 Lastly, when it comes to evaluating the adequacy of climate models for some purposes, especially predictive purposes, it can be difficult to know what sort of fit with past climate conditions would even count as evidence of a climate model’s adequacy-for-purpose (Parker 2009). How accurately should a climate model simulate various features of past climate if it is adequate for, say, predicting future changes in global mean surface temperature to within some specified margin of error? This can be very difficult to answer, not only because there is more than one way to achieve such predictive accuracy (e.g., by distributing error differently across contributing variables), but also because simplifications and idealizations in the model, as well as tuned components, might work better under some external conditions than others, e.g., under those of the recent past rather than those assumed in future scenarios (see ibid.). Katzav (2014) suggests that, in practice, attempts to determine what sort of fit with past conditions would count as evidence of a climate model’s adequacy-for-purpose will often rely on results from climate models themselves, in a way that is question-begging.