In a warming world, heat stress is becoming a growing public health concern.
The climate crisis has resulted in more frequent and intense heatwaves. In Europe – which is heating up much faster than the global average – the record-breaking heatwaves of this past June saw the number of heat stress days also break records.
Samantha Burgess, deputy director of the EU’s Copernicus Climate Change Service, said in July that “in a warming world”, heatwaves are likely to “impact more people across Europe”.
“June 2025 saw an exceptional heatwave impact large parts of western Europe, with much of the region experiencing very strong heat stress.”
Heat stress is a measure of how the human body responds to high temperatures. Symptoms include increased heart rate, excessive sweating, nausea and dizziness, with severe cases leading to heat exhaustion, heat stroke and death. According to the World Health Organization, heat-related deaths in its Europe region have increased by 30pc in the last 20 years.
To understand and monitor thermal stress, which includes heat stress and its opposite, cold stress, Copernicus and the European Centre for Medium-Range Weather Forecasts (ECMWF) have just launched the Thermal Trace application.
This app provides heat and cold stress information from a global dataset that starts at 1940 and includes data from up to five days before the present day.
“Thermal Trace was developed to provide key insights into heat and cold stress, which are critical for human health, particularly since heat stress is a leading cause of weather-related deaths, according to the WHO, and with heatwaves becoming more widespread, frequent and intense,” said Rebecca Emerton, the scientist at Copernicus who coordinated the development of the app.
Users can navigate maps and charts, selecting various locations, dates and variables, including ‘feels like’ and ‘peak stress’ options to explore for example the peak level of heat stress during a recent heatwave or how many tropical nights were experienced.
Screenshot of Thermal Trace app showing peak heat stress for June 2025. Image: C3S/ECMWF
The main data for the app comes from the ERA5-HEAT dataset, the first historical dataset to measure human thermal stress and discomfort in outdoor conditions, which is calculated from parameters including air temperature, humidity, wind speed and radiation. The dataset can be freely downloaded in the app.
The app is built using Zarr open-source archive technology that enables it to quickly process data updates.
The app is easy to navigate, and the charts can be downloaded for further study. The only drawback is that it doesn’t offer real-time event monitoring or forecasting because of the five-day data lag.
“Thermal stress is a critical issue which, in some cases, can become a literal matter of life or death,” said Copernicus principal scientific officer Chiara Cagnazzo.
“Thermal Trace and the post-processed dataset we have released can be a useful resource for the general public, policymakers, journalists, scientists and decision-makers from a range of sectors and organisations, to better understand it, to monitor the past and prepare for the future.”
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