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Deadly heat
Global warming is already responsible for one in three heat-related deaths
Dr. Ana Vicedo-Cabrera

“So far, the average global temperature has only increased by about 1°C, which is a fraction of what we could face if emissions continue to grow unchecked.”


The City of Bern also aims to combat heat stress

An international study coordinated by the University of Bern and the London School of Hygiene & Tropical Medicine caused a global stir in May 2021. This study shows that more than a third of all heat-related deaths between 1991 and 2018 were attributable to human-induced global warming. Its impact on urban populations is particularly harsh. The University of Bern is laying the groundwork for specific measures that the City of Bern can take to adapt to these conditions.


Global warming affects our health in different ways. One direct impact is apparent in the increased rates of heat-related illnesses and deaths. Scenarios predicting future climate conditions expect average temperatures to increase substantially and for extreme weather events, such as heat waves, to occur with growing frequency. This will also result in an increase in the associated health burden going forward. Previously, no study had ever investigated whether and to what extent effects such as these have already taken place in recent decades. The international study entitled “The burden of heat-related mortality attributable to recent human-induced climate change”, coordinated by the University of Bern and the London School of Hygiene & Tropical Medicine (LSHTM), has now proven for the first time that 37 percent of heat-related deaths between 1991 and 2018 can be attributed to human-induced changes in climate. The largest study of this kind, based on data from 732 cities in 43 countries around the world, was published on May 31 in the scientific journal “Nature Climate Change”.

Did you know?

“This study from Bern made huge waves internationally. According to ‘Carbon Brief’, a UK website that specializes in climate science and politics, it was the climate study most frequently mentioned in the media worldwide in 2021.”

The more global warming, the more deaths

The epidemiological investigation analyzed the consequences of human-induced global warming within the scope of something referred to as a “detection and attribution” study, which links global warming to human activities. Specifically, the researchers examined past weather conditions under scenarios both with and without anthropogenic emissions, which enabled them to distinguish between human-induced warming (and its corresponding health impact) and natural changes. “We expect the proportion of heat-related deaths to continue to grow if we don't do something about climate change or adapt,” says Dr. Ana Vicedo-Cabrera, first author of the study from the Institute of Social and Preventive Medicine and the Oeschger Centre for Climate Change Research at the University of Bern.

Did you know?

“Heat stroke causes a person’s brain and core body temperature to rise to over 40 degrees. If the temperature increases to much more than 42 degrees, proteins in the body start to denature (unfold), resulting in death.”

Enormous regional differences

While an average of more than one in three heat-related deaths are attributable to anthropogenic climate change, the impact varies enormously from one region to the next. The number of climate-related heat casualties in a specific location ranges from a few dozen to several hundred deaths each year, depending on the local changes in climate in each area and the vulnerability of its population. People living in low- and middle-income countries in Central and South America and South-East Asia, which have only been responsible for a minor part of anthropogenic emissions in the past, are those that are hardest hit.

According to Ana Vicedo-Cabrera, heat risks “should not be underestimated,” even in Switzerland, where the differences in living conditions are comparatively small and temperatures remain moderate. For example, the country-specific estimate for Switzerland was around 30%, meaning that one in three heat-related deaths can be attributed to climate change.

Cities hit particularly hard

Global warming is particularly problematic for cities, where something referred to as the urban heat island effect plays a major role: Temperatures in urban settings are often much higher than those in the surrounding areas, particularly at night. This is mainly attributable to surface changes: The greater the proportion of sealed surfaces in an area, the more solar radiation is absorbed. That means buildings and streets heat up during the day and act as thermal storage heaters, which then slowly radiate that stored heat at night. If climate change causes temperatures to rise, these thermal storage heaters are able to absorb even more energy, thereby causing people to suffer more and more under the urban heat.

As a result, researchers of the Oeschger Centre for Climate Change Research at the University of Bern are examining the impact of heat waves on Swiss cities within the scope of the “Urban Climate Bern” research project. The fourth measurement campaign was carried out in the summer of 2021.

Urban Climate Bern film series, part 1: Measurement campaign. Investigating urban heat with climate researchers Moritz Burger and Saba Baer as they set up the measurement network in Bern. © University of Bern

Large-scale measurement campaign in Bern

“High-resolution temperature measurements help us paint a detailed picture of urban heat stress,” explains climate scientist Moritz Burger, who is writing his PhD thesis as part of the project. The results of the measurement campaigns in 2018 and 2019 already demonstrated just how much variation there is in the urban heat island effect: In the most heavily impacted districts – the downtown area – nighttime temperatures during heatwaves are three to four degrees Celsius higher than in areas outside the city. In fact, one of the heat hotspots is Bern University Hospital, with measurements showing that the historic downtown as well as the Breitenrain and Mattenhof quarters being particularly hot, as well.

