In its first assessment in six years, the Intergovernmental Panel on Climate Change (IPCC) dealt a crippling blow to the shrinking body of opinion that claims higher temperatures in past decades have been driven by natural, not man-made, causes.

The United Nations' paramount scientific authority on global warming highlighted a range of changes that had taken place in Earth's ice cover, rainfall patterns and permafrost and declared that most of the temperature rise over the past 50 years had "very likely" been caused by human activity.

This term means a certitude of more than 90 percent and signals an increase on the IPCC's previous assessment in 2001, whose probability was more than 66 percent.

By 2100 global average surface temperatures could rise by between 1.1 and 6.4 C (1.98 and 11.52 F) depending on how much carbon dioxide (CO2), the principal greenhouse gas, is in the air. Within this range, the "best estimate" is Earth's surface temperatures will rise between 1.8 and 4.0 C (3.2 and 7.2 F), the IPCC said. In 2001, it had forecast 1.4-5.8 C (2.5 to 10.4 F).

These figures are contained in a "summary for policymakers" in the IPCC's fourth review of the scientific evidence for global warming. They are derived from computer models based on how much CO2 enters the atmosphere.

Greenhouse gases disgorged into the atmosphere this century will cause climate disruptions "for more than a millennium" to come because of the time it takes for these molecules to degrade, the summary warned.

France and Britain swiftly declared time was running out.

"We are on the verge of the irreversible," French President Jacques Chirac said.

"Faced with this emergency, the time is not for half measures. The time is for a revolution -- a revolution of our awareness, a revolution of the economy. A revolution of political action."

British Environment Minister David Miliband said the report "is another nail in the coffin of the climate change deniers."

To avoid dangerous climate change, he said, "we will require the engagement not just of environmental ministers but heads of state, prime ministers and finance ministers."

Achim Steiner, executive director of the UN Environment Programme (UNEP), agreed.

The report "enables the world to now respond to climate change not by debating the science any more but by figuring out how on Earth we are going to live in a world with an environment change scenario that is two, three, four degrees of global warming," he said. Two to four Celsius is equivalent to 3.6 to 7.2 Fahrenheit.

IPCC Chairman Rajendra Pachauri joined a chorus of UN organizations calling for an extraordinary summit of world leaders to map out a strategy.

European Environment Commissioner Stavros Dimas said industrialized nations should pledge to cut carbon emissions by 30 percent below 1990 levels by 2020.

Yvo de Boer, head of the UN Framework Convention for Climate Change (UNFCCC) underlined the importance of market mechanisms, especially carbon trading, for reining in CO2 levels. The exhaustive IPCC study, culled from work by 2,500 scientists in more than 100 countries, sounded alarms about the impact of carbon pollution, mostly from the burning of oil, gas and coal.

These fossil fuels release CO2, which traps heat from the sun instead of letting it radiate safely into space.

"We are in a sense doing things have not been done in 650,000 years," Pachauri said, referring to atmospheric concentrations of CO2. "You are able to see what the costs of inaction are."

The summary made these forecasts:

-- Sea levels would increase by 18 to 59 centimetres (7.1 to 23.2 inches) by 2100. In its 2001 report, the IPCC estimated a rise of 9.0 to 88 cms (3.5-35 inches). It says the revision is due to improved understanding as to how the oceans absorb heat.

-- Sea ice is predicted to shrink in both the Arctic and Antarctic. In some projections "Arctic late summer ice disappears almost entirely by the latter part of the 21st century".

-- Tropical cyclones are "likely" to become more intense, packing higher winds and rain.

-- Unseasonably warm weather, heatwaves and heavy rainstorms are "very likely" to become more frequent.

Green groups demanded action to cut greenhouse gas emissions, especially by the United States, which accounts for a quarter of the global total.

"The IPCC report embodies an extraordinary scientific consensus that climate change is already upon us and that human activities are the cause," said WWF International director general James Leape.

Jan Kowalzig, climate and energy campaigner for Friends of the Earth, said the document "scientifically confirms the extent of this man-made crisis already hitting people around the world and makes bleak predictions for the future".

He added: "We can no longer afford to ignore growing and compelling warnings from the world's leading experts."

Greenpeace climate and energy campaigner, Stephanie Tunmore, called the IPCC report a "screaming siren" of a warning.

"The good news is our understanding of the climate system and our impact on it has improved immensely. The bad news is that the more we know, the more precarious the future looks," Tunmore said.

In Washington, the White House said the report was "very valuable" and the "conclusions are significant."

But it offered no sign that President George W. Bush would shift focus away from voluntary, technology-driven initiatives to tackle climate change.

"The report will contribute to the body of knowledge that we have to study and understand the best way to meet the challenges," deputy White House spokesman Tony Fratto said.

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