So farewell then, El Nino

The continent-sized pool of warm water in the Pacific, which has been turning climates upside down, is finally fading away, but nobody knows what will follow it

THE PACIFIC Ocean is finally getting back to normal. That vast pool of warm water known as El Nino has all but faded away, though many parts of the world are still feeling its effects. And in parts of the world where they are not fighting the forest fires that follow El Nino- related drought, or the mudslides that follow El Nino-related storms, they wait in trepidation to see what will happen next.

Sometimes El Nino is followed by his little sister, La Nina, which produces more or less the opposite effects. But sometimes El is an only child and the weather systems of the world revert to normal. We just don't know.

The trouble is that the world's weather involves such a complex interaction between air and water currents operating on different time-scales. At the simplest level, we have the sun heating the earth and the earth giving off its heat to warm the air. (There is comparatively little direct heating of the air by the sun - which explains why the tops of mountains are such cold places, even though they are nearer the sun.) Equatorial areas of the earth receive more solar energy than polar areas, which leads to warmer air near the equator and colder at the poles. Warm air rises, cold air descends, and that accounts for one aspect of the continuous winds that circle the earth. The other main aspect is the earth's rotation, which provides a continuous slow-stirring effect.

Meanwhile, there is the warming and cooling effect of night and day to take into account, and then there is the problem of the earth taking such a long time to radiate its stored heat.

Have you ever wondered why the shortest day (when we receive the least heat from the sun) is in December, but the coldest day does not generally happen until February? It's because the earth is such a good storage heater. In December, it's still living off the heat it collected in summer. It's only when that is all used up that we feel really cold.

Then - and here we're finally catching up with El Nino again - there is the differential heat storage capacity of the oceans and the land. While the land may retain heat for a month or so, the oceans can do so for twice as long. And they can move it around in a way that the earth cannot.

The winds spread the warmth around at one rate, the earth radiates it at another, and the oceans at a third. No wonder it's all too complicated to predict. And what makes it even worse, is the motion of water in the oceans, not just from one location on the surface to another, but between different depths. Cold water sinks and warm water rises at a far slower rate than anything we have yet mentioned, and somewhere in all these processes lies the explanation of why, every five years or so, a great pool of warm water gathers unexpectedly in the Pacific and throws the world's weather into turmoil.

And as it passes, the world counts the cost: rebuilding after El Nino will cost Ecuador $2bn; Peru will spend $627m just to rebuild highways, bridges, homes and schools destroyed by floods and mud slides; Brazil's grain production has been cut by half, and some 10 million people are expected to go hungry; 10 per cent of Fiji's population will need government aid for up to a year to avert starvation and destitution. The roll of disaster goes on and on: from New Zealand we learn that 90 per cent of chicks of the world's rarest penguin species will starve to death because the El Nino-related drought has led to a scarcity of the yellow-eyed penguin's main food source; Washington anticipates an explosion of mosquito numbers; Los Angeles is bracing itself for an invasion of rats and killer bees. All because of El Nino.

Most momentous of all, last month, scientists calculated that at its peak, El Nino was responsible for a slowing of the earth's rotation by 800 microseconds. Well, at least that gives scientists a fraction of a second more to see if they can predict when this climate destroyer will be back again.

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