“The data delivered by the measurement campaigns let us model future developments – and serve as a decision-making basis for climate change adaptation measures,” says Moritz Burger. Climate scientists collaborate closely with real-world companies and organizations, such as with those in charge of public green spaces, Stadtgrün Bern and Energie Wasser Bern.

Combating urban heat with water from the Aar river

Walter Schaad, the sustainability expert at ewb, explains in the video that one option being discussed is a network of pipes that would transport cold water from the Aar river across the city to areas that will need substantial cooling energy in the future.

Urban Climate Bern film series, part 2: Measurements for real-life applications. What can we do about the increase in urban heat islands in Bern? In the field with climate researcher Moritz Burger and sustainability expert Walter Schaad from Energie Wasser Bern (ewb). © University of Bern

Research with a long-term perspective

The University of Bern isn’t alone in its efforts to research urban climates. According to climatology professor Stefan Brönnimann, current interest in urban climatology has prompted numerous research groups around the world to study the topic. All of which with different areas of focus. Bern’s special strength? “One of the unique aspects of our efforts is our long-term perspective: 50 years ago, our predecessors already had an urban climate measurement network in operation for a few years. We can now take that data and compare it with present-day measurements,” says Brönnimann. Another special aspect of Bern’s measurement network is that it works with low-cost sensors. The researchers built their measurement stations using something of a do-it-yourself approach, which cuts the cost to around CHF 60 per unit.

Urban Climate Bern film series, part 3: Science. In the field with students of Bern, who researched Bern’s urban climate during a summer field course in 2021.

Not only could this approach be copied in poorer countries, but measurement stations are few and far between here, as well, which is why a high-resolution series of measurements on urban climate is still missing. MeteoSchweiz only operates a single station in Bern – right outside the city gates in Zollikofen – and uses this station to calculate all its climate scenarios for the City of Bern. Assuming the IPCC scenario RCP 8.5 – which is that no climate protection measures are taken – there will “only” be between eight and ten tropical nights in Zollikofen toward the end of the century. The situation in downtown Bern, however, could be entirely different: If you factor in the already measurable difference between the various locations, models prepared by Urban Climate Bern show that the districts hardest hit by the heat will experience around 30 to 45 tropical nights. Those are conditions currently found in Southern Europe.

Urban Climate Bern film series, part 4: Interim assessment. Currently the final installation of this series of films, this video shows why the summer of 2021, even despite the heavy rainfall, was one of the warmest in Europe and how creating simple green spaces can help cool down even fully sealed urban areas.

The urban heat effect also became apparent in the summer of 2021

According to the data, however, 2021 was not a year of extremes. In fact, some people even thought it seemed unusually cool. Moritz Burger, however, says that “based on the measured data alone, the summer of 2021 didn’t deviate too far from the norm.” Temperatures during the summer as a whole averaged 18 degrees Celsius, which is still 0.6 degrees above the temperature that would have been expected based on the long-term average. The past summer was also only slightly below average in terms of sunshine duration: Coming in at 95 percent, the expected sunshine duration just barely missed the norm.

The climate researcher then points out, however, that the summer of 2021 was exceptionally warm for Europe as a whole. The Copernicus dataset, which has been around since 1979, shows that it was the hottest summer ever recorded in Europe. A new absolute temperature record was also set: A temperature of 48.8 degrees was measured near Syracuse, Sicily, on August 11. This extreme weather was brought by the anticyclone Lucifer, which also gave Southern Italy its the seventh heatwave in a row in the summer of 2021. 


The Oeschger Centre for Climate Change Research (OCCR) is one of the strategic centers of the University of Bern. It brings together researchers from 14 institutes and four faculties. The OCCR conducts interdisciplinary research right on the frontline of climate science. The Oeschger Centre was founded in 2007 and bears the name of Hans Oeschger (1927-1998), a pioneer of modern climate research, who worked in Bern. Research on climate change and health is one of the special focus areas of the Oeschger Centre for Climate Change Research. A corresponding research group was founded in 2019 together with the Institute for Social and Preventive Medicine (ISPM) of the University of Bern. It is headed up by Dr. Ana M. Vicedo-Cabrera, the first author of the international study on heat mortality and climate change.

Go to the Oeschger Centre for Climate Change Research