Oh, southern England, hot hectares of sunflowers under an azure sky

It is 2056 and Britain is unrecognisable. Peter Popham on life after global warming

Greg Smith rose before dawn, because this was the time when it was possible to do things. An hour or two later, with the sun fixed in the cloudless sky like a malign, fiery eye, the tar on the London streets would begin to bubble and melt and the few people still outside would scurry for shelter. And then the silence of the city summer would return, broken only by the chirping of cicadas and the metallic rustling of the pampas grass in the abandoned back gardens.

The year is 2056, and global warming is now so familiar a reality that nobody uses the term any more. It's merely the world we live in: a world of transformed climates and landscapes, of strange new geopolitical divisions and orientations, unfamiliar new groups of rich and poor. It's a world that would seem as bizarre to us, living 60 years before, as the world of the late Nineties would have seemed to people living before the outbreak of the Second World War.

Yet many of the changes that have transformed the world in the five and a half decades since the Millennium was celebrated in Birmingham were foreseen by scientists who tried, ultimately in vain, to alert the world's politicians to the dangers ahead.

For Greg Smith, one year into retirement now, London's fierce summer stretches formidably ahead. It's 20 years now since the great evacuation of the city began. First, the working-class populations of the low-lying districts in the east (along with the Docklands business community) were driven out. One dreadful winter when the bitter east wind never relented, the Thames Barrier was destroyed by a freak wave and the east of the city as well as much of Essex was inundated.

But then a more gradual exodus began, even from the upland parts of the capital that remained dry. Built for temperate conditions, the city became unbearable as each year the summer grew hotter. It became a city of intolerable extremes: bitterly cold winters punctuated by ferocious storms and wild floods; harsh, burning, pitiless summers, when the only recourse was to keep the air-conditioning on full blast - and swallow the massive fossil- fuel tax incurred. The only escape was to move northwards, to the zones where life could still be sweet.

The chief consolation of the exodus was that London's property prices had plunged, and Smith had managed to pick up for a song a splendid mansion in Hampstead once owned by a 20th-century pop singer called George Michael. It was in ruins, but it was still a splendid pile. Now, as he sat in his underwear looking down across London to where the swollen Thames - which had long since obliterated the Houses of Parliament and the South Bank arts complex - gleamed fiercely in the sunshine, and ate a breakfast of grapes sent down by his son in Macclesfield, he wondered idly how he was going to kill time that summer.

A submarine exploration of the sunken towns of the south coast? Brighton, its Royal Pavilion intact under the waves and glinting palely, was said to be particularly spooky and cooling. A trip along the new fresh-water canal built - in the teeth of massive and bitter Welsh protests - to feed the south of England with drinking water from Snowdon?

He leafed through the travel agency's brochures. Another journey north, to his son's place south of Manchester, was much the most tempting option, but he didn't want to abuse their hospitality. Besides, a lingering pride in his southern roots drove him to seek out things about the south still worthy of admiration. Though such things were, admittedly, harder and harder to find. His motoring tour of Kent last summer with an old friend had proved deeply depressing: all the hedgerows and woods torn down, all the ponds filled in. The homely little fields of his youth had been amalgamated into mighty prairies, where rows of tall, bushy sweetcorn plants marched towards the horizon, and hectare after hectare of massed sunflowers nodded under the azure sky.

At the beach (several miles inland from where he remembered it) he had got sunstroke, and was bitten by a scorpion, and nearly died. On the way back to London, he was further depressed by the sight of the newly established "temporary islander resettlement camps" baking behind high barbed-wire fences, where survivors of islands wiped off the map by the rise in the level of the oceans - the entire Maldives were the latest victim - were incarcerated indefinitely, pending an "international solution" to the problem. "Fortress Europe" had been a phenomenon even in his childhood in the late Nineties, but now that vast coastal-estuary and island areas of countries such as Bangladesh, India and Indonesia had been destroyed by the sea, Europe's attitude to would-be immigrants had become far tougher. The hard-nosed approach of the Hong Kong Chinese towards the Vietnamese boat people 60 years ago was always being cited by politicians as the prudent approach: lock 'em up, then ship 'em out.

