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Lovejoy and Varotsos (2016) (L&V) analyse the temperature response to solar, volcanic, and solarplus volcanic forcing in the Zebiak–Cane (ZC) model, and to solar and solar plus volcanic forcing in the GoddardInstitute for Space Studies (GISS) E2-R model. By using a simple wavelet filtering technique they conclude thatthe responses in the ZC model combine subadditively on timescales from 50 to 1000 years. Nonlinear responseon shorter timescales is claimed by analysis of intermittencies in the forcing and the temperature signal forboth models. The analysis of additivity in the ZC model suffers from a confusing presentation of results basedon an invalid approximation, and from ignoring the effect of internal variability. We present tests without thisapproximation which are not able to detect nonlinearity in the response, even without accounting for internalvariability. We also demonstrate that internal variability will appear as subadditivity if it is not accounted for.L&V’s analysis of intermittencies is based on a mathematical result stating that the intermittencies of forcingand response are the same if the response is linear.We argue that there are at least three different factors that mayinvalidate the application of this result for these data. It is valid only for a power-law response function; it assumespower-law scaling of structure functions of forcing as well as temperature signal; and the internal variability,which is strong at least on the short timescales, will exert an influence on temperature intermittence whichis independent of the forcing. We demonstrate by a synthetic example that the differences in intermittenciesobserved by L&V easily can be accounted for by these effects under the assumption of a linear response. Ourconclusion is that the analysis performed by L&V does not present valid evidence for a detectable nonlinearresponse in the global temperature in these climate models

earth system dynamics

Comment on "Scaling regimes and linear/nonlinear responses of last millenniumclimate to volcanic and solar forcing" by S. Lovejoy and C. Varotsos (2016)