CO2 as a primary driver of Phanerozoic climate: Reply

Royer et al. (2004), in their recent article in GSA Today, dispute the conclusion of Shaviv and Veizer (2003) that celestial forcing may have been the major driver of Phanerozoic climate, arguing instead for CO 2 as the dominant force.The δ 18 O of calcareous shells reflects the ambient temperature of seawater and the quantity of water locked in the polar ice caps, each contributing about one half to this signal, but lately it has been realized that seawater pH drives the δ 18 O in the opposite direction.Royer et al. utilized this for reconciling the GEOCARB III and the δ 18 O trend of Veizer et al. (2000) by assuming that any discrepancy of the two variables is due to pH.This is an interesting proposition that has some merit, but note that such a correction is entirely arbitrary, because we do not have any constraints for the pH of Phanerozoic seawater, except possibly some boron isotopes for the youngest portion of this record.Moreover, the pH correction itself basically reflects the GEOCARB III CO 2 model, such that the correlation obtained between the corrected δ 18 O and CO 2 cannot be claimed to be a CO 2 fingerprint.Furthermore, the model of Royer et al. does not consider the mitigating "ice volume" effect.Once included, the required pH correction (and GEOCARB III CO 2 levels) would have to be about doubled for CO 2 climate "driving" to be on par with the CRF.For all these, and the reasons listed in our detailed response (see www.gsajournals.org;go to "Online Journals," then "Online Forum"), we argue that the δ 18 O trend is still chiefly a reflection of the temperature history of the past oceans, controlled principally by the celestial driver.