Global droughts in a warming climate: Evaluation of SPI and SPEI under 1.5 , 2 , and 3 C global warming

Abstract

• Meteorological droughts compared using observations and CMIP6 GCMs. • GCMs show high bias in regions with strong historic drought characteristics. • SPEI with evapotranspiration shows larger increases than SPI in future scenarios. • Future droughts show larger increases in arid regions globally. Accurately predicting droughts, amidst rapid global warming, is crucial for effective resource management and policy development. Here, we employ CMIP6 models to analyze historical and future global meteorological drought patterns using the Standardized Precipitation Index (SPI) and Standardized Precipitation and Evapotranspiration Index (SPEI). We first assess model efficacy against historical observations (1951–2014) and then project changes at key IPCC warming thresholds. Our evaluation shows that even though CMIP6 models accurately capture global average drought characteristics, significant biases emerge in regions that commonly experience extreme historical drought conditions. Projections indicate increasing drought duration, frequency, and severity, particularly under higher warming scenarios. A more pronounced increase is noted with SPEI compared to SPI: under a 3 °C warming, drought severity is projected to increase across 91.8 % of land grids, based on SPEI, as opposed to 28.2 % for SPI. Similarly, increases in drought frequency are projected over 86.5 % (SPEI) versus 33.4 % (SPI), and increases in duration in 74.9 % (SPEI) versus 12.9 % (SPI) of land grids. This discrepancy highlights the importance of including temperature, through evapotranspiration, in drought assessments, especially because of the expected increase in evaporative demand driven by rising temperature. Our analysis further demonstrates that whereas humid and transitional areas may experience reduced drought severity and duration as per SPI, SPEI predicts a significant worsening of drought events in the future. On the other hand, both indicators project a worsening of drought conditions in arid regions—changes in SPI are small but that with SPEI are in the order of 2–3 months in duration, 3.5–4 events in frequency, and 4.3–5.2 units in severity. This study underscores the complexity of future drought patterns, which necessitates a range of strategies for mitigation and adaptation to safeguard against escalating drought risks in a warming world.