Contributions to uncertainty in projections of future drought under climate change scenarios

Abstract. Drought is a cumulative event, often difficult to define and involving wide reaching consequences for agriculture, ecosystems, water availability, and society. Understanding how the occurrence of drought may change in the future and which sources of uncertainty are dominant can inform appropriate decisions to guide drought impacts assessments. Uncertainties in future projections of drought arise from several sources and our aim is to understand how these sources of uncertainty contribute to future projections of drought. We consider four sources of uncertainty; climate model uncertainty associated with future climate projections, future emissions of greenhouse gases (future scenario uncertainty), type of drought (drought index uncertainty) and drought event definition (threshold uncertainty). Three drought indices (the Standardised Precipitation Index (SPI), Soil Moisture Anomaly (SMA) and Palmer Drought Severity Index (PDSI)) are calculated for the A1B and RCP2.6 future emissions scenarios using monthly model output from a 57 member perturbed parameter ensemble of climate simulations of the HadCM3C Earth system model, for the baseline period, 1961–1990, and the period 2070–2099 (representing the 2080s). We consider where there are significant increases or decreases in the proportion of time spent in drought in the 2080s compared to the baseline and compare the effects from the four sources of uncertainty. Our results suggest that, of the included uncertainty sources, choice of drought index is the most important factor influencing uncertainty in future projections of drought (60%–85% of total included uncertainty). There is a greater range of uncertainty between drought indices than that between the mitigation scenario RCP2.6 and the A1B emissions scenario (5%–6% in the 2050s to 17%–18% in the 2080s) and across the different model variants in the ensemble (9%–17%). Choice of drought threshold has the least influence on uncertainty in future drought projections (0.4%–7%). Despite the large range of uncertainty in drought projections for many regions, projections for some regions have a clear signal, with uncertainty associated with the magnitude of change rather than direction. For instance, a significant increase in time spent in drought is consistently projected for the Amazon, Central America and South Africa whilst projections for Northern India consistently show significant decreases in time spent in drought. We conclude that choice of which drought index (or drought indices) to use when undertaking drought impacts assessments is of considerable importance relative to choices relating to the other three included sources of uncertainty in this study. This information will help ensure that future drought impacts assessments are designed appropriately to account for uncertainty.