How come a big ice age happened when carbon dioxide levels were high? It's a question that climate sceptics often ask. But sometimes the right answer is the simplest: it turns out CO_2 levels were not that high after all.rnThe Ordovician ice age happened 444 million years ago, and until now records have suggested that CO_2 levels were relatively high then. But when Seth Young of Indiana University in Bloomington did a detailed analysis of carbon-13 levels in rocks from Canada and Estonia that were formed at the time, the picture that emerged was very different.
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