It is the hottest and humidtest heatwave ever recorded in Europe. According to reports from foreign media such as the British Broadcasting Corporation, France, Spain, Germany and other countries have all experienced extreme high temperatures in recent days. The temperature in some areas exceeded 40°C, with extreme high temperatures of 41.7°C in Germany and other places.
On June 28, local time, World Health Organization Director-General Tedros Adhanom Ghebreyesus stated on social media that since June 21, more than 1,300 excess deaths related to high temperatures have been recorded in Europe. French officials disclosed that in the three days from June 24 to 26 alone, more than 1,000 excess deaths were recorded, the vast majority of which were elderly people over 65 years old, and the number of deaths at home increased significantly.
There is still a fierce debate in Europe over whether air conditioning should be installed. At the same time, a new study in "Nature Climate Change" released a set of figures that are not easy to digest: compared with the 1970s, about 1 billion more people currently have to endure at least one day of "extreme heat stress," that is, a day with a Universal Thermal Climate Index (UTCI) ≥ 46°C.
The research was led by scientists from the European Center for Medium-Range Weather Forecasts. They analyzed a 75-year global heat stress data set from 1950 to 2024, breaking down daytime, nighttime, and periods between day and night. The conclusion is not complicated: no matter which time period, the heat stress phenomenon increases in frequency, intensity, and duration, and running is faster at night than during the day.
In the past, when we talked about “heat,” we mostly focused on the daytime maximum temperature and the number of heat wave days. But now, we have to experience the "unrelievable" heat stress at night and during the day and night. It also means that the global scale for quantifying "heat" is clearer.
The heat that people feel is real heat
The "maximum temperature today is 35°C" in the weather forecast measures the air temperature in a louver box 1.5 meters above the ground. It is completely different from standing under the sun, covered in moisture, with or without the wind. Heat stress refers to the net heat load endured by the human organism. Temperature is only one of them. Humidity, wind speed, and radiation must also be added. High humidity means that sweat cannot be discharged. If there is no wind, convection heat dissipation will collapse. Direct sunlight plus long-wave radiation from the ground can increase the body temperature by a few degrees.
This is why the UTCI standard comes. UTCI is calculated by treating the human body as a physiological model with active adjustment functions: heat production, sweating and evaporation, and respiratory heat transfer. Then it is combined with four meteorological variables: temperature, water vapor, wind speed, and radiation. The final number is calculated in degrees Celsius, but the official definition calls it "body-sensing equivalent temperature," which means "the load on the human body in this comprehensive environment is equivalent to the feeling at how many degrees in dry air."
The UTCI classification is also easy to remember: UTCI above 32℃ is called "strong heat stress", and above 46℃ is called "extreme heat stress". The group of "one billion people" in the newly released paper corresponds to the latter - UTCI ≥ 46°C.
You know, in many mid-latitude cities, the air temperature reaches 38°C in summer, which is enough. If there is high humidity and no wind, the UTCI may reach the 46°C line.
The ten hottest nights climbed 0.32°C per decade
There is a striking detail in the team's monitoring this time: since the 1970s, on a global average, the UTCI warming rate during the ten hottest nights of each year is 0.32°C per decade; the ten hottest days each year are slower, at 0.27°C per decade. In other words, in extreme events, the night runs faster than the day.
Why is it a night that is supposed to be cool? The urban heat island effect is one reason, but more important is humidity. The surface radiation cooling at night is trapped by cloud cover and atmospheric water vapor. Coupled with the increase in the number of calm wind days, there are more and more "not cool" nights.
This killing of people is actually more subtle than during the day. When it's hot during the day, people will seek shade, replenish water, and adjust their work and rest schedules. If it doesn't cool down at night, the cardiovascular system won't have a chance to recover, and the cumulative load will continue to pile up the next day. The paper specifically mentions that it is this part of nighttime heat stress and day and night continuous heat stress that has been insufficiently quantified in the past.
In terms of geographical distribution, the subtropics are the first to suffer: Southern North America, southern Europe, the northern and southern ends of Africa, and South America have about 50 more days with UTCI ≥32°C (strong) and ≥46°C (extreme) every year than in the 1970s. In other words, some subtropical cities are stuck in the severe heat stress line for nearly half of the year. In southern European countries such as Spain, Portugal, Italy, and France, the current body temperature is 5°C higher than in the 1970s.
At the same time, the scope of heat is also expanding outward: "very strong heat stress" with UTCI ≥38°C can now be pushed to parts of North America, the United Kingdom, and Scandinavia. These places may not have experienced this kind of heat at all in history.
Figures involving the population have also increased: in the 1970s, about 16% of the world's people experienced at least 1 day of UTCI ≥ 46°C, and now this has risen to 22%. Proportionately multiplied by the population base, it is "an additional 1 billion people."
Heat is no longer just a meteorological problem
The list of response suggestions given by scientists is not long, including formulating a heat health action plan, building an early warning system, engaging in urban cooling, and incorporating heat stress indicators into climate risk assessments. Behind each one, there actually corresponds to a current shortcoming.
High temperature is the driving force behind diseases such as cardiovascular and cerebrovascular, respiratory, urinary system, and mental and psychological diseases. Its impact on the entire chain has begun to rise simultaneously in middle-, high-income, and low-income countries. The vulnerability of outdoor workers, pregnant women, the elderly, and low-income residents is superimposed. To protect these groups, community health, summer resorts, flexible working hours, power supply guarantees, etc. in various countries need to keep up.
Nowadays, urban cooling methods such as reflective roofs, increased shading, water-retaining green spaces, and ventilation corridors are relatively mature. In the future, the construction of these facilities may be directly linked to heat stress indicators. For example, under the same air temperature, if there is an additional piece of permeable shade, the UTCI may be several degrees different.
The "Extreme Heat and Agriculture" report previously released by the Food and Agriculture Organization of the United Nations and the World Meteorological Organization provides a reference: 500 billion working hours are lost due to extreme heat every year around the world, impacting the livelihoods of hundreds of millions of people. Heat stress is evolving from a meteorological event into a compound risk factor, which is also linked to droughts, wildfires, power outages, crop yield reductions, etc.
The current research still has its shortcomings. For example, the global data set over the past 75 years is relatively macroscopic and may not reflect details such as urban differences, urban-rural differences, indoor and outdoor differences, etc., and subsequent urban micrometeorological observations will still be needed to supplement it. But there is no doubt that the signal is clear enough for policymakers.