Brian Fagan warns that global warming could shut down the Gulf Stream, locking Europe in a savage winter and the Middle East in a scorching drought for as long as ten centuries.

What would happen if the Gulf Stream shut down? Judging from events 13,000 years ago, Europe would be plunged rapidly into a near ice age and a savage drought would settle over the Middle East. Strange, perhaps, to contemplate an imminent cooling when everyone's talking about humanly caused global warming, but there's a possibility that warm water circulation in the North Atlantic is endangered, not because of global cooling, but because of persistent warming.

Giant conveyor-like cells circulate water through the world's oceans along what geochemist Wallace Broeker calls the "great ocean conveyor belt". The circulation has enormous power, equivalent to 100 Amazon rivers. In the Atlantic, warm upper-level water from the Gulf Stream flows northward past Europe, then westward until it reaches the vicinity of Greenland. Cooled by arctic air, the salt-dense surface waters sink and form a current that flows enormous distances at great depths to the South Atlantic, Antarctica and beyond. The salt water downwelling near Greenland keeps Europe warm. If the downwelling slowed, even shut off altogether, the Gulf Stream would no longer warm us and temperatures would plummet. What alarms climatologists is that this has happened before.

Thirteen thousand years ago, a huge glacial lake, Lake Agassiz, lapped a 1,125km front of the retreating continental ice sheet in North America. At its maximum extent, the lake covered parts of Manitoba, Ontario and Saskatchewan in Canada, and Minnesota and North Dakota in the US. A bulge of the ice sheet prevented the lake from draining into what is now the St Lawrence River Valley. As the ice retreated, so the lake grew, swollen by glacial mel****er.

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In about 11,000BC, the rising water broke through the bulge and burst into the St Lawrence River. Within months, perhaps weeks, Lake Agassiz ceased to exist, except as a few remnants, such as Lake Winnipeg. The huge burst of fresh water flowed into the Labrador Sea. The Agassiz mel****er floated atop the dense, salty Gulf Stream, forming a temporary lid that prevented warm water from cooling and sinking.

Like an electric switch, Lake Agassiz's fugitive waters reduced the power of the Atlantic conveyor belt. Temperatures fell rapidly. The vast Ice Age glaciers mantling Scandinavia advanced once again. A sea ice cap formed in short order, preventing the Gulf Stream from starting up again, helping trigger an intensely cold climatic regimen in northern latitudes.

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Breathtaking climatic changes rippled across Europe. The Netherlands saw winter temperatures plunge regularly below - 20C. Snow would fall any time from September to May, while summers were cool, averaging between 13C and 14C. Throughout much of Europe, tree cover retreated, replaced by cold-weather shrubs. Dramatic temperature fluctuations, wide annual climatic swings and severe winter storms pummelled Europe. Climatologists call this 1,000-year event the Younger Dryas, after a small polar flower that was then commonplace. Hundreds of carefully calibrated radiocarbon dates place the event between about 11,000 and 10,000BC.

The cold endured for ten centuries. Then, just as abruptly, the Gulf Stream started up again, possibly within a mere 50 years.

The Younger Dryas descended on a Europe inhabited by nomadic hunting bands, who subsisted off reindeer, forest animals and plant foods. Such people were accustomed to rapid climatic shifts; their mobility allowed them to move southwards to sheltered environments. But the people of the Middle East were less fortunate. A harsh and prolonged drought sank over southwestern Asia for ten centuries.

For 2,000 years after the end of the Ice Age, the Euphrates Valley and what is now Syria enjoyed plentiful rainfall. Oak and pistachio forests yielded vast harvests of nuts each autumn. In many places, people lived in crowded, if edible, landscapes, in settlements occupied over many generations. The very permanence of these communities, their close roots to wild grass stands or oak trees, was created not so much by population growth but by women and their plant-processing activities. Their work fed many more people, but it did so at a price - the loss of mobility, of a social flexibility that was as old as humanity itself.

Then the severe Younger Dryas drought came along. At Abu Hureyra, a village near the Euphrates River, botanist Gordon Hillman studied large plant samples from the occupation layers. He found that in about 11,000BC, people stopped gathering tree fruit and nuts from the forest fringe. Such trees no longer grew near the settlement. People now ate more wild cereals, including feather grass and asphodel seeds. Four hundred years later, in 10,600BC, even asphodel and wild cereal grains vanished from Abu Hureyra.

