"We can firmly say that today's concentrations of carbon dioxide and methane are 28 and 124 percent higher respectively than at any time during the last 800,000 years," said Thomas Stocker, an author of the report at the University of Berne.

Before the Industrial Revolution, levels of greenhouse gases were guided mainly by long-term shifts in the earth's orbit around the sun that have plunged the planet into ice ages and back again eight times in the past 800,000 years.

The UN Climate Panel last year blamed human activities, led by burning of fossil fuels that release heat-trapping gases, for modern global warming that may disrupt water and food supplies with ever more droughts, floods and heatwaves.

"The driving forces now are very much different from the driving forces in the past when there was only natural variation," Stocker told Reuters of the study in the journal Nature by scientists in Switzerland, France and Germany.

The experts, working on the European Project for Ice Coring in Antarctica, drilled down almost to bedrock in Antarctica. They recovered layers of ice formed by compressed snow, which can be counted much like the rings on trees.

DEEPER ICE

Stocker said Chinese and Australian scientists were examining possibilities for drilling in parts of Antarctica with even deeper ice, in some places 4,500 metres thick, that could yield atmospheric records dating back 1.5 million years.

The study also found big natural shifts in carbon dioxide levels. "We find very conspicuous natural oscillations of carbon dioxide 770,000 years ago that bear the fingerprint of abrupt climate change during ice ages," Stocker said.

And the Nature report also set a new record low for carbon dioxide at 172 parts per million (ppm) in the atmosphere about 667,000 years ago, about 10 ppm below the previous known low and giving an ancient natural range of 172 to about 300 ppm.

The study suggested that the low might be a sign that the oceans once soaked up more carbon dioxide. Carbon dioxide levels are now at about 380 ppm.

Taken together, the data "allow us to learn more eventually about the carbon cycle and its responses to climate change."

Temperatures in an ice age are about 5-6 Celsius (9 to 11 Fahrenheit) colder than now, already a mild period in earth's history. Climate change could add a "best guess" of 1.8 to 4.0 Celsius this century, according to the UN panel.

The study also linked variations in methane to monsoons.