Atmospheric CO2 doesn’t drive temperature, and temperature doesn’t drive CO2. They drive each other, in a feedback loop. Each relationship involves integration – CO2 accumulates temperature changes through mechanisms like forest growth and ocean uptake, and temperature is the accumulation of heat flux controlled by the radiative effects of CO2.
This has been obvious for at least decades, yet it still eludes many. A favorite counter-argument for an influence of CO2 on temperature has long been the observation that temperature appears to lead CO2 at turning points in the ice core record. Naively, this violates the requirement for establishing causality, that cause must precede effect. But climate is not a simple system with binary states and events, discrete time and single causes. In a feedback system, the fact that X lags Y by some discernible amount doesn’t rule out an influence of Y on X; in fact such bidirectional causality is essential for simple oscillators.
A newish paper by Shakun et al. sheds some light on the issue of ice age turning points. It turns out that much of the issue is a matter of data – that ice core records are not representative of global temperatures. But it still appears that CO2 is not the triggering mechanism for deglaciation. The authors speculate that the trigger is northern hemisphere temperatures, presumably driven by orbital insolation changes, followed by changes in ocean circulation. Then CO2 kicks in as amplifier. Simulation backs this up, though it appears to me from figure 3 that models capture the qualitative dynamics, but underpredict the total variance in temperature over the period. To me, this is an interesting step toward a more complete understanding of ice age terminations, but I’ll wait for a few more papers before accepting declarations of victory on the topic.
Predictably, climate skeptics hate this paper. For example, consider Master Tricksed Us! at WattsUpWithThat. Commenters positively drool over the implication that Shakun et al. “hid the incline” by declining to show the last 6000 years for proxy temperature/CO2 relationship.
I leave the readers to consider the fact that for most of the Holocene, eight
millennia or so, half a dozen different ice core records say that CO2 levels were rising pretty fast by geological standards … and despite that, the temperatures have been dropping over the last eight millennia …
But not so fast. First, there’s no skepticism about the data. Perhaps Shakun et al. omitted the last 6k years for a good reason, like homogeneity. A spot check indicates that there might be issues – series MD95-2037 ends in the year 6838 BP, for example. So, perhaps the WUWT graph merely shows spatial selection bias in the dataset. Second, the implication that rising CO2 and falling temperatures somehow disproves a CO2->temperature link is yet another failure to appreciate bathtub dynamics and multivariate causality.
This credulous fawning over the slightest hint of a crack in mainstream theory strikes me as the opposite of skepticism. The essence of a skeptical attitude, I think, is to avoid early lock-in to any one pet theory or data stream. Winning theories emerge from testing lots of theories against lots of constraints. That requires continual questioning of models and data, but also questioning of the questions. Objections that violate physics like accumulation, or heaps of mutually exclusive objections, have to be discarded like any other failed theory. The process should involve more than fools asking more questions than a wise man can answer. At the end of the day, “no theory is possible” is itself a theory that implies null predictions that can be falsified like any other, if it’s been stated explicitly enough.