Celestial driver of Phanerozoic climate?

Atmospheric levels of CO₂ are commonly assumed to be a main driver of global climate. Independent empirical evidence suggests that the galactic cosmic ray flux (CRF) is linked to climate variability. Both drivers are presently discussed in the context of daily to millennial variations, although they should also operate over geological time scales. Here we analyze the reconstructed seawater paleotemperature record for the Phanerozoic (past 545 m.y.), and compare it with the variable CRF reaching Earth and with the reconstructed partial pressure of atmospheric CO₂ (pCO₂). We find that at least 66% of the variance in the paleotemperature trend could be attributed to CRF variations likely due to solar system passages through the spiral arms of the galaxy. Assuming that the entire residual variance in temperature is due solely to the CO₂ greenhouse effect, we propose a tentative upper limit to the long-term "equilibrium" warming effect of CO₂, one which is potentially lower than that based on general circulation models.