Climate change is no longer a distant threat or a mere hypothesis for speculation. Heat waves are becoming longer, more intense, and, above all, more treacherous , because many of their effects go unnoticed if you only check the thermometer once a day. Research led by Sun Yat-sen University in China and published in the journal Nature Climate Change has focused on a previously little-explored aspect: the exact duration, hour by hour, of extreme heat episodes. The findings are cause for concern, but also for action.
The study, which analyzed global data from 1981 to 2023, concludes that if greenhouse gas emissions remain high, hot days could accumulate more than four additional hours of extreme heat by the end of the 21st century . This is no small matter: the persistence of heat for several consecutive hours, especially at night, prevents the body from recovering from thermal exposure and multiplies health risks. In fact, researchers estimate that 28% of extreme heat episodes occur on days that, according to traditional metrics, would not be considered extreme. In other words, a day that seems manageable on the whole can conceal several hours of exceptionally high temperatures.
More hours of extreme heat: what daily averages hide
The authors of the study combined hourly temperature records with climate projections for the rest of the century. The result is striking: more than 80% of the Earth's land surface analyzed recorded an increase in these temperatures between 1981 and 2023 , with an average of about 62 additional hours of extreme heat per decade. Currently, land areas around the world experience approximately 269 hours of extreme heat each summer, a figure that continues to rise.
The most worrying scenario emerges under very high emissions. For the period 2070-2099, models project that each day identified as hot could incorporate more than four additional hours of extreme temperatures compared to the current reference period (2001-2023) . Even days not classified as hot could add up to about three hours of extreme heat that daily indicators tend to overlook. José Miguel Viñas, a meteorologist at Meteored and a consultant for the World Meteorological Organization (WMO) in Spain, highlights that this research has focused on a novel aspect: the hourly assessment of temperature distribution during extreme heat waves. This will allow for a better characterization of the impact of persistently high temperatures on human health , both during daytime and nighttime peaks.
The study also reveals a worrying gap: low- and middle-income countries currently bear more than three-quarters of global exposure to extreme heat , and will continue to bear a similar proportion in the future. Younger cohorts, especially in lower-income countries, will face a much higher cumulative lifetime exposure than previous generations. Scientists argue that this new way of measuring heat should be incorporated into risk assessments, because average daily temperatures can mask brief but very intense episodes capable of producing significant consequences.
Cities being redesigned: from Kiel to Pamplona, ​​adaptation is urgent
While science refines its measurements, cities have already begun to take action. Kiel, in northern Germany, became one of the first European cities to declare a climate emergency in 2019. Following the worst storm in 150 years, which in October 2023 devastated dikes, cliffs, and marinas along the Baltic Sea coast, the city established a dedicated climate adaptation division in 2025. Martina Baum, head of that division, sums it up clearly: the impacts of climate change are hardly abstract in Kiel . The city is testing natural solutions to limit air conditioning use, such as covering building facades with vegetation and redesigning parks to channel fresh air into the city center. It is also experimenting with 20 tree species to assess which are best suited to withstand drought, heat, and the salinity of sea air. Trees provide shade, cool the streets by approximately 1,5°C, and improve air quality.
In Spain, Pamplona has taken a step in the same direction. The Socialist Party of Navarre has proposed a Municipal Plan for Prevention and Action against Extreme Heat, which coordinates social, health, civil protection, and environmental services to protect the most vulnerable people. The proposal includes creating a network of climate shelters using municipal buildings such as libraries, community centers, and sports centers , as well as expanding the number of drinking fountains and hydration stations in parks, squares, and around schools. It also proposes a Local Shade Plan that analyzes areas with the greatest need for shade and allows for action based on clear criteria, prioritizing school and healthcare environments and areas with high pedestrian traffic. The plan calls for more trees and vegetation, but also pergolas and awnings when trees cannot provide immediate shade.
Adaptation is not without its contradictions. Installing air conditioning in nursing homes or schools protects vulnerable populations from extreme heat, but it increases energy consumption , which runs counter to climate protection goals. The energy grid still relies partially on non-renewable sources, resulting in more emissions. That's why cities like Kiel are opting for nature-based solutions and redesigning public spaces so that rainwater gradually infiltrates the ground instead of accumulating. One concrete idea: building soccer fields slightly lower than the surrounding area so that, during periods of heavy rain, the water flows there and either evaporates or infiltrates the soil. The drawback is that the field could be unusable for up to 60 days a year, but residents would have to accept this.
Changing clouds and shifting malaria: other fronts of global warming
As cities adapt, the sky is changing too. Low clouds, which act as a natural umbrella by reflecting sunlight back into space, tend to disappear in certain tropical and subtropical regions . Conversely, at higher latitudes, high, thin, and almost translucent clouds are gaining ground. This difference matters a great deal: where low clouds protect, high clouds trap heat, like swapping a parasol for a blanket. Scientists consider clouds the biggest unknown in climate models because small variations in their distribution can make the difference between moderate warming and much more severe warming.
Global warming is also reshaping the map of infectious diseases. A study published in the journal Nature by the Climate Impact Lab offers the most comprehensive assessment to date of the impact of global warming on childhood malaria in sub-Saharan Africa . Far from simply intensifying transmission across the board, rising temperatures are causing a geographical shift of the disease to new latitudes and altitudes. In traditionally the most affected and hottest areas, such as West African countries, the extreme heat is beginning to work against the mosquito that transmits the disease, projecting reductions of up to 5% in cases. Conversely, in the cooler, high-altitude regions of the south and east of the continent, such as the highlands of Ethiopia, Kenya, and Burundi, a 5% increase in incidence is already being recorded. The authors emphasize that limiting global warming to the 2°C threshold would prevent at least five additional cases per thousand children in the southern part of the continent.
The combination of all these factors paints a complex picture. The scientific community agrees that adaptation cannot wait, but neither should it replace mitigation . Reducing emissions remains the first line of defense, but cities, health systems, and communities must prepare for a climate that is already changing. As Martina Baum points out, investing in both adaptation and mitigation simply pays off. The question is not whether we can afford to act, but whether we can afford not to.
The next time we look at the sky or check the thermometer, it's worth remembering that extreme heat is no longer an exception, but a chronic condition . The decisions made today in cities, in the countryside, and in laboratories will determine how many hours of extreme heat we will have to endure tomorrow. And that is a responsibility that cannot be delayed.
