Drought and heatwaves are among the most visible—and at the same time, most complex—impacts of climate change. Frequently mentioned together, they are in fact distinct phenomena, which only in certain cases interact. But when they do, the consequences can be severe and long-lasting.
In this analysis, we aim to better understand what happens when the climate amplifies—when water scarcity and extreme heat combine, feed into each other, and intensify. We do so by exploring the different types of drought, the operational definitions of heatwaves, and the scientific indicators that allow us to detect otherwise invisible signals.
Compound hazards
The World Drought Atlas, the most comprehensive global study on the impacts of drought, is clear: extreme heat and drought are not synonymous, but they can interact synergistically, generating compound hazards. Heatwaves can worsen an ongoing drought or even trigger one, especially when affecting an already vulnerable system. Conversely, a pre-existing drought can amplify thermal effects by reducing evaporation and accelerating warming.
The consequences manifest at multiple scales—from crop stress to urban water shortages, up to the collapse of entire ecosystems. The feedback loops between extreme heat and drought become particularly evident when observing the different types of drought in relation to atmospheric and hydrological phenomena.
Meteorological drought is primarily linked to precipitation deficits: heat does not directly cause it, but can result from or be intensified by it. Agricultural drought, on the other hand, occurs or may be exacerbated by high temperatures, which increase evapotranspiration and reduce soil moisture, negatively affecting crops and ecosystems. Under prolonged conditions, heat also impacts basin-scale water balance, leading to hydrological drought: streamflows decrease, water temperatures rise, and aquatic fauna is endangered.
In mountain regions, thermal anomalies can generate snow drought: rainfall replaces snow, or snow reserves melt too early, disrupting the seasonal water availability cycles.
Finally, even in the absence of a declared drought, heatwaves can trigger water stress conditions in populations and urban systems. This is known as socioeconomic drought, where water may not be scarce in absolute terms but becomes insufficient to meet demand and reduce vulnerability.
What do we mean by heatwave (and why it matters)
When we talk about heatwaves, we do not simply refer to “very hot days.” In meteorology, a heatwave is defined as a prolonged period (typically at least 3 consecutive days) during which both maximum and minimum temperatures exceed the 90th percentile of the typical temperatures for that location and reference climatological period. This threshold is calculated based on a standard climatological baseline (e.g., 1991–2020) and varies from city to city.
In the Copernicus drought monitoring system (EDO/GDO), heatwaves are also identified and mapped by analyzing simultaneous anomalies in daily maximum and minimum temperatures, as described above. This approach makes it possible to detect extreme phenomena even outside of the typical season, such as winter or spring heatwaves, which nonetheless have severe impacts on ecosystems, crops, and water resources.
In urban contexts, however, it is also relevant to refer to perceived heatwaves, which account for the combined effects of temperature, humidity, and environmental conditions on human well-being. Here, the concept of tropical nights becomes significant—nights in which the minimum temperature does not fall below 20 °C. These events not only hinder the body’s thermal recovery but also increase water demand, energy use, and physiological stress, especially among vulnerable individuals.
Indicators to detect the invisible
To understand the heat–drought relationship, it is necessary to use integrated indicators capable of capturing the complex interactions among temperature, soil, and precipitation. The Standardized Precipitation Index (SPI) measures only precipitation anomalies relative to a climatological reference period and is useful for identifying meteorological droughts, but it does not account for heat effects.
For this reason, the Standardized Precipitation-Evapotranspiration Index (SPEI) is more informative. It also includes temperature data to estimate the balance between water availability and demand and is often used to identify agricultural or hydroclimatic drought.
Another key indicator is the Soil Moisture Anomaly (SMA), which detects deviations in soil moisture from historical averages, providing a direct assessment of water deficits available to plants and ecosystems. Finally, the Combined Drought Indicator (CDI) is a composite index that integrates SPI, SMA, and anomalies in vegetation productivity (fAPAR), offering a more robust assessment of drought conditions.
As explained by Luca Trotter and Michel Isabellon, researchers in the Drought Department of CIMA Research Foundation: “It is essential to use indicators that incorporate the thermal component. Heat-induced evapotranspiration can degrade soil water status in just a few days. If we looked only at the SPI, we would miss half the picture.”
The story of a changing climate
Understanding the relationship between drought and heatwaves means engaging with a complex system of interactions, where environmental variables influence each other in ways that are not always predictable. It requires clarity in definitions, careful attention to thresholds, and above all, the ability to interpret data within its context. Indicators such as SPI, SPEI, SMA, and CDI are not just acronyms—they are essential tools to understand whether we are entering a critical phase, or whether we are already beyond it. That is why it is important to explain not only what happens when it’s hot or when it rains too little, but what happens when these events overlap and reinforce one another. That is when the climate truly changes.