The limitations of weather forecasting have been exposed by the ice storms in Montreal, which have brought a major industrialised city to a standstill. This and other weather disasters around the world show that the power of El Nino is, paradoxically, so huge that we cannot predict its effects.

Almost a quarter of a century ago, the American mathematician and meteorologist Edward Lorenz introduced Chaos Theory with the powerful image of the flap of a butterfly's wings in Brazil resulting in a tornado in Texas.

The essential point was not that a small flap can build up into huge consequences, but - far more profoundly - that a theoretically deterministic system can be totally unpredictable. In certain circumstances, the tiniest change in the initial conditions can lead to vastly different consequences. So even if we took into account the butterfly's wing-flap, it could still throw all our predictions off course by coughing, or even breathing.

What the world's weather is experiencing now, however, is no butterfly. Compared with the fabled wing-flap, the current El Nino is the stampede of a herd of elephants. Yet the consequences are just as unpredictable. I call it the Elephant Effect. You start with a deterministic, yet delicately balanced system, where small changes can be measured, and generally lead to predictable consequences. And spring follows winter, and summer follows spring, and every place has its climate, which ensures that temperatures, and wind, and rainfall fall within known limits.

Then an elephant, in the form of El Nino, comes along, and gives the whole system such a huge jolt that all previous bets are off. Quite apart from the places in the southern hemisphere that are directly affected by the mass of warm water in the Pacific, we are now seeing Montreal brought to its knees by the worst ice storms in its history, while England enjoys fine spring weather in January.

I am sure that it cannot be too difficult to explain, with hindsight, why a mass of warm water in the Pacific should lead to unprecedentedly icy conditions in North America, but to predict that effect in advance seems to be beyond our understanding of the weather system.

Events such as this expose how vulnerable we all are to changes in climate. The clothing we wear, the crops we plant, and the provisions we make for the supply of our energy requirements - vital parts of our civilisation are built on the assumption that our weather will continue behaving in much the same way it always has.

While the Butterfly Effect provides a perfect excuse for a forecaster to miss the occasional hurricane, however, the Elephant Effect exposes a more serious defect in our understanding of weather systems. It is bad enough not knowing, from year to year, how bad the next El Nino will be, or when it will arrive. What is really discouraging is not even knowing what it is going to do when it gets here.