We compare the statistical properties of solar, volcanic and combined forcings over the range from 1 to 1000 years to see over which scale ranges they additively combine, a prediction of linear response. The main findings are (a) that the variability in the Zebiac–Cane model and GCMs are too weak at centennial and longer scales; (b) for longer than ≈ 50 years, the forcings combine subadditively; and (c) at shorter scales, strong (intermittency, e.g. volcanic) forcings are nonlinear.