IF YOU stroll along the "infinite shingle" of Chesil Beach in Dorset, England, as Ian McEwan did while composing his new novel, you will find that millennia of tides and winds have "graded the size of pebbles" along its 18-mile length, "with the bigger stones at the eastern end". The writer went to check this out, and felt - as he weighed the pebbles in his palms - that it was true.

Already, critics have lauded On Chesil Beach as a major achievement from a painstaking microhistorian of the inner life. Edward and Florence, its loving but fatally innocent couple, stumble into a wedding-night disaster in the "buttoned-up", respectable England of July 1962, the victims not merely of "their personalities and pasts" but of "class, and history itself".

Yet long-haul admirers of McEwan will detect some even deeper rhythms at work here. Once again, he traces the ominous crossing of a threshold from one human state to another: a step into the dark framed - as often in his fiction - by the inexorable onward movement of maturing and ageing bodies, of biological evolution, of climate and even geology itself.

We talk in a restaurant in Fitzrovia, a short walk for McEwan from the handsome house in a Georgian square that he fictionally lends to the neurosurgeon Henry Perowne in Saturday. Upstairs, there seems to be a meeting of the revived Bonzo Dog Doo-Dah Band, exactly the kind of wacky pop pranksters that Edward - in the lonely hippie-era limbo where McEwan's epilogue leaves his stubborn hero - might have promoted in his Camden record shop.

Outside, the sunshine signals another kind of transition, from winter into spring. And McEwan, a model of quietly spoken exactitude with words and ideas alike, stresses that On Chesil Beach aims at more than just the scrutiny of that early-'60s cusp of change between - as Philip Larkin and almost all the reviewers have put it - "the end of the Chatterley ban/ And the Beatles' first LP".

For all the pin-sharp evocation of a time when "youthful energies were pushing to escape, like steam under pressure", this last gasp of British sexual inhibition gave his story a starting point and not a terminus. "I never really thought of it as a historical novel, " he explains, "because I was interested in another aspect: which is when young people cross this line - the Conradian shadow-line - from innocence to knowledge.

You're also dealing with a human universal. So I was rather interested to discover what young people would make of this. And I was quite relieved, for example, that my sons took to it avidly - even though they're living at a time when they not only have girlfriends, but they have lots of friends who happen to be girls:

another world."

The book also survived a testrun beyond McEwan's family (his wife is the journalist and author Annalena McAfee, and he has two early-20s sons from his first marriage). He read an extract at Hunter College in New York, to the sort of student body who might have been forgiven for failing to sympathise with the bedroom blunderings of a pair of virginal Home Counties 22-year olds in the summer before the Cuban missile crisis. "This is a community college, " the author says, "and the kids are - tough is not the word, they're really lovely, but they're not protected. They've clearly been out there." Would this streetsmart audience think: why don't Edward and Florence "just get on with it? What's the problem? On the contrary: they seemed deeply engaged.

"So there have to be two elements running side by side, " McEwan continues. "One is that, this is particular: these are characters frozen in history, limited by psychology, by class, by private experience. But on the other hand, this is a universal experience that is differently dressed up by different people at different times." Youth always has to cross that line, even if it would no longer run through the starched sheets of a marriage bed in a dowdy Dorset hotel.

Always the punctilious realist, McEwan nonetheless skirts the seas of parable, or myth. Yet for this, the 12th work of fiction since his 1975 debut with the luridly memorable tales of First Love, Last Rites, he wanted to avoid wading in too deep. "This particular beach offered so many metaphorical possibilities, " he says. "They could kill the novel! So I really had to row back quite hard on that."

McEwan's fiction strikes so hard and lingers so long in the imagination precisely because he keeps the interpretative volume down.

"Readers will rebel, " he believes, "when they spot an overriding, determining metaphor." Or, perhaps, a determining cause. On Chesil Beach hints at a specific reason for Florence's "visceral dread" of sexual experience, one that throws a line from this work back to the toxic households of those earliest stories. Her creator reveals that "in an early draft, it was all too clear". The finished work allows more space for the reader: we can join the dots through the past ourselves, just as we can fill in the futures to be enjoyed or endured by both after the act, or failure to act, that will mould them. Edward, the promising historian, now seems headed for a life of amiable countercultural drift; Florence, the driven violinist, stands on the brink of a solitary musical destiny.

Florence plays in a rising string quartet, and the novel that tells her story has a densely wrought, compacted, chamber-music quality. A central movement - the wedding night itself - is interspersed with chapters that delve into the characters' past and, at the finale, the future as well. "One of the first things that I wrote about it when I was making notes, " McEwan recalls, "was a simple direction:

five times eight - five chapters of about 8,000 words. A wedding night seemed to me perfect for a short novel."

The author of other compressed but resonant pieces, such as The Comfort of Strangers, Black Dogs and the Booker-winning Amsterdam, points out that "I've always liked that form: the novel that can be read in three hours, at a sitting, like a movie or an opera." A chamber opera will be McEwan's next project, due for its premi�re at next year's Hay festival. He has almost completed a small-scale, "easily exportable" collaboration with the composer Michael Berkeley (who was his partner more than 20 years ago on the anti-nuclear oratorio Or Shall We Die? ). It has a Don Giovanni-style seducer for its protagonist: "We thought that sexual obsession would be a very good subject for an opera."

