Inferring and Explaining

110 t 0 . High CO 2 concentrations cause global temperature variations. InferrIng and exPlaInIng Given that the correlation is real and not simply a fuke or coincidence—for the modern social scientist, it is statistically signifcant—we must now determine whether t 0 is the best explana- tion of the correlation. We must compare it to some rival explanations. Perhaps, as some skep- tics have claimed, the direction of causation is reversed: t 1 . Global temperature variations cause vary- ing CO 2 concentrations. Tis rival is probably a better account of the historical data because many believe that we see the changes in temperatures before we see changes in CO 2 level in the historical record. In addition, before the advent of the Industrial Revolution, it was hard to see what else could initiate such large-scale changes in the CO 2 concentrations. It is likely that the temperature variations . . . drove the CO 2 variations, not the reverse. Tat might have occurred, for example, when warmer temperatures increased the rate of bacterial breakdown of plant material, releasing CO 2 to the atmosphere as it warmed. Tis historical relationship does not, how- ever, refute the modern relationship of human addi- tions of CO 2 to the atmosphere driving increases in temperature. 3 Why, youmay ask, doesn’t the “reverse cause” rival, t 1 , refute the anthropogenic hypoth- esis? Here comes the surprise possible causal relationship between two things, A and B , that I promised earlier. [One] potential explanation for the observedwarming of the Earth is human activity. Tere are several rea- sons to think that this can account for some portion of the observed warming. We know that human activi- ties have been increasing the concentration of CO 2 and other greenhouse gases in the atmosphere for at least the past century or two. Measurements show the concentration of CO 2 has increased about 30 percent over that time . . . while other greenhouse gases have increased by similar or larger amounts. Basic phys- ics provides strong theoretical reasons to believe that such an increase in greenhouse gases should warm the Earth. 4 It now seems likely that the best explanation of the correlation is that the causal relationship between CO 2 and global warming actually points in both directions; increased CO 2 concentrations cause increased temperatures, and simultane- ously, increased temperatures cause increases in CO 2 concentrations. We probably have a kind of feedback loop . t 2 . Increased CO 2 concentrations cause increased temperatures, while increased temperatures cause increases in CO 2 concentrations. In a way, of course, t 2 does not really contra- dict Gore’s original causal hypothesis in t 0 ; it merely ofers more detail about the complicated causal relationship between CO 2 and global tem- peratures. So in the sense that we are using the term in the inference-to-the-best-explanation (IBE) recipe, t 2 does not even count as a rival