No abstract

Recent studies have purported to show a closer correspondence between reconstructed Phanerozoic records of cosmic ray flux and temperature than between CO 2 and temperature. The role of the greenhouse gas CO 2 in controlling global temperatures has therefore been questioned. Here we review the geologic records of CO 2 and glaciations and find that CO 2 was low (<500 ppm) during periods of long-lived and widespread continental glaciations and high (>1000 ppm) during other, warmer periods. The CO 2 record is likely robust because independent proxy records are highly correlated with CO 2 predictions from geochemical models. The Phanerozoic sea surface temperature record as inferred from shallow marine carbonate δ 18 O values has been used to quantitatively test the importance of potential climate forcings, but it fails several first-order tests relative to more well-established paleoclimatic indicators: both the early Paleozoic and Mesozoic are calculated to have been too cold for too long. We explore the possible influence of seawater pH on the δ 18 O record and find that a pH-corrected record matches the glacial record much better. Periodic fluctuations in the cosmic ray flux may be of some climatic significance, but are likely of secondorder importance on a multimillionyear timescale.

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