More than one-third of summer deaths caused by heat can be attributed to human-caused climate change, according to a new study of 43 countries over nearly three decades. Its authors said their findings highlight the "urgent need" for climate action that limits harm to the health of the public.

In a paper published Monday in Nature Climate Change, an international collaboration of 70 scientists analyzed more than 700 locations to determine their relationships between temperature andheat-related deaths. The study is the first to analyze the effects of climate change-driven heat on historical public health at a global scale.

Although the analysis excluded most of Africa and much of Asia due to missing or inaccessible data, it still demonstrates how global warming has already been causing death with rising temperatures, according to Antonio Gasparrini, the study's senior author.

The study ultimately attributed to climate change an annual average of 9,702 heat-related deaths across the 43 countries, or a total of about 270,000 over the 28-year period. This is certainly an undercount of such deaths in the entire world, given that the paper's scope did not capture countries home to billions of people.

"You don't have to wait until 2050 to see something — it's already happening," said Gasparrini, a professor of biostatistics and epidemiology at the London School of Hygiene and Tropical Medicine. "This should ring the alarm bell even louder to policymakers, and also to the politicians and also the general public."

Heat-related illnesses develop when the body cannot self-regulate at high temperatures, and can lead to dehydration, cardiovascular and respiratory problems and heatstroke, which sometimes kill. In the U.S. alone, as many as 1,300 people may die each year at least in part from heat, with low-income communities and people of color on average being exposed to higher temperatures.

Climate change and the rising temperatures that accompany it are expected to significantly increase heat stress experienced throughout the world, driven in part by more frequent and extreme heat waves.

Many scientific investigations about the public-health impacts of climate change look into the future, Gasparrini said, but the newest study found evidence that such change has already been taking lives with heat.

Gasparrini and the other authors studied data from 732 locations in 43 countries between 1991 and 2018, and determined that 1.56% of global summer deaths are heat-related; of those, 37% can be blamed on climate change. 

Heat mortality from global warming struck hardest in southern Europe, southern and western Asia and parts of Latin America, where it often constituted more than half of heat-related deaths — and where in some countries, heat from any cause was responsible for more than 5% of all deaths in the warm season. In contrast, heat's share of overall deaths was below 1% for the U.S., Canada, U.K. and eastern Asia.

Gasparrini said policymakers can take two routes to address the risk of heat: mitigation of future warming, which includes reducing greenhouse-gas emissions to slow rising global temperatures — something that needs to happen more rapidly, according to other research; and adaptation that makes the public less vulnerable, which can take the form of better air conditioning, weather forecasting and public awareness.

The detection-and-attribution study was conducted by first determining each location's relationship between heat and death by analyzing mortality and temperature data. Faced with extremely high temperatures, some places experience more deaths than others: Berlin experienced a 57% increase in deaths of all causes during the top 1% of hottest temperatures, while that kind of heat was associated with only a 9% increase in Johannesburg.

The researchers also modeled two global temperature scenarios between 1900 and 2018: one with human-caused climate change and the other without, with the latter excluding human impacts such as greenhouse-gas emissions. The modeled world with climate change began getting warmer than the other around 1991, diverging from the roughly steady temperatures in the scenario driven only by natural forces.

Both models were plugged into all the locations, and the difference in deaths between the scenarios was attributed to climate change as a cause.

The research project was a "momentous task" that "advances understanding of the heat-health burden due to climate change" at these locations, Dann Mitchell, a met office joint chair in climate hazards at the University of Bristol, said in an article in the same issue of Nature Climate Change.

The scientists did not include countries where high-quality or easily accessible data was not available, so the study received no data from all but one nation in Africa and many in Asia, including the entire Middle East. Subsequently, many low- and middle-income countries representing billions of people were omitted from the analysis, including highly populous and growing regions such as India and Nigeria. 

Follow-ups to the massive research project have not been planned out yet, Gasparrini said, but there are ongoing discussions on investigating how risks vary across different geographical regions, as well as expanding the analysis to countries that were excluded.

