Heat waves have become one of the most characteristic meteorological phenomena of recent summers, with increasingly intense, prolonged, and frequent episodes across much of the planet. What was considered exceptional just a few decades ago is now becoming a recurring pattern, with direct impacts on health, the economy, and the functioning of cities, as several analyses of summers in Europe in the face of climate change show.
In Europe, and particularly in Spain and other southern European countries , heat waves have become almost a fixture of summer. Meteorological services and public health agencies issue a series of warnings and emergency protocols, while scientific studies indicate that these types of events will continue to become more frequent in the coming decades if greenhouse gas emissions are not drastically reduced.
A warming planet and more frequent heat waves
Recent meteorological data show that the last few years have been the warmest since reliable global records began . This overall warming of the planet translates into longer summers and an increase in heat waves, which are no longer limited to a few isolated days but can last for weeks; recent studies and new findings about the atmosphere are providing further insight into these changes.
Heat waves are generally defined as periods of several consecutive days with maximum and minimum temperatures well above the values ​​considered normal for a given region and time of year. In Europe, these episodes are increasingly accompanied by tropical nights, when the temperature rarely drops below 20°C, making it difficult to sleep and increasing heat stress. The early prediction of heat waves is also a growing area of ​​research.
Behind this increase lies the advance of climate change driven by human activities , primarily the burning of fossil fuels. A warmer atmosphere favors the formation of extremely hot and dry air masses, which linger longer over certain regions, causing daytime and nighttime temperatures to soar.
This global context not only makes heat waves more frequent, but also raises the thermal floor : that is, what was previously perceived as a very hot day can now be considered almost normal in the middle of the summer season in many areas of southern Europe.
What can we expect for European cities in the coming decades?
Climate projections based on international models, which organizations such as AEMET and CSIC translate to local scale , point to a future in which heat waves will be a central element of the European climate if emissions are not drastically reduced.
These scenarios are grouped into different possible paths, which depend on how much climate pollution can be reduced. Under the assumption of very high emissions and a lack of decisive action , it is estimated that the average temperature in many cities could rise several degrees by the end of the century. This would imply not only hotter summers, but also more prolonged episodes of extreme heat.
For an Atlantic city like Santiago de Compostela , currently associated with a temperate and rainy climate, high-emission models paint a picture of an increase of nearly 5°C in the average temperature by the end of the century . This jump would significantly alter the usual perceived temperature, with much longer and more stifling summers.
In this context, heat waves would not only be more frequent, but also significantly longer . Simulations indicate that the maximum duration of these episodes could increase from just over two weeks to more than two consecutive months in the most extreme cases, posing an enormous challenge to healthcare systems, infrastructure, and daily life.
Duration of heat waves and tropical nights in cities like Santiago
Beyond daytime temperatures, a key aspect of heat waves is what happens at night. When nighttime lows don't fall below certain thresholds, the body doesn't recover from the heat stress accumulated during the day, and health problems increase, especially among the elderly, children, and patients with pre-existing conditions.
In a global scenario of very high emissions, projections for cities in the northwest of the Iberian Peninsula, such as Santiago, indicate that the number of warm nights would skyrocket . Currently, nights when the temperature remains well above 20°C are rare; by the end of the century, they could number in the dozens each year, with a considerable presence even during traditionally cooler periods.
At the same time, the duration of heat waves would increase significantly. Models used by the scientific community suggest that, in the worst-case scenario, the maximum number of days with extreme heat could multiply several times compared to the current situation . What is now perceived as an exceptional two-week episode could become a sustained period of more than a month of abnormally high temperatures.
In an intermediate scenario, more in line with the climate policies being implemented in the European Union, the increase would still be significant but less drastic: an increase of around 1,5°C in the average temperature is projected , with heat waves lasting a maximum of just over a month. Even in this more moderate scenario, warm nights and prolonged heat waves would be considerably more frequent than they are now.
Rainfall, intense episodes and their relationship to heat
Rainfall patterns will also change in a warmer climate, with direct implications for how heat waves are experienced. In many parts of Europe, including much of Spain, models suggest that total annual rainfall may not vary as much as the distribution of heat waves.
This means a shift from a greater number of days with moderate rain to fewer days with showers, but more intense ones . In northern regions of the peninsula, where many days with drizzle and light rain are currently common, projections anticipate a significant decrease in the number of rainy days, although the average daily rainfall may remain the same or even increase slightly in certain scenarios.
This combination of long periods of heat and dryness with occasional episodes of heavy rain complicates water resource management and increases the risk of extreme phenomena, such as flash floods or damage to infrastructure; it also raises the risk of fires in the subsequent dry weeks.
