Geoengineering of Earth is remarkably easy. The reason it’s not already being done is political, not technical or even necessarily economic. For less than NASA’s budget, it’s possible to do things like stratospheric solar radiation management. See: Mount Pinatubo. Some places (Florida, etc) have already made laws prohibiting it.

As far as being science fiction… obviously? Terraforming Venus is a very long term project. It’s scientifically possible but hasn’t already been done. I guess I don’t understand what “science fiction” is supposed to mean. Like, Jules Verne writing about long distance underwater submarines? Trips to the Moon launched from Florida?