These swings, researchers believe, are associated with changes in ocean currents in the North Atlantic. And, as reported on page one, there are ominous signs that such changes are beginning to happen.
The warning has been uncovered by an international team of scientists who have drilled deep into the ice sheet to find a deep-frozen record of the world's past climate. The record is contained in tiny bubbles of air that get trapped as each year's snowfall lays down a new layer of ice. The air contains two different isotopes of oxygen, which vary with temperature, making it possible to chart warm or cool years tens, hundreds or thousands of years ago.
Over the last few decades, scientists have drilled cores from the ice in both the Arctic and the Antarctic that have allowed them to read the climate of the last Ice Age, which lasted about 100,000 years. They found it was surprisingly changeable, "flickering" between relatively warm and relatively cold periods. But it was assumed that the periods between Ice Ages, like the one we live in now, were as stable and benign as ours.
But new ice cores from the centre of the Greenland ice-sheet have given a detailed record of the last "interglacial", which ran from about 135,000 to 115,000 years ago. They show that during this period, the climate oscillated between three states, instead of remaining in one, as in the whole of civilised human history. The middle state was like our own, but the others were much colder or warmer.
Worse, it seems that the climate can flip from one condition to another very rapidly. "It apparently took very little time, perhaps less than a decade or two, to shift between the states," wrote Dr J C W White of the Institute of Arctic and Alpine Research at the University of Colorado in the scientific journal, Nature. "We humans have built a remarkable socio-economic system during perhaps the only time when it could be built, when the climate was stable enough to let us develop the agricultural infrastructure required to maintain an advanced society.
"We do not know why we have been so blessed. But if the Earth had an operating manual, the chapter on climate might begin with a caveat that the system has been adjusted at the factory for optimum comfort - so don't touch the dials."
Unfortunately, we have been twiddling the knobs for decades. In December the official Intergovernmental Panel on Climatic Change (IPCC), which represents the work of 2,000 top meteorologists, concluded that global warming due to human activities is probably already taking place.
Global warming, as John Gummer, the Environment Secretary, has pointed out, sounds deceptively benign, raising visions of Malaga coming to Manchester. In fact, the world would struggle to cope with the effects of even a steady gradual warming.
Last Thursday, Sir John Houghton, chairman both of the Royal Commission of Environmental Pollution and of one of the main IPCC working groups, spelled it out to the Royal Society. He painted a picture of seas flooding much of Egypt, southern China and Bangladesh, making "many millions" of people homeless; of hordes of "environmental refugees" and of wars breaking out over dwindling water supplies as world rainfall patterns changed.
This would be bad enough, but there is at least a chance that the world could adapt to steady warming if it happened slowly enough. However, while a few scientists have received a lot of publicity for saying the IPCC was exaggerating, far more believe that it has been too cautious.
Many, like Professor Paul Crutzen, last year's Nobel prizewinner for Chemistry, fear that climate change will bring "unpleasant suprises". By definition, these are hard to predict. But when scientists have asked giant computers to make forecasts, they have pointed the digital finger at changes in ocean currents of the North Atlantic with remarkable regularity. Ominously, these predictions seem to be coming true. The European Sub- Polar Ocean Programme has found that the Odden Feature, a tongue of ice that acts as a natural pump to drive currents in the North Atlantic, has failed for the past three years in succession. Never before has this happened.
Until now, seawater turned to ice to form the feature; salt was left behind and the more saline, heavier water was sucked 5,000 metres down to the ocean floor. The phenomenon, which occurs in only four other places in the world, powers a vast, deep, cold current that wanders around the world's oceans and helps control their circulation.
The programme, financed by the European Commission, recently discovered that it began to falter in 1972. Gradually, the water was drawn less deep into the ocean. It ceased completely in 1994 and has failed to function ever since.
The co-ordinator of the programme, Dr Peter Wadhams of the Scott Polar Research Institute in Cambridge, says: "Global warming is the culprit. It has reduced the area of ice in the Greenland Sea and cut off the process."
Nobody knows what the effect on the world's oceans will be, but Dr Wadhams says that scientists believe it will weaken the Gulf Stream, which keeps Britain and much of northern Europe far warmer in winter than other places on the same latitude. The Gulf Stream is also driven by winds, but these, too, are expected to change as the climate heats up.
The effects may be far worse than just the arrival of harsh winters in Britain. Researchers believe that the deep water current in the North Atlantic, largely controlled by the Odden Feature, may be the trigger that sets off violent switches in our climate: there is abundant evidence that temperature flips and changes in currents have occured at the same time.
Human civilisation, it seems, has flourished during a 10,000-year climatic ceasefire. Hostilities may be about to resume.