I am a science fiction fan. I even have an annual convention I attend. Until very recently, my for-pleasure reading list consisted almost entirely of space ships and aliens, dragons and magic. Science fiction and fantasy are two very different genres that are often lumped together in readers' heads because they are found in the same section of bookstores and libraries. Authors will often go back and forth between the two as well. However, they generally fall on two ends of the extreme on the environmental spectrum.
Fantasy stories, with some very notable exceptions, tend to feature in lands that are agrarian, forested, and with a pre-Industrial Revolution level of technology: a environmental utopia, if you will, before humanity invented the steam engine. Also interesting to note is that most fantasy novels take place in wholly imaginary worlds with fanciful names and elaborate maps that, while having attributes that might resemble Earth, are not found anywhere on this planet.
Science fiction, on the other hand, tends to exist in the realm of what could be: advanced technology and different planets and races that are discovered through the use of space travel. Where fantasy takes place in a vibrant world whose natural resources are largely untouched, science fiction occurs in a (no less vibrant) world that is metal and machines.
Both settings, if used widely, can be wonderful mediums for conveying green-tinged messages. Perhaps the most famous fantasy story of all, Tolkien's The Lord of the Rings is also one of the most environmentally-themed but done so subtly that most do not notice. In science fiction, the comparisons between a green world and one that is full of cities and technology can be highly effective. The settlers in Anne McCaffrey's Doona trilogy leave Earth, which has been developed to the point where there is only one-square mile of open green space, to eke out a life on a (supposedly) uninhabited planet.
Because science fiction tends to focus on the what-ifs associated with outer space, the planet Earth tends not to play a major role, mentioned only in passing if at all. This has led to a tendency, I believe, for science fiction fans to forget the importance of Earth in the drive to reach out there via the space program, a space elevator, or colonies on the Moon or Mars. But how productive is that? By looking outward to where we cannot yet go, are we turning our backs on where we are now? At my science fiction convention, there is a heavy science program, mainly focused on some aspect of space exploration. It seems slightly... off to me to put so much effort and attention on something that may not ever be, when there is so much to fix right here on good old Terra.