As noted earlier, this chapter focuses only on those variables important in determining the transient response of climate models (see Section 8.6). Due to space limitations, much of the analysis performed for this section is found in the Supplementary Material (Figures S8.12 to S8.15). An assessment of the modes of natural, internally generated variability can be found in Section 8.4. Comparisons of the type performed here need to be made with an appreciation of the uncertainties in the historical estimates of radiative forcing and various sampling issues in the observations (see Chapters 2 and 5). Unless otherwise noted, all results discussed here are based on the MMD at PCMDI.