It’s a question climate sceptics often ask: How come a big ice age happened when carbon dioxide levels were high?

Now, scientists claim to have solved the mystery of high-carbon ice age — carbon dioxide levels at the time of the Ordovician ice age were not that high after all, the New Scientist reported.

The Ordovician ice age happened 444 million years ago and records have suggested that CO2 levels were relatively high then.

But Seth Young of Indiana University in Bloomington did a detailed analysis of carbon-13 levels in rocks formed at the time, the picture that emerged was very different. Young found CO2 concentrations were in fact relatively low when the ice age began.

According to Lee Kump of Pennsylvania State University in University Park, earlier studies missed the dip as they calculated levels at 10-million-year intervals and the ice age lasted only half a million years.

The dip was triggered by a burst of volcanic activity that deposited new silicate rocks. These draw CO2 out of the air as they erode.

As the ice spread, however, it gradually covered the silicate rocks, slowing the erosion and so allowing CO2 to build up in the atmosphere once more. This eventually would have warmed the atmosphere enough to end the ice age, Kump was quoted as saying.

The findings have been published in the latest edition of the Palaeogeography, Palaeoclimatology, Palaeoecology journal.

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