Sergio Vicente-Serrano
The report is sound because it covers various relevant variables relating to the hydrological cycle. Some of these involve considerable uncertainty, such as the study of evaporation, which is based on global models that have limitations and problems. It also provides useful observational data from specific locations, such as the soil moisture anomalies in the network available in the United States or the general assessment of global river flows. Although the latter has significant spatial gaps, given the limited availability of data in some regions of the world —such as Africa or much of Asia—, it represents a valuable contribution on a global scale.
It should be borne in mind that this assessment analyses a specific year and does not provide an overview of long-term trends. We may experience a very wet year, such as this winter in Spain, but this forms part of a very different context. In other words, this report does not record interannual variability, potential trends or possible future scenarios. Its purpose is to provide an overview of the situation in 2025, both in terms of the magnitude of the various variables used to measure water resources and in terms of any anomalies.
Drought is a consistent feature of our Mediterranean climate and, by extension, also in areas situated on the boundary between regions with subtropical characteristics and those with more temperate climates. Both geological and biological records, as well as documentary sources, demonstrate that drought is an inherent feature of our climate.
The current focus of interest is: are these droughts becoming more frequent? Are they becoming increasingly severe? How is climate change affecting these droughts? In this regard, we must distinguish between dynamic and thermodynamic mechanisms. Dynamic mechanisms are those related to atmospheric circulation. For example, Azores high-pressure systems that are longer-lasting, more persistent and more intense, preventing the entry of storms associated with the polar front and leading to more widespread dry conditions. On the other hand, there are the thermodynamic processes, which relate to temperature. Currently, what we are observing is that rainfall patterns are not changing substantially, but increasingly higher temperatures are leading to ever-greater atmospheric demand.
When we have a situation of precipitation deficit, such as a dry year or season, this increases stress on both water resources and vegetation. Above all, in the case of water resources, there are times when every drop of rain counts, and there is greater evaporation from reservoirs and river systems. As for vegetation, if the soil is already low in moisture due to a rainfall deficit, a temperature of 42 °C is not the same as one of 35 °C. Vegetation suffers much more because the atmosphere demands more water from it, and physiological processes occur that cause plants to assimilate less carbon or even die or wither due to this lack of moisture.