Climate change and Australian wheat yield

Nicholls replies — Godden et al. state that I proxied non-climate factors “by a linear trend in yields”. I did not and my conclusions were not dependent on the functional form of any relationship between non-climate factors and yield. They suggest that forcing the intercept to zero and fitting with ordinary least-squares biases my estimates. I repeated the analysis without forcing a zero intercept, with no significant effect on the results. The climate-yield change relationship is so strong (see Fig. 2 of Ref. 1) that other forms of fitting produce a result very similar to ordinary least-squares. This relationship dominates the effects of other variables and provides a way to estimate the effect of climate on yield change. Godden et al. believe that detrending with first differences is inappropriate. As I noted, repeating the analysis on residuals from fitted non-linear trends produces very similar results.