Many models predict that summer rainfall will decline over northern mid-latitude continental regions due to anthropogenic forcing. However, this drying is less robust over North America than over Europe, and the ensemble mean drying is less severe. This study aims to better understand the reasons for this important regional difference, and utilises data from both the IPCC-AR4 multi-model ensemble and the Met Office Hadley Centre ‘perturbed physics' ensemble in which modelling uncertainties are sampled by perturbing uncertain physical parameters. It is shown that the vast majority of individual models also exhibit a weaker summer rainfall decline over North America. It appears that one or both of the two dominant mechanisms of future mid-latitude continental summer drying acts more weakly over North America than over Europe. These mechanisms are: (a) an earlier and more rapid decline of soil moisture during spring, and (b) reduced continental relative humidity due to the enhanced land–sea contrast in surface warming. These mechanisms are compared between the two continents, using data from the perturbed physics ensemble, which consists of 66 different versions of the HadCM3 model. In particular, this leads to a subjective judgement of the reliability of the predicted differences between the two continents. Conclusions differ between the mountainous western part of North America and the tropically-influenced eastern part.