evidence: As noted by Meehl et al. (2003), solar was likely the dominant forcing that produced the temperature increases in the first part of the twentieth century. Thus, in PC1 and PC2, explaining 90% and 8%, respectively, of the variance of the original time series, the elements of the two major features of the globally averaged surface air temperature over the twentieth century are represented: the early century warming in the model associated mainly with solar forcing, and the late-century warming produced mainly by increased GHGs in the model. PC3, representing only 1% of the variance, shows low-amplitude variations that have elements of both the early century and late-century warming and cannot be associated directly with any single forcing time series in Fig. 1. Had the PC analysis shown a gradual decay of variance explained among the 13 PCs with no obvious cutoff, we would not advocate this approach over a simple mean standard error computation between original forcings and residuals. But the obvious dominance of just three components seems to us to justify this simple additional step, and adds insight to the analysis of linearity. [...] Meehl, G. A., W. M. Washington, T. M. L. Wigley, J. M. Arblaster, and A. Dai, 2003: Solar and greenhouse gas forcing and climate response in the twentieth century. J. Climate, 16 , 426–444.