The study, adding powerfully to evidence of human interference in the climate system, appears in the runup to a key conference on global warming which opens in Montreal next Monday.

The evidence comes from the world's deepest ice core, drilled at a site called Dome Concordia (Dome C) in East Antarctica by European scientists who battled blizzards and an average year-round temperature of minus 54 Celsius (minus 65 Fahrenheit) and made a thousand-kilometer (650-mile) trek to bring up supplies.

The core, extracted using a 10-centimetre (four-inch) -wide drill bit in three-metre (10-feet) sections, brought up ice that was deposited by snows up to 650,000 years ago, as determined by estimated layers of annual snowfall.

Analysis of carbon dioxide trapped in tiny bubbles in the ancient ice showed that at no point during this time frame did levels get anywhere close to today's CO2 concentrations of around 380 parts per million (380 ppm).

CO2 levels began to rise with the Industrial Revolution, when coal began to be burned in large quantities, and have surged in recent decades as more countries become industrialised and millions more cars take to the road.

As a result, billions of tonnes of CO2 are now being released into the air each year from fossil fuels that previously were underground. In pre-industrial times, the CO2 concentration was just 278 ppm.

Today's rising CO2 concentrations are 27 percent higher than at the highest level seen over the 650,000-year time scale, according to the study, which appears in the weekly US journal Science.

The Dome C core, extracted by the 10-country European Project for Ice Coring in Antarctica (EPICA), outstrips by 210,000 years the previous record-holder, drilled at an Antarctic site called Vostok.

"We have added another piece of information showing that the time scales on which humans have changed the composition of the atmosphere are extremely short compared to the natural time cycles of the climate system," said lead author Thomas Stocker of the University of Bern's Physics Institute in Switzerland.

Skeptics about man-made global warming point out that Earth has been through many periods of higher and lower temperatures in its history as a result of natural processes.

Volcanic eruptions that disgorge CO2 and other greenhouse gases, oscillations in the planet's axial spin and minor changes in its orbit can have a major impact on surface temperatures, sometimes plunging Earth into prolonged Ice Ages, the last of which ended some 11,000 years ago.

But over the past decade, a mountain of scientific evidence has accumulated about Man's impact on temperatures through the unbridled burning of fossil fuels.

In the past five years, the average global temperature has risen by 0.2 C (0.36 F) -- 100 times higher than is normal for such a short time scale -- and 2005 is on course for being the hottest year on record.

Glaciers in the Alps, Greenland and the Himalayas are shrinking and ice shelves are cracking in the Antarctic peninsula in what appear to be early signs of dangerous climate change, according to recent studies.

The 12-day Montreal talks, gathering members of the UN Framework Convention on Climate Change (UNFCCC), will focus on the future of the Kyoto Protocol after this pact, aimed at curbing carbon pollution, runs out in 2012.

Scientists say political progress for tackling the problem falls miserably short of what is needed to avoid long-term damage to the climate system.

In the most extreme scenarios, global warming could drive up sea levels and drown coastal cities, cause floods, droughts and freak storms, and create tens of millions of "climate refugees."

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