The inhabitants turned to less palatable foods, to drought-resistant clovers and medicks that were far from nutritious and much harder to process.

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In about 10,000BC, the inhabitants of Abu Hureyra took the next logical step - growing wild grasses to expand the wild harvest. The first domesticated seeds appeared in the village - rye, einkorn and lentils - but they were not enough to feed everyone. After years of good living, the village had swelled to perhaps 300 or 400 inhabitants, a population density far beyond the constraints imposed by a mobile existence. We can imagine the cold months of winter, hungry families huddled in their dwellings, with even firewood in short supply in an arid, no longer forested landscape.

Despite the experiments with cereal grasses, Abu Hureyra was a community under stress from seemingly unending drought. But within a few generations, the habit of repeated planting and harvesting changed the genetic make-up of wild grasses and altered the course of history. The hunters of the Euphrates Valley became full-time farmers. When wetter climatic conditions returned after 10,000BC, the new economies spread rapidly throughout southern Asia, to the shores of what is now the Black Sea, in the Jordan Valley, and as far away as the Nile.

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Within a remarkably short time, farming replaced hunting and gathering over an enormous area of the eastern Mediterranean. By 3,000BC, there were many more farmers than hunter-gatherers in the Old World.

But what would happen today if another huge discharge of glacial mel****er were to cascade into the North Atlantic, perhaps from the implosion of the Greenland ice sheet? Europe could well see another ten centuries of intense cold, much shorter growing seasons, advancing glaciers in Scandinavia and the Alps and decimated forests. A doomsday scenario, perhaps, but one the Pentagon is taking seriously. A recently released report on a Younger Dryas-type scenario recommends that the possibility be moved beyond a mere scientific debate to a potential security concern for the US that would affect food and water supplies in the northern hemisphere. All this is while we worry about the ongoing melting of the Greenland ice sheet which a team of scientists recently predicted would flood London and raise sea levels more than 7m within a millennium.

The potential scale of disaster is almost unrecognisable in historical terms. Judging from the Little Ice Age, a period of highly variable climatic shifts between 1300 and 1860, Alpine and Scandinavian glaciers would advance and growing seasons throughout northern Europe would be shortened. European vineyards would go out of business - and Little Ice Age cold pales beside that of the Younger Dryas. The southern limits of pack ice would move southwards, fish populations would adjust to much colder sea water. The Baltic and much of the North Sea would freeze over for months on end. Cereal crops would no longer grow throughout much of Europe; temperate forests would be devastated. How would we feed ourselves? Agriculture is less visible to us now - the number of people growing food has shrunk from 90 per cent of the labour force in Europe 500 years ago to less than 3per cent in the US today - but we still need to eat. The European Union would have to import millions of tons of grain from less affected regions.

But our vulnerability extends far beyond growing food: our crowded coastlines with densely packed high-rise offices and apartment buildings, our communication and transport systems, our abstract worlds of finance and scholarship and entertainment are beholden to the world's climate in ways both obvious and hidden. Like many civilisations before us, we've simply traded up in scale, accepting vulnerability to the rare big disaster in exchange for a better ability to handle the smaller common stresses such as short-term droughts and exceptionally rainy years. But what would happen if the disaster lasted for ten centuries? The course of human history would change in frightening, unimaginable ways.

We are a supertanker among human societies, navigating in ever more uncertain climatic seas. Only a tiny fraction of the people on board are engaged with tending the engines. The rest are buying and selling goods among themselves, entertaining each other or studying the sky or the hydrodynamics of the hull. Those on the bridge have no charts or weather forecasts and cannot even agree that they are needed; indeed the most powerful among them subscribe to a theory that says storms do not exist, or if they do, their effects are entirely benign, and the steepening swells and fleeing albatrosses can be taken only as a sign of divine favour.

Few of those in command believe that the gathering clouds have any relation to their fate or are concerned that there are lifeboats for only one in ten passengers. And no one dares to whisper in the helmsman's ear that he might consider turning the wheel. But ahead of us looms the possibility of catastrophic, sudden climatic shipwreck.

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Brian Fagan is emeritus professor of anthropology at the University of California, Santa Barbara, and author of The Long Summer , published by Granta on May , ?20.00.