And it had all been foreseen, Smith told himself wonderingly. Many of the warnings given by scientists before the end of the last century had been astonishingly clear and detailed. Looking back through his father's yellowing newspaper archive, it had startled him how prescient they had been. As long ago as June 1996, the Intergovernmental Panel on Climate Change had predicted many of the particulars of the changes that actually occurred: the rising sea levels, the swallowing up of islands and deltas, the increasing incidence of storms and floods, more hunger and disease, the return of menaces such as malaria to formerly temperate countries like Britain. All this had been predicted - and the underlying reason was as familiar now as the ABC: the build-up in the atmosphere of the "greenhouse gases", particularly carbon dioxide, the consequent trapping of infra-red radiation which then warmed the atmosphere unrelentingly. Separately there was the destruction of the ozone layer. Together they spelt drastic trouble.

In Newsweek, in January 1996, James E Hansen, director of Nasa's Goddard Institute for Space Studies in New York, was quoted as saying, "As you get more global warming, you should see an increase in the extremes of the hydrologic cycle - droughts and floods and heavy precipitation." Quite right, Smith thought: monsoon-like rain has become normal in London. "The more rapidly we force change in the [climate] system," said Stephen Schneider of Stanford University in the same article, "the more likely it is to exhibit inscrutable behaviour." Hence the strange weather first noted in the late Nineties - and it has been consistently strange ever since.

Smith sighed heavily, and dabbed at the sweat on his brow. Yet although the warming of the planet, as the environmental organisations long and vociferously predicted, had had many devastating consequences, not everyone had suffered. Many people's lives had in fact been transformed for the better. His thoughts turned longingly towards Macclesfield - Macclesfield and the glorious north.

Again, Smith realised, it had all been foreseen: Professor Martin Parry of the Jackson Institute of Climate Research in London and a member of the IPCC, had predicted 60 years before that, while the whole of Britain would get hotter, the south would at the same time get drier and the north wetter. The consequences were foreseeable, the professor had declared: irrigation, prairie-size fields, semi-tropical crops in the south. And in the north?

The slopes of the hills around Macclesfield town centre were terraced for vines 20 years back, under the eye of a French master of viniculture from Bordeaux, fleeing the desertification of his own region. Greg Smith's son Max had had to go abroad to find lucrative work, like most ambitious children of his generation. He had mined coal in China for five years in his early twenties - yes, he knew it was adding to the warming problem, but that's where the money was, especially now the Chinese refused to do such work themselves. He had made so much money (when translated into euros) that on his return he had been able to buy one of the up-and-coming vineyards of southern Manchester, with a handsome villa - built 150 years before, during the town's previous surge of economic success - set in the middle of it.

Now he could only congratulate himself on his sound instincts. For Manchester had decisively taken over many of London's functions when the capital became too hot to bear - the City, in particular, had favoured Manchester's lusher, more merciful climate, not to mention its excellent wines - and the green suburbs to the south were experiencing an apparently endless boom, as the changing climate greened its valleys and fruited its hillsides and gradually transformed it into the Provence of the North.

And the good fortune was not restricted to the inland areas. More than half a century before, seemingly crazed city fathers insisted on building marinas, for no conceivable purpose, in unlikely places such as Hartlepool. But now these mad acts were revealed as stunningly far-sighted: the same marinas were throbbing night and day with Mediterranean-style life, their brilliant blue harbours alive with pretty sails. Liverpool readily re- made itself in the image of Cannes, Blackpool in the likeness of Monte Carlo.

It all had a strange, circular sort of justice to it. It was here in the north of England that the industrial revolution had been born; here for the first time greenhouse gases had been unleashed on the atmosphere in quantities previously unimagined. Decades, centuries passed, and while the north of England languished in increasing poverty and obscurity, the revolution it had made went around the world and did its work, and the greenhouse gases went up into the heavens. Finally, today, the north was reaping the benefits of what it had originally sown. It was almost enough to make one believe in a benign God - though this was a view with which the environmental organisations were likely to disagree.

Down in Hampstead it was nine o'clock in the morning, and the cicadas shrieked and the pampas grass creaked and another infernal day was under way. Greg Smith swallowed his pride and picked up the phone. He'd go north again, after all.