Droughts, periods of significantly below-average moisture, are major climatic impact drivers affecting society and ecosystems. Their future variations under climate change are crucial to study. Droughts stem from precipitation deficiency, impacting the water cycle and influenced by factors like vegetation and land management. Classifications include meteorological, agricultural, hydrological, and socio-economic droughts. Due to difficulties in quantifying these, indices based on atmospheric variables have been developed, such as the Standardized Precipitation Index (SPI) and the Standardized Precipitation-Evapotranspiration Index (SPEI). SPI uses only precipitation, while SPEI incorporates temperature, providing a more comprehensive assessment of drought conditions. Aridity, on the other hand, measures long-term water deficits by comparing average precipitation to potential evapotranspiration (PET), often using the aridity index. The Aridity Change Index (ACI) compares aridity in different periods. Previous studies using observational data and global climate models (GCMs) have shown global increases in arid areas and drought frequency/severity, particularly in regions like the Mediterranean. This study focuses on the Canary Islands, a mountainous region where elevation influences climate change impacts. Previous research indicates a more pronounced temperature increase and precipitation decrease at higher elevations in the islands. This study utilizes the Weather Research and Forecasting (WRF) regional climate model to downscale data from three CMIP5 GCMs (GFDL-ESM2M, IPSL-CM5A-MR, MIROC-ESM) under RCP4.5 and RCP8.5 scenarios to analyze drought and aridity at various altitudes (0–400 m, 400–1100 m, 1100–2100 m, >2100 m).

Existing literature highlights the global increase in arid lands and drought frequency and severity, especially in regions such as the Mediterranean, Southern South America, and Southern Africa. Studies utilizing SPI and SPEI indices from observational datasets consistently reveal a robust increase in drought events between 1951-1980 and 1981-2016 in several regions. Projections using GCMs indicate further expansion of drylands by the end of the 21st century under high emission scenarios (RCP8.5), particularly in North America, the Mediterranean, Southern Africa, and parts of South America. Studies concerning small islands, particularly vulnerable to climate change, suggest increased aridity in many regions including the Canary Islands. Previous research on the Canary Islands notes observed elevation-dependent warming and projected precipitation decline, particularly at higher elevations.

The study used the Weather Research and Forecasting (WRF) model with a 3x3 km resolution to downscale data from three CMIP5 Global Climate Models (GCMs): GFDL-ESM2M, IPSL-CM5A-MR, and MIROC-ESM. Simulations were conducted for the recent past (1980-2009) and the end of the 21st century (2070-2099) under RCP4.5 and RCP8.5 emission scenarios. The Standardized Precipitation Index (SPI) and Standardized Precipitation-Evapotranspiration Index (SPEI) were calculated at 3-month and 12-month time scales to assess meteorological and hydrological droughts, respectively. The Aridity Change Index (ACI) was also computed to evaluate changes in aridity. The Penman-Monteith method was used for potential evapotranspiration (PET) calculations. A bias correction method (Scaled Distribution Mapping or SDM) was applied using data from a previous WRF simulation driven by ERA-Interim data as a reference. The four elevation intervals (0–400 m, 400–1100 m, 1100–2100 m, >2100 m) were analyzed. Robustness tests were performed using a bootstrapping technique to assess the statistical significance of the projected changes. Model results were validated against observational data from ECA&D for precipitation and PET at three stations representing different elevations on Tenerife.

The WRF simulations successfully reproduced the annual cycle of precipitation and evapotranspiration in the recent past, although some differences were observed, particularly at coastal stations. Future projections revealed a significant decrease in 12-month accumulated precipitation at higher elevations, with the most drastic decreases seen at elevations above 2100 meters. Cumulative distribution functions showed a shift towards drier conditions in the future, with precipitation levels associated with extreme drought in the recent past becoming typical under the high emission scenario (RCP8.5). Both SPI and SPEI indices indicated worsening drought conditions with altitude, with the SPEI index showing a stronger signal due to the inclusion of evapotranspiration. The most severe drought conditions were projected for autumn, when evapotranspiration is high and precipitation is low. The percentage of the island area affected by drought was projected to increase substantially, particularly at higher elevations, with the increase being more pronounced under the RCP8.5 scenario. The Aridity Change Index (ACI) showed a clear increase in aridity, particularly at higher altitudes, with the highest increases seen in the RCP8.5 scenario. Spatial maps revealed that drought frequency may decrease in some areas but that duration and severity would increase significantly. The inclusion of evapotranspiration in the drought indices proved crucial for accurately assessing drought conditions in the Canary Islands, as using SPI alone could mask the severity of projected droughts.

The findings confirm that climate change will significantly exacerbate drought conditions in the Canary Islands, with the impact being particularly pronounced at higher elevations. The inclusion of evapotranspiration is vital for accurate drought assessment in semi-arid regions, as demonstrated by the differences between SPI and SPEI projections. The results highlight the importance of high-resolution climate projections for adequately capturing the complex topographic influences on drought patterns in mountainous regions. The projected increases in drought duration, severity, and spatial extent pose significant challenges for water resource management, agriculture, tourism, and ecosystem health in the Canary Islands. The increased aridity, especially at higher elevations, will likely impact vulnerable ecosystems and may exacerbate the risk of wildfires.

This study provides high-resolution climate projections of drought and aridity for the Canary Islands, demonstrating a substantial increase in drought severity and extent, particularly at higher elevations. The inclusion of evapotranspiration in drought indices is crucial for accurate assessment. The results emphasize the need for adaptive strategies to mitigate the impacts of intensified droughts, considering the specific vulnerabilities of different sectors and ecosystems. Future research could investigate the impact of climate change on fog formation and its contribution to water resources at different elevations, further refine water demand projections, and explore specific adaptation measures tailored to the diverse topographic settings of the Canary Islands.

The study's limitations include the use of only three GCMs and the inherent uncertainties associated with climate model projections. The spatial resolution of the WRF model, while higher than that of GCMs, may still not capture all the microclimatic variations within the complex terrain of the Canary Islands. The study focuses solely on meteorological and hydrological droughts and does not consider socio-economic impacts directly. Furthermore, the study does not account for potential changes in human water management strategies, which could influence future drought impacts.