"Is the climate changing? For the past few years there is no longer any doubt about it," said Herve le Treut, one of the world's top climate scientists who muster in Paris on Monday.

"Is the climate changing due to human activity, the response is more and more certainly, yes," le Treut, the director of the dynamic meteorology laboratory at the Pierre-Simon Laplace institute in eastern France, told AFP in an interview.

On Friday the hundreds of experts at the Paris meeting of the Intergovernmental Experts Group on Climate Change (GIEC) will issue the first update in six years of the scientific evidence for global warming.

It is expected to be the bleakest assessment yet about global warming and its effects on climate.

"The major risk is that of the speed of climate change," he said, pointing to the increased risks posed by the changing nature of society.

"If we were in a very stable world it would not be so bad if it were a bit hotter or a bit colder.

But the risk is of a very quick transition that we can have from one world to another, because it is very destabilising at every level, including socially and politically," he said.

He said scientists were becoming more confident in their evaluations as events had backed up previous predictions.

"Every period that passes strengthens the previous conclusions," he said.

"Science is developing in the direction of confirming the first simulations. For the past 20 years the level of confidence that we can have in what is said and the way in which things are backed up from the scientific point of view is enormous," he said.

But consensus between scientists is sometimes difficult to reach, he said.

In particular, he says he is personally opposed to the publication of forecasts of temperature increases according to different economic scenarios.

According to the GIEC's last analysis in 2001 average temperatures would increase by 1.4 to 5.8 degrees C (2.5-10.4 F) by the end of the century, compared to 1990.

The wide difference was down to uncertainties over the development of society and economies, and the extent to which they consumed energy, rather than scientific uncertainties.

Le Treut said the ranges had been chosen somewhat arbitrarily.

"If they give us different scenarios we can have different results but that does not mean that the danger for the planet is any different," he said.

He said an element of uncertainty had to be built into any prediction.

"When you measure the complexity of natural systems you can add a certain number of risks which we did not have such a good knowledge of previously," he said.

"Because some processes are likely to increase the climate change beyond a certain threshhold... such as the destruction of plant life, which can no longer hold CO2, the main greenhouse gas, or the quick melting of the ices."

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