Read in the late 1990s in e-book format; don't remember the acquisition source.
A new ice age has started, and the ice is down to roughly the US/Canada border. The US government - and by implication, much of the rest of the world - is essentially controlled by the Greens, and most of what we would consider modern technology is considered unacceptable (though some is "acceptable" to the extent that it allows the government to keep control over the populace). Science fiction, while not actually illegal, is taken as a sign of viewing "unacceptable" technology favorably, and can lead to "re-education" (in a distinctly Soviet style).
A pair of astronauts is attempting to scoop atmosphere to sustain their space habitat, and get shot down, landing on the ice. They are rescued by some "underground" SF fans, and the story centers around the attempts to keep the rescued astronauts out of the government's hands - and eventually morphs into a project to get them home to the space habitat, in spite of the government. There is a "Fen underground railway" of sorts, with signs that fen will recognize but non-fen wouldn't, which are references to what I'd consider 'classic' SF.
I never actually finished the book; that's as far as I'd gotten before a computer crash took out my copy of the e-book. Thus I ask...
What book was I reading?