Scientists have recently focused not on how high daytime temperatures climbed, but on the "heat load" that people actually must endure. At the same temperature, the danger the human body feels grows much greater when humidity is high, wind is weak, and radiant heat emanates from asphalt and buildings. When temperatures do not drop at night either, the body loses time to release the heat accumulated during the day and recover. This is why the limits of human endurance against extreme heat are being put to the test.
Reuters reported on the 12th of this month, citing mortality surveillance data from EuroMOMO, the European mortality monitoring system, that excess deaths during the European heat wave in late June this year surpassed 10,000. More than 9,000 of them were aged 65 or older. EuroMOMO is a surveillance system that tracks excess deaths related to public health threats such as infectious diseases and heat waves and cold spells, based on mortality data from European countries. This is not a problem for Europe alone. The World Health Organization (WHO) estimated that from 2000 to 2019, about 489,000 people died from heat-related illnesses worldwide each year. In the past, summer heat was studied mainly from the perspective that global warming threatens the future global environment and the foundation of human survival. Recently, however, the scientific community sees extreme heat as already directly threatening human life and health, and is shifting toward analyzing the heat load humans must bear and their physiological limits.
An international research team, including researcher Rebecca Emerton of the European Centre for Medium-Range Weather Forecasts (ECMWF), published in the international journal Nature Climate Change last month an analysis of changes in global heat stress from 1950 to 2024. Heat stress refers to the physiological burden the human body bears to maintain a constant body temperature in high-temperature environments. The team used the Universal Thermal Climate Index (UTCI), which reflects temperature, humidity, wind, radiant heat, and human response together. The analysis found that in the 1970s, 55% of the world's population experienced "strong heat stress" for more than 90 days each year, but in the recent climate that proportion rose to 70%. The proportion of the population exposed to "extreme heat stress" for at least one day each year also increased from 16% to 22%. Factoring in population growth, this means about 1 billion more people are exposed to extreme heat stress than in the 1970s.
The dangerous change was more pronounced at night. In the team's analysis, since the 1970s the perceived temperature on the hottest days rose 0.27 degrees per decade, while the perceived temperature on the hottest nights rose 0.32 degrees per decade. Nighttime heat has intensified faster than daytime heat. Night is when the human body sheds the heat accumulated during the day. If temperatures and perceived temperatures do not fall sufficiently, the heart, blood vessels, and sweat-secretion systems remain under strain.
The limits of heat that humans can endure are also being redefined. A team led by Professor Sarah Perkins-Kirkpatrick at the Australian National University (ANU) reported in a paper published in Nature Communications this March that fatal heat stress is already appearing even under conditions below the wet-bulb temperature of 35 degrees previously known as the limit of human survival. Wet-bulb temperature is an indicator that reflects both temperature and humidity, showing whether sweat can evaporate to cool the body. When the team analyzed recent heat wave cases using a human physiology model, none of the cases studied reached a wet-bulb temperature of 35 degrees. Even so, under some conditions, "unsurvivable" levels of heat stress appeared for elderly people aged 65 or older. This means wet-bulb temperature alone cannot sufficiently explain the danger the human body actually faces.
Cities are spaces that amplify such dangers. A research team including researcher Marzie Naserikia and Professor Negin Nazarian of the University of New South Wales (UNSW) in Australia published in May in Communications Earth & Environment an analysis of satellite-based surface temperatures and population exposure for 1,400 cities worldwide. Over the past 20 years, urban surface temperatures rose 0.03 to 0.2 degrees per year during the day and 0.01 to 0.15 degrees per year at night. This is the result of concrete, asphalt, glass facades, and densely packed buildings absorbing heat during the day and releasing it slowly at night.
However, experts believe that even as heat waves intensify, damage can be reduced if response systems are in place. An international joint research team including researcher Aleš Urban of the Czech University of Life Sciences and Professor Antonio Gasparrini of the London School of Hygiene & Tropical Medicine (LSHTM) analyzed daily temperature and mortality data from 102 regions in 14 European countries between 1990 and 2019, and reported this January that deaths from extreme heat fell by an average of 25% after the introduction of heat wave prevention plans. This means that combining early warnings, public guidance, protection of vulnerable groups, and coordination of medical services can reduce a significant portion of the damage. The WHO Regional Office for Europe also pointed out in a statement on the 11th of last month that more than 200,000 people had died from heat in the EU and related countries over the past four years, but most of these deaths were preventable.
To prevent damage in advance, warning standards must also change. Even at the same temperature, actual mortality risk varies depending on a region's population structure, housing environment, and access to medical care. Chinese researchers, including those from the Chinese Center for Disease Control and Prevention, presented a health-risk-centered, staged early-warning model using temperature and mortality data within China. Rather than simply whether the maximum temperature exceeded a certain level, the approach issues warnings based on how much actual mortality risk rises. Using this model, annual heat-related deaths in China's 337 major cities were estimated at 94,008, far exceeding the 14,858 captured by the existing heat wave warning system of the China Meteorological Administration.
