One consequence of the various gloomy cinematic forecasts of our planet's future is that browbeaten audiences might start to think: "Come on, it's not the end of the world!" So it's no footling achievement that the film of Cormac McCarthy's novel The Road manages to overcome Apocalypse Apathy Syndrome, reproducing on-screen the book's marrow-deep dread; it makes decay feel fresh. The Australian director John Hillcoat showed with his outback western The Proposition that he has a sophisticated understanding of the correlation between psychology and landscape. This is vital to The Road, which proposes equivalence between the devastated terrain and the stooped figures trudging through it.

Among them are a nameless man (Viggo Mortensen), who makes Boudu look like GQ cover material, and his son (Kodi Smit-McPhee), who looks to his father to reassure him that they are still the good guys, the ones "carrying the fire". They are heaving a shopping trolley across a desolate America towards the coast. Finding food is one concern; not ending up on someone else's plate is another. Evidence of cannibalism is glimpsed in a brace of blunt, sickening scenes. One, a variation on the old "Don't-go-into-the-cellar" horror-movie staple, is lifted from the book but proves infinitely more upsetting on-screen. The second, in which a woman and child are hounded by hunters, is the invention of the playwright-turned-screenwriter Joe Penhall. The horror of both episodes lies less in what is shown than in the sense that mankind has reached a point where the cataclysmic has become matter-of-fact and no atrocity is beyond comprehension.

This topsy-turvy dynamic is present in every frame of the film. In one of the candlelit flashbacks that provide necessary visual relief from the largely colourless palette, the boy's mother (Charlize Theron) insists they should all shoot themselves before they are murdered. "Other families are doing it," she urges her husband, as coolly as though she were putting the case for holidaying in the Cotswolds. Suicide is presented as a domestic necessity, while the discovery of a can of soda or a stash of tinned fruit acquires a significance of religious proportions.

Penhall's script is so faithful to the novel that he might well have used satnav to find his way along The Road. Any detours he takes are minor and judicious. One is to steer our reading of the material more resolutely in the direction of global warming. McCarthy was ambiguous about the cause of nature's depreciation, whereas the film sides with the consensus that this is a nightmare about climate change. "I knew this was coming," claims a scabrous old coot (Robert Duvall) with whom father and son share a campfire meal. That line is from the book but Penhall allows him to elaborate: "Some said it was a con. Not me."

This could have been an imposition if Hillcoat and Penhall didn't display a delicate understanding of how to translate McCarthy's ruined world into film language. Locations across North America, including hurricane-racked parts of New Orleans and disused mines in Pittsburgh, reportedly required little cosmetic adjustment to stand in for an earthly hell. One pity is that the film-makers couldn't have shot a token scene in Copenhagen.

It is the rigorous tone of The Road, rather than its images of dereliction, that makes it worthy of respect. If the score (by Nick Cave and Warren Ellis) and the perfunctory narration are concessions to audience comfort, there are other points at which the film ramps up the harshness. Man and boy are no longer robbed of their possessions when they leave their cart unattended, as they were on the page: now it happens while the child is ill, with the tent swiped from around his sleeping body.

The scene could scarcely be any more heartless if the kid were subjected to a Chinese burn for good measure. It might have been the case that this cruelty was added to mitigate the man's callous revenge on the thief, if that eventual confrontation were not the piercing core of the film, the point at which hope dwindles almost to nought. That's when you start to understand why the trees uproot themselves from the earth, crashing to the forest floor in despair at what they've witnessed.