evidence: In the late-20th-century warming period (1970–2005) the observed trend is consistent with the BEST6 ensemble with the BEST6 trends dominated by GHG warming, as is also true of the WRST6 ensemble. In contrast to an earlier study7 using past generation climate models together with an empirical fit to global temperatures we find no evidence in the CMIP5 multi-model ensemble that changes in aerosols since the 1970s (here part of OTH) have contributed to recent Arctic warming. [...] Finally, using this set of current climate models run without natural and anthropogenic forcings it is possible to estimate the probability that the observed variation in Arctic-average surface temperature was due to internal climate variability alone. To accomplish this we have taken all available 106-year long independent segments from 17 of the above climate models with available control (i.e. externally-unforced) simulations. In total this yielded 92 such segments which were each masked with the observational coverage before computing piecewise linear approximations as above. Figure 6 shows that only one such segment, or realization, has early-20th-century warming as large as observed and that only two realizations have middle-20th-century cooling as large as observed – no realizations have late-20th-century warming as large as observed. The evidence, therefore, suggests that internal climate variability cannot account for the observed trends and this is supported by statistical tests of the null hypothesis that observed trends are consistent with zero29. Differences between observed and control–average trends in the early-20th-century and middle-20th-century periods have p-values equal to 0.01 and 0.02, respectively and near zero for the late-20th-century period (assuming the models are exchangeable with each other29).