We examine the sensitivity of the biogeography of nitrogen fixers to a warming climate and increased aeo-lian iron deposition in the context of a global earth system model. We employ concepts from the resource-ratio theory to provide a simplifying and transparent interpretation of the results. First we demonstrate that a set of clearly defined, eas-ily diagnosed provinces are consistent with the theory. Us-ing this framework we show that the regions most vulnera-ble to province shifts and changes in diazotroph biogeogra-phy are the equatorial and South Pacific, and central Atlantic. Warmer and dustier climates favor diazotrophs due to an in-crease in the ratio of supply rate of iron to fixed nitrogen. We suggest that the emergent provinces could be a standard diagnostic for global change models, allowing for rapid and transparent interpretation and comparison of model predic-tions and the underlying mechanisms. The analysis suggests that monitoring of real world province boundaries, indicated by transitions in surface nutrient concentrations, would pro-vide a clear and easily interpreted indicator of ongoing global change.