Aridity and Drought in a Non-stationary Climate

Abstract

Both drought and aridity indicate an imbalance in water availability. While drought is a natural temporal hazard, aridity is a constant climatic feature. The purpose of this thesis is to reveal how global warming affects drought and aridity in order to find a proper method to estimate aridity and drought under a changing climate. First, past probable changes in aridity are investigated using published literature, trend analysis, and clustering. The results suggest that arid zones are becoming slightly more humid and vice versa. The trend in aridity changed, or even reversed, around 1980 in most parts of the world. We speculate that the reason for this is the dramatic change (rise) in global temperature around 1980 which, in turn, caused similar trends for Potential Evapotranspiration (PET). Possible changes in the future aridity are then analysed by employing bias-corrected simulations of 22 global climate models. The results suggest that climate change will alter the expansion of aridity zones in the future which is mostly due to positive PET trends, albeit the slight increase in precipitation will lessen that. Next, to verify the importance of PET as a means to better assess drought in a warming climate, as observed for aridity, drought trends in different aridity zones are surveyed with and without PET consideration. Given the known changes to PET in observed records, and expected increases as global warming intensifies, the results suggest that in the face of climate change, PET should not be ignored in drought modelling. Finally, we show that while precipitation-based drought indexes can underestimate future drought, PET-based indexes overestimate drought, as they assume greater evaporation than actually occurs. We introduce the Actual evapotranspiration Drought Index (ADI) that uses precipitation and actual evapotranspiration to quantify drought. ADI offers a more realistic representation of drought occurrence than other measures, and assumes special importance in a warming climate, given its consideration of the changed evaporation profile a warmer future will bring. In conclusion, the thesis presents a new framework to appropriately assess droughts in a non-stationary climate. The basic concepts of the approach can also be used for aridity assessments.