Multi-type global drought projection using multi-model hydrological simulations

A multi-drought study that covers several draught types is required to better understand future drought. It is anticipated that drought will be exacerbated under climate change due to altered precipitation patterns and/or increased evapotranspiration. However, IPCC AR5 and SREX report stated with barely medium confidence that drought is expected to intensify over several regions in the world by the end of the 21st century, while elsewhere there is overall low confidence.

One of the reasons for these confidence levels stems from a definitional issue. As drought is a complex phenomenon and involves several processes, there are multiple hydrological variables and relevant-indicators used to quantify drought. Nonetheless, very few studies have comprehensively discussed future drought considering several drought types within a single study, hence leaving a gap on the holistic picture of future drought. Besides, most studies referred to in AR5 and SREX are based on coarse general circulation model (GCM) or regional climate model projections which have inherent model biases. Also, scenario uncertainties need to be examined more on drought projections, using the latest greenhouse gas emission scenarios.

This study presents a comprehensive multi-drought-type assessment on a global-scale until 2099. Using a set of multiple state-of-art global hydrological model (GHM) simulations forced by four bias-corrected GCM projections, meteorological (precipitation), agricultural (soil moisture) and hydrological (runoff) droughts are investigated by using the Standardized method at monthly-scale and another hydrological drought (discharge) by using a variable threshold method. The multi-model data set, which was developed in the Inter-Sectoral Impact Model Inter-comparison Project phase2b under a consistent simulation protocol, provides finer and detailed hydrological simulations at 0.5°x0.5° resolution. To explore potential pathways of drought changes, this study examined the Representative Concentration Pathways (RCP) 2.6, 6.0 and 8.5 scenarios. For each case, four drought features; drought intensity, spatial extent, the number of events, dry spell length, were studied, compared to those of the period before the 1960s.

The results highlight the hotspots of future droughts and show the development of each drought type for each RCP scenarios. As well as consistencies, differences among drought types were found in change trends and drought features. For instance, meteorological drought will decrease in some parts of middle-latitude in the northern hemisphere but the other two drought types will increase due to an increase in evapotranspiration over the regions. Or, dry spell length tends to be longer in runoff > soil moisture > precipitation drought in this order. These differences indicate that it is crucial to clearly define drought in discussing the phenomenon and it is critical to properly select drought types and index for one’s interest. Also, differences among RCP scenarios pose a question for mitigation discussions from the viewpoint of drought. Two types of uncertainties in this projection concerning model (GHMs and GCMs) uncertainty and parameter uncertainty in the drought analysis methods are also presented along with the drought projections.