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To the Editor —

Fyfe et al.¹ showed that global warming over the past 20 years is significantly less than that calculated from 117 simulations of the climate by 37 models participating in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). This might be due to some combination of errors in external forcing, model response and simulated internal variability¹. Meanwhile, Kosaka and Xie² used an earlier-generation climate model to show that such a difference is substantially reduced if eastern tropical Pacific sea surface temperatures are prescribed to follow the observed rather than the simulated evolution. Kosaka and Xie² concluded, therefore, that “accounting for recent cooling in the eastern tropical Pacific reconciles climate simulations and observations”. It is in this light that we revisit the findings of Fyfe and colleagues¹.

Figure 1 shows observed³ (red) and simulated (black) trends over the past 20 years (1993–2012) in global mean surface temperature plotted against corresponding trends in eastern tropical Pacific sea surface temperature. As pointed out by Fyfe and colleagues¹, the observed rate of global warming over this period is less than that simulated in all but two of 117 CMIP5 simulations. Figure 1 shows an even more pronounced discrepancy over the eastern tropical Pacific, with the observed cooling trend being substantially more negative than that in any of the 117 CMIP5 simulations. The observations in Fig. 1 lie on the straight line that best fits the simulated global and eastern tropical Pacific temperature trends over the period from 1993 to 2012 — indicating that the observed global mean trend could be inferred from the observed tropical Pacific trend and the relationship between these two variables in the models.

Observed trends (red) are averages over 100 reconstructions of the HadCRUT4 dataset³. Simulated trends (black) are from 117 realizations of the climate from 37 CMIP5 models and their 5–95% ranges are shown with the black ellipse. The straight line is the best fit to the simulated global mean and eastern tropical Pacific trends, with a correlation of 0.63. As in Kosaka and Xie² the eastern tropical Pacific is defined as the region east of the dateline and between 20° S and 20° N, and as in Fyfe et al.¹ the simulations are sampled only where corresponding observations exist.