Sensitivity of water scarcity events to ENSO driven climate variability at the global scale

Globally, freshwater shortage is one of the most important risks for society. Changing hydro-climatic and socioeconomic conditions have aggravated water scarcity over the past decades. A wide range of studies show that water scarcity will intensify in the future, as a result of both increased consumptive water use and in some regions climate change However, less attention has been paid to the impacts of climate variability on water scarcity, despite its importance for adaptation planning. Therefore, we present the first global scale sensitivity assessment of water scarcity and water availability to El Niño-Southern Oscillation (ENSO), the most dominant signal of climate variability. We show that over the time period 1961-2010, both water availability and water scarcity conditions are significantly correlated with ENSO-driven climate variability over a large proportion of the global land area (> 28.1%); an area inhabited by more than 31.4% of the global population. We also found, however, that climate variability alone is often not enough to trigger the actual incidence of water scarcity events. The sensitivity of a region to water scarcity events, expressed in terms of land area or population impacted, is determined by both hydro-climatic and socioeconomic conditions. Currently, the population actually impacted by water scarcity events consists of 39.6% (water stress) and 41.1% (water shortage) of the global population whilst only 11.4% (water stress) and 15.9% (water shortage) of the global population is at the same time living in areas sensitive to ENSO driven climate variability. These results are contrasted however by differences in found growth rates under changing socioeconomic conditions, which are relatively high in regions affected by water scarcity events. Given the correlations found between ENSO and both water availability and water scarcity, and the relative developments of water scarcity impacts under changing socioeconomic conditions, we suggest that there is potential for ENSO-based adaptation and risk reduction which could be facilitated by more research on this emerging topic.