Some scientists in the past have said that during the Phanerozoic Eon, which spans the last 541 million years, CO2 was the atmospheric control knob driving temperature change. But others have said the two weren’t correlated at all. Now with the benefit of a large number of new data sets covering the Phanerozoic beginning with the “Cambrian explosion” of multicellular plant and animal life, the evidence is in. As Professor Jackson Davis of the Environmental Studies Institute at Boulder and the University of California-Santa Cruz says: “I report here that proxies for temperature and atmospheric CO2 concentration are generally uncorrelated across the Phanerozoic climate, showing that atmospheric CO2 did not drive the ancient climate. The concentration of CO2 in the atmosphere is a less-direct measure of its effect on global temperature than marginal radiative forcing, however, which is nonetheless also generally uncorrelated with temperature across the Phanerozoic.” Scientist says.
You read that right. Over a 541 million year span, temperature and CO2 did not move together. The term “marginal radiative forcing” refers to the hypothesized mechanism connecting CO2 changes to temperature changes (T). (See our video on the Simple Physics slogan to find out more about the concept of CO2 forcing.) Professor Davis computed that measure too, and it likewise didn’t correlate with temperature. What does that mean? Either the data are wrong or the theory is. As he writes,
Correlation does not imply causality, but the absence of correlation proves conclusively the absence of causality. The finding that atmospheric CO2 concentration and [marginal radiative forcing] are generally uncorrelated with T, therefore, implies either that neither variable exerted significant causal influence on T during the Phanerozoic Eon or that the underlying proxy databases do not accurately reflect the variables evaluated.
He does caution that a positive correlation between T and marginal radiative forcing appears over the last 36 million years, which might mean CO2 has an effect when levels are very low. But before you get too excited, he adds that the correlations disappear if you look just at the last 26 million years, so it’s probably a spurious result.
His “The Relationship between Atmospheric Carbon Dioxide Concentration and Global Temperature for the Last 425 Million Years” is a lengthy and technical paper, citing 93 sources. But the author is clear that his results come down on the side of previous studies arguing CO2 and T are uncorrelated. “The present findings corroborate the earlier conclusion based on study of the Paleozoic climate that ‘global climate may be independent of variations in atmospheric carbon dioxide concentration.’” Scientists say.