Maximum Ice — Kay Kenyon

Zoya Kundara has lived on the space vessel Star Road for two hundred fifty years. As its Ship Mother, kept alive in a state of pseudoimmortality, she has provided wisdom and counsel to succeeding generations of its crew, self-exiled survivors of earth's great plague.

But now, to escape the ravages of space radiation, the giant starship has returned to earth, only to discover a world on the verge of extinction, its barren surface blanketed in a crystalline substance that resembles ice and that is slowly, inexorably encapsulating the planet. Zoya is chosen as emissary to this strange new earth, and now she must approach its denizens and find a suitable home for her desperate crew among the shrinking lands.

But what she finds shakes Zoya to her core: groups of humans huddled like moles in underground techno-warrens called preserves, and a pseudospiritual order known as the Ice Nuns, who seek control of the physics-defying crystals and enslave their disciples in their crazed quest for truth. For on this once green land, Ice and the science behind it are now the only Godâ€"and mastering this grand ecology of information the only higher calling. Allies are few and far between, but somehow Zoya must uncover the secrets of Ice and halt its expansion.

That is, if the snow witches don't get her first...