Fagan’s novel balances the oncoming climate disaster with the human-scale stories of these characters, focusing especially on Stella, whose feelings about her sexual identity are refreshingly resolute. Her confidence about her girlhood, and the pleasure she takes in it, as well as the way she stands her ground when dealing with the hurtful rejection of schoolyard bullies, reveal that she possesses a kind of resilience that may serve her well in uncertain times. She is supported by her mother, although not by her birth father, a fact that causes her to hope for the romance between Constance and Dylan she senses brewing.

Stella’s intrepid and sometimes dangerous attempts at self-care, and her ­coming-of-age under the pressure of ­societal disapproval and global threat, are the emotional anchors of the narrative. The interior lives of the adults in the novel are not quite as precisely drawn. We don’t learn much more about Constance than that she is an adept survivor who keeps her emotional entanglements at a safe distance. Dylan is a somewhat unformed man whose lack of direction and self-­knowledge might be ascribed to the fact that his lineage has been kept from him, information that he will uncover during the course of the novel and that he fears will threaten his relationship with Constance and Stella. This conventional narrative ruse — the unearthed secret — is not wholly persuasive in a book that admirably avoids melodrama, especially since the revelation does not have the weight of meaningful consequence. What does matter, and something Fagan handles with deceptively effortless prose, is the way in which ordinary, even banal, life dramas unfold while the existential noose is tightening. The girl’s sense of the dislocation is tender. “Her voice is sending her odd notes. Her body is becoming a strange instrument,” Fagan writes evocatively, and then, easing from the distanced poetry of the writer’s omniscience into the mind of this witty child, “Any day now a tiny man is going to set up a loudspeaker in her throat and his voice will make declarations in a baritone and everyone will think it is her speaking, but it won’t be.”

The mercury plummets, ultimately reaching an unfathomable and unsurvivable minus 56 degrees. As the days grow short and most of life must be spent inside the confines of a trailer, the claustrophobia Stella feels inside a body that might soon betray her is mirrored by what is happening in the world. When she takes an ill-advised bike ride into the freezing weather, we feel not only her physical desire to break out of her trailer home but also her desperation to escape the gender she was born into. Fagan joyfully summons the sheer jubilance of the girl’s physical power as well as her fear when she realizes she’s out of her depth in the freeze. The evocation of that maturational tipping point where wisdom trumps desire is one of the novel’s wrenching explorations. There is so much for this young girl to lose. That she receives news of frozen bodies and devouring sinkholes, of food shortages and economic collapse from the internet makes her isolation that much more devastating. A young Italian transgendered man who is Stella’s online consigliere suddenly disappears from the web, and we, like Stella, can only wonder if he has fallen victim to the freeze.

“The Sunlight Pilgrims” is a stylistically quieter novel than Fagan’s bravura debut, “The Panopticon” — a fiery and voice-­driven effort that landed her on Granta’s 2013 list of the best British novelists under 40 years old — but it is no less critical in its portrayal of marginalized people under the pressure of society’s norms. When Stella and her mother visit a doctor in hopes of getting Stella started on hormone therapy, the unhelpful man suggests antidepressants. At a community meeting, the nuns who run Stella’s school, and who do nothing to support or accommodate the girl’s gender transition, greet the oncoming freeze with educational leaflets and announcements about community preparedness plans including “ideas on how to insulate and heat your homes,” information that will be useless when the temperature drops and resources become scarce. At the meeting’s closing prayer, “Stella gazes around at the bended heads and Mother Superior is looking at her . . . and there is a faint distaste in her eyes.” It is easy to imagine that this young, marginalized girl and her anti-authoritarian mother will get no special help from their local church, insulation or otherwise. The difference in the argument from Fagan’s first novel to her second is that with the world tipping into disaster, intolerance will seem like a petty thing. No one, not even those who hew to ideas of perceived “normalcy,” will be spared. In this way, the satire in this novel is even sharper. A church that largely recoils from embracing difference can do nothing when it comes to protecting the earth from human abuses and vanities.

Fagan is a poet as well as a novelist, and many of her images of this unbidden winter are shot through with lyric beauty. Early on, we are told that in this worst of winters “icicles will grow to the size of narwhal tusks or the long bony finger of winter herself.” Later, when the threesome venture out of the caravan to witness an iceberg’s arrival, they observe “all those peaked figures of ice, like all of their ancestors have been caught by the elements on the long walk home, their souls captured by ice and snow, and below them the North Sea cracks and groans as ice floes creak and collide.” Strange beauty can be found in destruction, and Fagan is fearless and wise to allow her characters to be as entranced by nature’s awesome power as they are terrified of it. The mythic reach of such imagery mirrors the way Fagan overlays elements of the tribal onto the quotidian. The novel’s three central characters are as much recognizable humans as they are visitations from a folk narrative. Dylan is an orphaned giant whose mother, to obscure his troubling lineage, told him he was the product of a fallen angel and a mortal woman. To ward off the worst of the chill, Constance frequently dons the pelt and preserved head of a wolf, so at times she seems a strange hybrid. Stella is a double, both a boy and a girl. The local lore about the sunlight pilgrims of the novel’s title tells of a race who drink light to live through the darkest times. But Fagan does not use metaphor as poetic immunity for her characters or her readers. The novel leaves them — and us — in a deeply troubling and unresolved moment. The world looks like a place of our darkest imagination, but it is all too real.