Evidence of anthropogenic impacts on global drought frequency, duration, and intensity

Droughts pose significant threats to ecological, agricultural, and economic systems, impacting wildlife habitats, crop production, and energy generation. Observed trends reveal increasing meteorological drought frequency, duration, and intensity in various regions globally. While previous research has attributed changes in mean and extreme temperature and precipitation to anthropogenic emissions, the global influence of anthropogenic forcing on diverse drought characteristics remains understudied. This work aims to quantify the impact of anthropogenic forcing on global meteorological drought characteristics using the Coupled Model Intercomparison Project Phase 6 (CMIP6) climate model simulations. Specifically, it examines the changes in drought frequency, duration, and intensity using standardized precipitation indices (SPI) and standardized precipitation-evapotranspiration indices (SPEI) over the 19th and 20th centuries, considering both historical and historical natural-only scenarios. By analyzing the individual contributions of greenhouse gases (GHGs) and anthropogenic aerosols (AERs), the study seeks to elucidate the regional variations in drought response and improve our understanding of historically observed trends and future projections.

Existing literature demonstrates the severe impacts of droughts across various sectors. Studies using observations from the Global Precipitation Climatology Center (GPCC) and the Climatic Research Unit (CRU) have indicated positive trends in drought characteristics in several regions. Previous detection and attribution studies have successfully linked increasing trends in mean and extreme temperature and precipitation to anthropogenic emissions. However, the global impact of anthropogenic forcing on various drought characteristics remains incompletely understood. While some studies have examined changes in drought types within specific regions (e.g., the United States) or utilized indirect indicators like soil moisture, a comprehensive global quantification of anthropogenic forcing on drought features such as duration, frequency, and severity has been lacking. Previous research has touched upon the complexity of this relationship, highlighting the roles of greenhouse gases, aerosols, and the interaction between them in influencing drought patterns.

This study employed monthly precipitation data from CMIP6 historical and historical natural-only simulations from nine models, each with three ensemble members. Data covered the period 1851-2005, and analyses focused on land areas between 60°N and 60°S. To analyze droughts, a non-parametric approach was used to create standardized indices, avoiding the computational challenges of parametric methods. Six-month SPI and SPEI values were calculated for each month, and drought events were defined using thresholds (SPI or SPEI < -1.5). The study quantified drought frequency, maximum duration, and maximum intensity, comparing historical and historical natural-only conditions. Probability ratios (PR) were calculated to assess the likelihood of droughts under historical conditions relative to natural-only conditions. Individual contributions of GHGs and AERs were analyzed separately to understand their distinct impacts. Additionally, bivariate distributions of drought duration and intensity were generated for IPCC regions using a 2-dimensional Kolmogorov–Smirnov test to assess whether the dependence between these drought characteristics differs between the historical and historical natural-only conditions. Spatial aggregated distributions were generated to summarize changes and probability density functions were used to compare the distributions.

The analysis reveals statistically significant hotspots of increased drought frequency, duration, and intensity in several regions under historical conditions compared to historical natural-only conditions. These hotspots include Southern Europe, Central and South America, western and southern Africa, and eastern Asia. Globally aggregated distributions showed stronger increases in drought characteristics under historical conditions compared to the historical natural-only conditions, indicating a significant influence of anthropogenic forcing. Probability ratio (PR) maps showed that regions such as southern and eastern Europe, northern and western Africa, and India experienced greater increases in drought likelihood under historical conditions than what is suggested by simple comparison of historical shifts. Analyzing GHG-only and AER-only forcings revealed that greenhouse gases significantly increased drought likelihood in southern Europe, northern and southern Africa, Central America, and parts of South America. Conversely, anthropogenic aerosols played a substantial role in South and East Asia, Central America, the Sahel region, and higher latitudes. Bivariate distributions of drought duration and median intensity showed significant changes in dependence structure between drought duration and intensity in most IPCC regions, indicating increased drought intensity levels compared to pre-industrial conditions. The inclusion of evapotranspiration in the SPEI analysis showed a further intensification of drought impacts, particularly in tropical and subtropical regions, driven primarily by the influence of greenhouse gases on potential evapotranspiration.

The findings address the research question by providing strong evidence that anthropogenic forcing has significantly impacted global drought characteristics. The observed increases in drought frequency, duration, and intensity are not solely attributable to natural climate variability, highlighting the significant role of human activities. The regional variations in drought response demonstrate the complex interplay between GHGs and AERs, indicating that the drying patterns in certain areas are largely determined by the regional balance of these forcings. The results align with current climate models' projections of changes in mean precipitation and underscore the persisting influence of greenhouse gases. The significant changes in bivariate distributions further emphasize the intensity of drought events in many regions. These results have significant implications for water resource management, agriculture, and disaster risk reduction.

This study provides compelling evidence of anthropogenic influence on global drought characteristics. The results highlight the substantial contribution of greenhouse gases to increased drought likelihood in many regions, while also showing the influence of aerosols in specific regions. The inclusion of evapotranspiration further emphasizes the impact of climate change on net water availability. Future research should explore the impacts on different drought types and investigate regional responses to varying forcings to develop effective adaptation strategies.

The study relies on CMIP6 model simulations, which have inherent limitations, including biases in tropical SSTs and the double-ITCZ issue. These biases can impact the accuracy of drought simulations, particularly in regions strongly influenced by ENSO. Additionally, the models might not fully capture aerosol feedbacks or vegetation responses, which can influence regional hydroclimatic conditions. The study acknowledges the potential impact of model uncertainties and biases on results. However, the focus on differences between model experiments and large temporal scales minimizes these effects.