One Element to Rule them all
Current climate models fed on emissions and more recently green house gas concentrations depict temperature trends with a far greater certainty than any other climate variable. Regardless of one’s view on the causes of global warming, this effect is foreseen at a more or less uniform way around the planet. On the other hand, future changes in precipitation distribution and thus runoff (also affected by snowmelt) is expected to be uneven with a significant inherent uncertainty in frequency and intensity. This uncertainty barrier may also be one of the reasons that temperature is seen as the overall driver of change, including that of the global water cycle.
According to a new article by Rach et al. (2014), the water cycle is a major driver of controlling other climate variables including temperature, thus being a staple to climate change. Evidence for this observation was found in organic remains extracted from western Germany lake sediments that act as proxy rain gauges for paleoclimate research.