"I have heard three kinds of argument claiming that it is not necessary to combat climate change. The first is based on saying that scientists are wrong," Stern told a conference in Paris hosted by French President Jacques Chirac.

"After the report of the IPCC released today, this position is untenable," said the former World Bank chief economist.

The assessment released on Friday by the UN's Intergovernmental Panel on Climate Change (IPCC) said global warming was almost certainly caused by humans, and carbon pollution disgorged this century would disrupt the climate system for a thousand years to come.

"To do nothing is to run a risk that we cannot afford to take," said Stern, as he called for "urgent, resolute and concerted" action in Europe and the international community as a whole to tackle the challenge.

He also rejected the argument that -- even accepting the science -- mankind can easily cope with rising temperatures.

"That is an irresponsible position, because it does not take into account the real risks linked to a very high rise in temperatures, for example in the case of a world where temperatures rise by five or six degrees." Five or six degrees Celsius is nine to 10.8 Fahrenheit.

Those who dismissed the consequences of global warming as a remote, long-term problem were "indefensible from an ethical point of view," he said.

In a report commissioned by the British government last year, Stern warned that without urgent action, the fallout of climate change could be on the scale of the two world wars and the Great Depression of the 1930s.

Singling out current and rising economic powerhouses the United States, China and India, he said the world must be prepared to pay now -- in the form of green taxes or emissions trading schemes -- to prevent economic disaster.

The IPCC, the United Nations' paramount scientific authority on global warming, on Friday predicted Earth's surface temperatures will rise between 1.8 and 4.0 degrees Celsius (3.2 and 7.2 degrees Farenheit) by 2100.

It described this as a "best estimate" within a range of 1.1 to 6.4 degrees C (1.98 to 11.52 degrees F). In 2001, it had forecast a rise of 1.4 to 5.8 C (2.5 to 10.4 F).

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