sure your conclusion follows from your premise. For instance, it could be the case that global warming is caused entirely by increased sun activity, but that concerted human effort to decrease atmospheric greenhouse gasses could still cool the earth off. Combinatorily, I see 7 possibilities combining global warming (exists/doesn't), and is (entirely/partially/not at all) anthropogenic, and (can/cannot) be ameliorated by human effort.

*sponsored by the committee for strong syllogisms