The Dyson Critique (the Lucas Critique for the macro-climate) says that climate models that assume constant feedback effects with the increase of CO2 are unreliable predictors of climate change. Here’s another great discussion of the issue:
If that is not enough, changes in CO2 in the real world would almost certainly be associated with other changes in the atmosphere – sulfur dioxide, mineral aerosols (dust), ozone, black carbon, and who knows what else would vary through time and complicate the “all else held constant” picture. By the way, the Sun varies its output as well. And when discussing climate change over the next century, even more uncertainties come from estimations of economic growth, adoption of various energy alternatives, human population growth, land use changes, and … you get the message.
He goes on to say there have been recent estimates of, what economists would call, the reduced form parameter — the sensitivity of the atmosphere in terms of temperature increases to increasing CO2 levels. Basically, by looking at levels of atmospheric dust, and their correlation with CO2 levels, during previous warming periods, scientist were able to estimate the reduced form parameter. He reports that their conclusion is that climate sensitivity is lower than previous estimates which means we should expect less warming than previously estimated.
There is no discussion, however, of what economists call the structural parameters. Why is there a connection between CO2 levels and dust levels, for example? If there’s a third force that is driving both CO2 levels and dust levels, then its not clear the new estimates of sensitivity are applicable to the climate today. My understanding is that in the past, CO2 wasn’t driving climate change, climate change was driving CO2 levels and CO2 was acting as a positive feedback. The earth warmed, the ocean’s emitted their stores of CO2 causing further warming. Given the climate didn’t become unstable, I assume there were also negative feedbacks that brought the system back into equilibrium.
Its not clear though that those negative feedbacks had anything to do with CO2 levels. The pertinent question then is: can historical data tell us anything about the climate feedbacks from increasing CO2 levels today?