Aridity on the Rise: Spatial and Temporal Shifts in Climate Aridity in Spain (1961–2020)

Climate aridity (the long-term balance between water availability through precipitation and the atmospheric evaporative demand) has a fundamental role in determining water availability and the geographic distribution of ecosystems and agricultural regions, and plays a crucial role in shaping ecological transitions under current climate change. We computed the Aridity Index, computed as the ratio of precipitation to reference evapotranspiration, over Spain for the period 1961–2020. Here we present spatially detailed climatologies of the Aridity Index, at the annual and the monthly scales, and an assessment of changes between the normal periods 1961–1990 and 1991–2020. The results show a transition towards reduced values of the Aridity Index (i.e., towards drier conditions) at the annual scale, which was more intense in the Canary Islands (where 16.3% of the territory transitioned towards more arid climate categories) than in mainland Spain and the Balearic Islands (11.6%). At the monthly level, the most striking changes over mainland Spain occurred in June, with 39.7% of the territory transitioning towards more arid categories, while transitions towards more humid conditions have only been relevant in March (23.5%) and October (13.0%) and did not compensate for the aridification trend when the whole year is considered. In the Canary Islands, the strongest changes occurred in May (22.6%) and September (19.4%), although drying trends were found almost in all months except the summer.