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Brook thought so-called “blue ice” might offer a chance to go back further in time.

If continuous ice cores are like books, blue ice cores are untitled chapters presented out of order, with lines missing.

But not one blue ice sample older than 1 million years exceeded 300 ppm, says Julia Marks Peterson, a paleoclimatologist at OSU who performed the greenhouse gas analysis.

But the snapshots across the transition found in the blue ice suggest CO2 levels held steady between about 220 ppm and 250 ppm.

That means the warmer Pliocene climate—and its clues to our warming future—will be the sole preserve of blue ice deposits.