In terms of thermal comfort, intense but highly concentrated rainfall doesn't always alleviate the feeling of muggy conditions, especially when it occurs within the context of warm, humid air masses . In many cases, after a severe storm, the air remains heavy, with high humidity and temperatures that quickly rise again.
Impact on health and urban life during heat waves
Heat waves have a direct impact on public health. The increase in heat stroke, dehydration, dizziness, and fainting is a pattern that repeats itself whenever several days of very high temperatures occur, especially if nighttime lows are also high; therefore, it is crucial to check forecasts, health risks , and local recommendations.
European healthcare systems, including emergency and primary care services in Spain, activate protocols when extreme heat events are declared. Warnings to at-risk populations are reinforced, measures such as frequent hydration, avoiding the hottest hours of the day, and adapting physical activity are recommended , and special attention is given to the most vulnerable: elderly people living alone, chronically ill patients, children, and those who work outdoors.
In urban areas, the combination of heat waves with the so-called urban heat island effect exacerbates the problems. The high percentage of paved and built-up areas, the scarcity of green spaces, and heavy traffic mean that the temperature in city centers is several degrees higher than in the surrounding rural areas, especially at night.
Recent studies conducted in various cities show that temperature differences between neighborhoods and the outskirts can reach five or six degrees Celsius during periods of intense heat. This contrast translates into much more difficult nights for sleep in densely built-up areas, with an increase in cardiovascular and respiratory problems associated with the lack of nighttime heat relief.
Furthermore, a significant portion of the urban population with fewer economic resources is concentrated in neighborhoods with less tree cover, parks, and well-insulated housing, which increases inequality in exposure to extreme heat and in the ability to cope with it using means such as air conditioning.
Energy, electricity demand and infrastructure stress
Heat waves also put infrastructure to the test, particularly electrical and water supply systems . In Europe, extreme heat episodes are often accompanied by peak electricity demand, as the use of air conditioners, fans, and cooling systems skyrockets in homes, offices, and shopping centers; in this context, district heating and energy efficiency projects become crucial as part of the response.
This sudden and sustained increase in demand can stress distribution networks , generate overloads, and lead to localized power outages, especially in areas where the infrastructure is older or less adapted to these peak consumption levels. Electricity companies and network operators are forced to plan for reinforcements and have additional capacity available to handle these peaks; air conditioning in public and educational spaces is also key to reducing strain on the system.
At the same time, heat waves often coincide with periods of drought , which affects the levels of reservoirs and aquifers. This forces local authorities to launch water conservation campaigns, monitor water reserves, and, in some cases, implement temporary restrictions on domestic and recreational water use when the situation persists. They also promote aid programs and projects to combat energy poverty, targeting the most vulnerable groups.
In Spain, where a large part of electricity generation comes from sources such as solar energy and, to a lesser extent, hydropower, these episodes of extreme heat pose a double challenge: on the one hand, managing a very high demand concentrated in a few hours of the day ; on the other, ensuring that renewable production and interconnections with other countries can keep up with these peaks without compromising the stability of the system.
Urban adaptation: shade, vegetation and planning
Given a scenario in which heat waves will become more frequent, the adaptation of European cities becomes a priority. Urban planning can make a significant difference in how extreme heat is experienced or mitigated , even if baseline temperatures continue to rise.
Among the most effective measures is the strengthening of green infrastructure : increasing the number of trees in streets and squares, creating well-connected parks, promoting the presence of green roofs, and prioritizing species adapted to the local climate that provide shade and evapotranspiration. These elements help reduce ambient temperature and improve thermal comfort, both day and night.
Other strategies include the use of building materials and pavements with a greater capacity to reflect solar radiation, the energy rehabilitation of buildings to improve their insulation, and the creation of more climate-friendly public spaces, such as shaded areas, drinking fountains, or climate shelters open during the hottest times.
Public policies can also promote specific emergency plans for heat waves , coordinating health services, local councils, nursing homes, and social service organizations to identify and assist the most vulnerable groups. Many European cities are already developing heat risk maps that combine climatological, urban planning, and socioeconomic information.
This combination of planning, investment, and social awareness will be key for Spanish and European cities to remain livable and safe in a context of increasingly frequent and severe heat waves.
All indications suggest that heat waves will continue to play a more prominent role in the European climate , with longer summers, less cool nights, and more frequent episodes of extreme heat, even in regions that until recently were considered temperate. The difference between a future of uncontrolled impacts and one in which, although the heat increases, remains manageable will depend largely on how quickly emissions are reduced and on the ability of cities to adapt intelligently, taking advantage of solutions such as vegetation, urban design, and efficient water and energy management.
