Measurements of ancient air bubbles trapped in Antarctic ice offers evidence that humans have been changing the global climate since thousands of years before the industrial revolution.

From 8000 years ago, atmospheric levels of carbon dioxide began to rise as humans started clearing forests, planting crops and raising livestock, a scientist said on Tuesday. Methane levels started increasing 3000 years later.

The combined increases of the two greenhouse gases implicated in global warming were slow but steady and staved off what should have been a period of significant natural cooling, said Bill Ruddiman, emeritus professor at the University of Virginia.

The changes also disrupted regular patterns that dominated the 400,000 years of atmospheric history that scientists have teased from samples of ancient ice.

"You have 395,000 years of history, which sets some rules, and 5000 years that break those rules," Professor Ruddiman said.

He briefed reporters on his theory at the autumn meeting of the American Geophysical Union on Tuesday. Further details appear in the December issue of the journal Climatic Change.

Previously, scientists assumed widely it was only with the onset of the factory age that human activity had any significant effect on the global climate. The prehistoric changes in carbon dioxide and methane levels have been noted before but were attributed to natural causes, Professor Ruddiman said.

"It's a great new idea we need to talk about and evaluate," said Bette Otto-Bliesner, a paleoclimate expert at the National Centre for Atmospheric Research, who was not connected with the research.

Atmospheric levels of carbon dioxide and methane naturally fluctuate, in part because of changes in the orbit of the Earth and the resulting variations in the amounts of sunlight.

But human activity apparently thwarted expected decreases in the atmospheric concentrations of both gases.

Leading the change was the revolutionary adoption, across both Europe and Asia, of agriculture and animal husbandry, Professor Ruddiman said.

Analysis of air trapped in ice cores drilled from the Antarctic ice sheet show anomalous increases in carbon dioxide levels beginning 8000 years ago - just as crop lands began to replace previously forested regions across Asia and Europe.

About 5000 years ago, the ice cores reflect a similarly anomalous rise in methane levels, this time tied to increased emissions from flooded rice fields, as well as burgeoning numbers of livestock, Professor Ruddiman said.

The prehistoric practices apparently overrode a build-up of ice that models predict should have occurred from 5000 years ago.