Carbon dioxide (CO2) was the big driver that ended the last Ice Age, scientists said in a study that undermines a key argument by global-warming sceptics.

About 10,000-20,000 years ago, earth started to emerge from a quarter of a million years of deep freeze as the terrestrial ice sheet rolled back and warmer temperatures helped man to spread and conquer the planet.

What caused the end of this age, known as the Pleistocene, has long been debated.

Until now, the main evidence has come from ice cores drilled in Antarctica whose air bubbles are a tiny time capsule of our climate past.

Traces of CO2 − the principal greenhouse gas that traps solar heat − show that carbon concentrations in the atmosphere rose after temperatures were on the rise, not before.

The timing has been seized upon by sceptics as proof that man-made carbon gases either do not cause global warming or at least do not make it as bad as mainstream scientists say.

Instead, natural changes in earth’s orbit, bringing the planet closer to the sun, caused the warming, according to the dissident view.

But the new study says a far wider picture shows orbital change merely started things going.

Real responsibility for warming lay with CO2 it contends.

“Our study shows that CO2 was a much more important factor and was really driving worldwide warming during the last deglaciation, ” said researcher Jeremy Shakun of Harvard University.

Published in the British journal Nature, the investigation looked at 80 ice cores and sedimentary samples taken from Greenland, lake bottoms and sea floors on every continent.

A rise in carbon dioxide “actually precedes global temperature range, which is what you would expect if CO2 is causing the warming.”