Flight Behaviour, By Barbara Kingsolver

Until a certain point, early on in Flight Behaviour, it seems as if Barbara Kingsolver has written the perfect novel. Her prose is elegant, urgent and rich with beauty, depth and feeling.

The story opens as her main character, Dellarobia Turnbow, takes flight up a mountain path in Tennessee, away from the compromises of her family life and into the arms of a waiting lover. Then she stops dead in her tracks in a moment of quasi-religious rapture – she glimpses a "burning bush" which turns out not to be what it first seems - that leads her back down the mountain to her husband, Cub, her two small children, Preston and Cordelia, and her burdened life as a hard-up farmer's wife, low down in the family pecking-order.

The fine prose maintains itself through Kingsolver's evocation of the natural world: the autumnal trees that "dropped their tresses in clumps like a chemo patient losing her hair", and the flock of migrating butterflies that rove like "a rush of air, a river in flood". Yet somewhere in its telling, Flight Behaviour swerves off the road of literary perfection and gets lost in the fog of a sprawling story which has at its heart a global-warming message delivered with a heavy, heavy hand.

The butterflies encompass a terrible kind of beauty. Mistaken as a "miracle" sign by the church-going community, their freakish presence on the mountain clearly indicates climate change, potentially catastrophic to the species. While Kingsolver's talent for descriptive prose makes the most of the ironies inherent in this beautiful harbinger of doom, the environmental implications are conveyed in unwieldy fashion, mainly in conversations between a butterfly expert, Ovid Byron, and Dellarobia, the latter decoding his science for the reader.

The climate story unravels alongside Dellarobia's marital trajectory, from accidental pregnancy at 17 to shotgun wedding and now disappointment and despair. Kingsolver's last, Orange Prize-winning novel, The Lacuna, had at its heart a psychologically elusive protagonist, but here Dellarobia's inner world is far more fully fleshed out. Yet in its weakest passages, Kingsolver slips into the trite language of romantic fiction. We are informed that Dellarobia is a petite Southern beauty but Kingsolver will not let us forget the fact.

Ovid is summed up as "Tall, dark and handsome, but extra tall, extra dark. Okay extra all three." At a sheep-shearing, Dellarobia admires the rippling arms of a grandfather at work with a desire that borders on comic. Slapdash descriptions reduce her sexual longing to corny sound-bites, utterly at odds with the extraordinary writing in other sections. We are reminded of the message – that Dellarobia's life is an impoverished one despite her feistiness and ambition – with tedious consistency.

This is a novel as compelling and epic as it is maddening. One wonders whether, had Kingsolver chosen brevity over heft, she might not have ended up with a better book.