And sexual obsession, in the form of longing or loathing rather than action, makes an equally compelling motif for On Chesil Beach. For McEwan, the book's microscopically observed convergence of social embarrassment and erotic misery "is not great tragedy. But it's something I always have an interest in: how something small, like not saying the right thing or not making the right gesture, could then send you down a slightly different path in life. It must happen to us countless times, but we barely notice."

In the pre-permissive shadowland of 1962, McEwan himself was a 13-year-old schoolboy, the itinerant, Aldershot-born son of a career army officer from Glasgow.

Famously, his father's ordeal at Dunkirk helped to shape the wartime scenes of Atonement, the 2001 novel that, for many of his readers, ranked McEwan firstamong-equals in that gifted cohort of novelists (Amis, Barnes and Rushdie among them) born into the aftermath of global war.

Now, a few readers wonder if the poignant road-not-taken theme in On Chesil Beachmight connect with his rediscovered brother, David Sharp. The son born to McEwan's parents while his mother was still married to her first husband (later killed in action), David was given up for adoption in 1942. McEwan first encountered him in 2002, and they periodically meet, but he says that this reconfigured family history has not (yet) found its way into his work.

The novelist may not enlist people into fiction so directly, but he does recruit places. Just as Saturdaymore or less gave Perowne his creator's own address, so On Chesil Beach has Edward grown up in a Chilterns cottage that McEwan once almost rented, while Florence's chilly family occupies the north Oxford house he lived in during the 1980s. McEwan conjures up his terrain with a walker's close-to-the-ground eye.

Plants thrust, creatures breed (or refuse to), and even hills or beaches shift according to the overlapping cycles that push on beyond the limited history that persons or societies know. He is also deeply immersed in ecological debates.

In 2005, he joined a trip to the Svalbard archipelago, 79� north in the Arctic, for the Cape Farewell project led by the artist David Buckland, which aims to raise cultural awareness of the issues of global warming. He reads widely in scientific literature and, just before we met, had travelled to Hamburg for a public dialogue with John Schellnhuber, the German government's adviser on climate change.

Yet McEwan the engaged intellectual and McEwan the novelist remain separate beings. "Fiction hates preachiness, " he affirms.

"Nor does it much like facts and figures or trends or curves on graphs. Nor do readers much like to be hectored." He says that in spite of "all the reading that I've done around climate change, none of it suggests anything useful in the way of approaching this novelistically".

Still, he can just about envisage a fiction that would do artistic justice to a perilously warming world: "Something small and fierce, that would unwind in a way that's intrinsically interesting. . .

It's got to be fascinating, in the way that gossip is. It's got to be about ourselves. Maybe it needs an Animal Farm. Maybe it needs allegory. But if you're going in that direction, then you need a lot of wit."

Meanwhile, ventures such as Cape Farewell may trigger an urge to cherish as well as to lament. In an Arctic cold snap, "I did two or three long hikes that just took my breath away, " he says.

"Many others have thought this too: that one way forward is not doom-and-gloom but celebration;

of what we are, what we have, and what we don't want to lose."

On his return, he wrote a fable about the boot-room of the expedition ship, with its all-too-human rows over purloined kit. Artists may not refine the theory or advance the technology that will grapple with climate change, but they can deepen the self-knowledge of the selfish but potentially co-operative beasts who have crossed a fateful, collective shadow-line. "How do you talk about the state we've got ourselves into, " he asks, "as a very successful, fossil-fuel-burning civilisation? How do we stop? That really does become a matter of human nature. There's all the science to consider, but finally there is a massive issue of politics and ethics."

McEwan, who shadowed a leading neurosurgeon while researching Saturday, likes the company and outlook of scientists as an antidote to lazy arts-faculty despair. "Among cultural intellectuals, pessimism is the style, " he says with a tinge of scorn. "You're not a paid-up member unless you're gloomy."

For all that he draws from science however, McEwan keeps faith in the special tasks of art. "I hold to the view that novelists can go to places that might be parallel to a scientific investigation, and can never really be replaced by it:

the investigation into our natures;

our condition; what we're like in specific circumstances." On Chesil Beach, it strikes me, shows at its infinitely sad conclusion an example of self-punishing "negative altruism" (a term McEwan has learned from the world of science) at work. Here, a vengeful righteousness that wrecks the injured party takes shape not in colour-coded neural maps, but through a vigilant writer's heartbreaking empathy with the twisted feelings of a child in its time.

If human communication and solidarity can founder so totally in this novel's little pool of fear and frustration, what are its prospects in the great ocean of social behaviour? We talk of the carboncutting, resource-saving sacrifices this generation may have to make on behalf of its successors, and McEwan comments that such long-term altruism "does go against the grain a bit. . . It is difficult to do favours for people you have never met, " he says. "But we give money to Oxfam, to charities, to victims of the tsunami and so forth. These are not people who are ever going to repay those favours, or even know who bestowed them."

Unlike his characters, doomed to a kind of soul-extinction in their solitude, McEwan believes in making the last-ditch gesture that might save a world. "The worst fate would be to conclude that there's nothing we can do about this, and so let's party to the end."

See review, page 18 'On Chesil Beach' is published by Jonathan Cape (�12.99)