Climate over decadal timescales, forced by added greenhouse gases, could either change independently of internally generated variability or interact with it. For hypothesis 1, the atmosphere warms gradually, affected by random climate noise. For hypothesis 2, warming is nonlinear and step-like. Two statistical models, step and trend, are used to analyse observed and modelled temperatures; the results are subject to six tests. In conclusion, externally forced warming is step-like at these scales.