SummaryDrought is a natural hazard that can have severe and long-lasting impacts on natural and human systems. Although increases in global greenhouse forcing are expected to change the characteristics and impacts of drought in the 21st century, there remains persistent uncertainty about how changes in temperature, precipitation and soil moisture will interact to shape the magnitude – and in some cases direction – of drought in different areas of the globe. Using data from 15 global climate models archived in the Coupled Model Intercomparison Project (CMIP5), we assess the likelihood of changes in the spatial extent, duration and number of occurrences of four drought indices: the Standardized Precipitation Index (SPI), the Standardized Runoff Index (SRI), the Standardized Precipitation–Evapotranspiration Index (SPEI) and the Supply–Demand Drought Index (SDDI). We compare these characteristics in two future periods (2010–2054 and 2055–2099) of the Representative Concentration Pathway 8.5 (RCP8.5). We find increases from the baseline period (1961–2005) in the spatial extent, duration and occurrence of “exceptional” drought in subtropical and tropical regions, with many regions showing an increase in both the occurrence and duration. There is strong agreement on the sign of these changes among the individual climate models, although some regions do exhibit substantial uncertainty in the magnitude of change. The changes in SPEI and SDDI characteristics are stronger than the changes in SPI and SRI due to the greater influence of temperature changes in the SPEI and SDDI indices. In particular, we see a robust permanent emergence of the spatial extent of SDDI from the baseline variability in West, East and Saharan Africa as early as 2020 and by 2080 in several other subtropical and tropical regions. The increasing likelihood of exceptional drought identified in our results suggests increasing risk of drought-related stresses for natural and human systems should greenhouse gas concentrations continue along their current trajectory

The effect of global climate change on infectious disease remains hotly debated because multiple extrinsic and intrinsic drivers interact to influence transmission dynamics in nonlinear ways. The dominant drivers of widespread pathogens, like West Nile virus, can be challenging to identify due to regional variability in vector and host ecology, with past studies producing disparate findings. Here, we used analyses at national and state scales to examine a suite of climatic and intrinsic drivers of continental-scale West Nile virus epidemics, including an empirically derived mechanistic relationship between temperature and transmission potential that accounts for spatial variability in vectors. We found that drought was the primary climatic driver of increased West Nile virus epidemics, rather than within-season or winter temperatures, or precipitation independently. Local-scale data from one region suggested drought increased epidemics via changes in mosquito infection prevalence rather than mosquito abundance. In addition, human acquired immunity following regional epidemics limited subsequent transmission in many states. We show that over the next 30 years, increased drought severity from climate change could triple West Nile virus cases, but only in regions with low human immunity. These results illustrate how changes in drought severity can alter the transmission dynamics of vector-borne diseases.