There has been a good deal of talk lately, much of it set off by a recent article in the Economist, about the fact that global temperatures have now been nearly constant for over a decade despite large increases in atmospheric CO2. Looking at the situation as a non-expert observer, it seems to me there are three possible explanations both for the recent pause and for the earlier pause from about 1940 to 1970.
1. The theory is wrong. Warming over the past century+ is due to something other than the greenhouse effect of CO2 produced by human action.
This is possible but does not strike me as very likely.
2. The theory and models are right. Anthropogenic CO2 produces warming at about the rate implied by the IPCC models. There are, however, other factors that also affect global temperature. Recent periods of stable or falling temperature occurred because those other factors were pushing temperature down about as fast as the greenhouse effect was pushing it up.
3. The theory is right but the models are wrong—they substantially overestimate the sensitivity of temperature to CO2.
This is, I think, a more plausible variant of the previous explanation because of two of the latter's implications. The first is that the other factors are about as important, at least in the short run, as CO2, since otherwise they could not have cancelled its effect. The second is that the other factors were not adequately included in the IPCC models, since if they had been those models would not have seriously over predicted global temperatures, and they did.
As best I understand the relevant theory, sensitivity is an open question—we do not have climate models good enough to calculate it with confidence, so rely instead on deducing it from statistical data. But if global temperature depends on both CO2 and other things and the models left out or underestimated those other things, then calculations when the non-CO2 effect was pushing temperature up instead of down would tend to overestimate the CO2 effect. That seems to me consistent with the fact that temperature change over the past century is significantly positive, but at a rate much lower than what the IPCC models imply for the rest of this century. And, of course, it would explain why global temperatures for the past decade are lower than would be expected from the IPCC models and the increase in atmospheric CO2.