The study "highlights how important these issues are on a country-by-country level," Mitchell said. "It is only by expanding the network of countries involved that we can start to understand the true global burden of heat-related mortality from our changing climate."

The study, "The burden of heat-related mortality attributable to recent human-induced climate change," published May 31 in Nature Climate Change, was authored by A. M. Vicedo-Cabrera, University of Bern and London School of Hygiene & Tropical Medicine; N. Scovronick, Emory University; F. Sera, London School of Hygiene & Tropical Medicine and University of Florence; D. Royé, University of Santiago de Compostela and CIBER de Epidemiología y Salud Pública; R. Schneider, London School of Hygiene & Tropical Medicine, European Space Agency and European Centre for Medium-Range Weather Forecast; A. Tobias, Spanish Council for Scientific Research and Nagasaki University; C. Astrom and B. Forsberg, Umeå University; Y. Guo and S. Li, Monash University; Y. Honda, University of Tsukuba; D. M. Hondula, Arizona State University; R. Abrutzky, Universidad de Buenos Aires; S. Tong, Shanghai Jiao Tong University School of Medicine, Anhui Medical University, Queensland University of Technology and Nanjing Medical University; M. de Sousa Zanotti Stagliorio Coelho, P.H. Nascimento Saldiva and S. Osorio, University of São Paulo; E. Lavigne, Health Canada and University of Ottawa; P. Matus Correa and N. Valdes Ortega, Universidad de los Andes; H. Kan, Fudan University; J. Kyselý and A. Urban, Institute of Atmospheric Physics of the Czech Academy of Sciences and Czech University of Life Sciences; H. Orru and E. Indermitte, University of Tartu; J. J. K. Jaakkola, University of Oulu and Finnish Meteorological Institute; N. Ryti, University of Oulu; M. Pascal, Santé Publique France; A. Schneider, Helmholtz Zentrum München; K. Katsouyanni, National and Kapodistrian University of Athens and Imperial College London; E. Samoli, National and Kapodistrian University of Athens; F. Mayvaneh and A. Entezari, Hakim Sabzevari University; P. Goodman; Technological University Dublin; A. Zeka, Brunel University London; P. Michelozzi and F. de'Donato, Lazio Regional Health Service; M. Hashizume, The University of Tokyo; B. Alahmad, A. Zanobetti and J. Schwartz, Harvard University; M. Hurtado Diaz and C. De La Cruz Valencia, National Institute of Public Health; A. Overcenco, National Agency for Public Health of the Ministry of Health; D. Houthuijs and C. Ameling, National Institute for Public Health and the Environment; S. Rao and F. Di Ruscio, Norwegian Institute of Public Health; G. Carrasco-Escobar, Universidad Peruana Cayetano Heredia; X. Seposo, Kyoto University; S. Silva, Instituto Nacional de Saúde Dr Ricardo Jorge; J. Madureira, Instituto Nacional de Saúde and Universidade do Porto; I. H. Holobaca, Babes-Bolay University; S. Fratianni and F. Acquaotta, University of Torino; H. Kim and W. Lee, Seoul National University; C. Iniguez, CIBER de Epidemiología y Salud Pública and Universitat de València; M. S. Ragettli, Swiss Tropical and Public Health Institute and University of Basel; Y. L. L. Guo, National Taiwan University and National Health Research Institutes; B. Y. Chen, National Health Research Institutes; B. Armstrong and A. Gasparrini, London School of Hygiene & Tropical Medicine; A. Aleman, University of the Republic; T. N. Dang and D. V. Dung, University of Medicine and Pharmacy at Ho Chi Minh City; N. Gillett, Environment and Climate Change Canada; A. Haines, London School of Hygiene & Tropical Medicine and European Space Agency; M. Mengel, Potsdam Institute for Climate Impact Research; and V. Huber, Potsdam Institute for Climate Impact Research and Universidad Pablo de Olavide.