A combination of global warming and the El Niño weather system is set to make 2007 the warmest year on record with far-reaching consequences for the planet, one of Britain's leading climate experts has warned.

As the new year was ushered in with stormy conditions across the UK, the forecast for the next 12 months is of extreme global weather patterns which could bring drought to Indonesia and leave California under a deluge.

The warning, from Professor Phil Jones, director of the Climatic Research Unit at the University of East Anglia, was one of four sobering predictions from senior scientists and forecasters that 2007 will be a crucial year for determining the response to global warming and its effect on humanity.

Professor Jones said the long-term trend of global warming - already blamed for bringing drought to the Horn of Africa and melting the Arctic ice shelf - is set to be exacerbated by the arrival of El Niño, the phenomenon caused by above-average sea temperatures in the Pacific.

Combined, they are set to bring extreme conditions across the globe and make 2007 warmer than 1998, the hottest year on record. It is likely temperatures will also exceed 2006, which was declared in December the hottest in Britain since 1659 and the sixth warmest in global records.

Professor Jones said: "El Niño makes the world warmer and we already have a warming trend that is increasing global temperatures by one to two tenths of a degrees celsius per decade. Together, they should make 2007 warmer than last year and it may even make the next 12 months the warmest year on record."

The warning of the escalating impact of global warming was echoed by Jim Hansen, the American scientist who, in 1988, was one of the first to warn of climate change.

In an interview with The Independent, Dr Hansen predicted that global warming would run out of control and change the planet for ever unless rapid action is taken to reverse the rise in carbon emissions.

Dr Hansen said: "We just cannot burn all the fossil fuels in the ground. If we do, we will end up with a different planet.

"I mean a planet with no ice in the Arctic, and a planet where warming is so large that it's going to have a large effect in terms of sea level rises and the extinction of species."

His call for action is shared by Sir David King, the Government's chief scientific adviser, who said that 2006 had shown that the "discussion is now over" on whether climate change is happening. Writing in today's Independent, Sir David says progress has been made in the past year but it is "essential" that a global agreement on emissions is struck quickly. He writes: "Ultimately, only heads of state, working together, can provide the new level of global leadership we need to steer the world on a path towards a sustainable and prosperous future. We need to remember: action is affordable - inaction is not."

The demands came as the World Meteorological Organisation (WMO), the United Nations agency that deals with climate prediction, issued a warning that El Niño is already established over the tropical Pacific basin. It is set to bring extreme weather across a swath of the planet from the Americas and south-east Asia to the Horn of Africa for at least the first four months of 2007.

El Niño, or "the Christ child" because it is usually noticed around Christmas, is a weather pattern occurring every two to seven years. The last severe El Niño, in 1997 and 1998, caused more than 2,000 deaths and a worldwide damage bill of more than £20bn.

The WMO said its latest readings showed that a "moderate" El Niño, with sea temperatures 1.5C above average, was taking place which, in the worst case scenario, could develop into an extreme weather pattern lasting up to 18 months, as in 1997-98. The UN agency noted that the weather pattern was already having "early and intense" effects, including drought in Australia and dramatically warm seas in the Indian Ocean, which could affect the monsoons. It warned the El Niño could also bring extreme rainfall to parts of east Africa which were last year hit by a cycle of drought and floods.

Its effect on the British climate is difficult to predict, according to experts. But it will probably add to the likelihood of record-breaking temperatures in the UK.

The return of El Niño

* Aside from the seasons, El Niño and its twin, La Niña, are the two largest single causes of variability in the world's climate from year to year.

Both are dictated by shifts in temperature of the water in the tropical Pacific basin between Australia and South America. Named from the Spanish words for "Christ child" and "the girl" because of their proximity to Christmas, they lead to dramatic shifts in the entire system of oceanic and atmospheric factors from air pressure to currents.

A significant rise in sea temperature leads to an El Niño event whereas a fall in temperature leads to La Niña.

The cause of the phenomenon is not fully understood but in an El Niño "event" the pool of warm surface water is forced eastwards by the loss of the westerly trade winds. The sea water evaporates, resulting in drenching rains over South America, particularly Peru and Ecuador, as well as western parts of the United States such as California.

Parts of the western Pacific, including Indonesia and Australia, suffer drought. The effects can last for anything from a few weeks to 18 months, causing extreme weather as far afield as India and east Africa.

The co-relation with global warming is as yet unclear. Archaeological evidence shows El Niños and La Niñas have been occurring for 15,000 years. But scientists are investigating whether climate change is leading to an increase in their intensity or duration.