The current El Niño episode, often described as a "super El Niño", raises concerns about the potential impact of this phenomenon on global temperatures and related health impacts. A new study by the Barcelona Institute for Global Health (ISGlobal), a centre supported by the "la Caixa" Foundation, has examined the relationship between this climate phenomenon and temperature-related mortality worldwide. The study, published in Nature Medicine, estimates that very strong El Niño episodes are associated with 107,035 additional heat-related deaths and 112,681 fewer cold-related deaths per year compared with neutral conditions.
El Niño–Southern Oscillation (ENSO) is a natural climate phenomenon in the tropical Pacific, which has two opposite phases: a warm phase (El Niño) and a cool phase (La Niña). During El Niño, surface waters in the central and eastern tropical Pacific warm unusually for several months, altering global atmospheric circulation. The release of ocean heat into the atmosphere causes an increase in global temperatures, especially in tropical countries, differently modifying the patterns of heat and cold related mortality in different seasons and parts of the world.
The research combines climate and mortality data with statistical models to investigate the relationship between ENSO and heat- and cold-related deaths during 1980–2023, a period that includes particularly intense episodes, such as those of 1982–1983, 1997–1998 and 2015–2016. The research team used mortality records from 1,667 regions in 125 countries, and used state-of-the-art techniques to extrapolate epidemiological associations between temperature and mortality for a total of 3,331 regions in 189 countries, representing more than 8 billion people. They then estimated the mortality burden related to heat and cold, and how these burdens varied with ENSO variability (El Niño and La Niña) and intensity (weak and strong events).
Heat-related deaths increase while cold-related deaths decline
During years with very strong El Niño episodes, the study estimates a total of 107,035 additional heat-related deaths per year and 112,681 fewer cold-related deaths compared with neutral conditions. "The fact that these two changes largely offset each other globally does not mean that El Niño is beneficial to health. Combined heat- and cold-related mortality increases in countries with fewer resources and, in an increasingly warm climate, future episodes could exacerbate these inequalities. Furthermore, El Niño's effects extend beyond temperature anomalies to include extreme weather events, such as floods and periods of drought," says Natalia Shartova, ISGlobal researcher and first author of the study.
Unequal impacts across regions and levels of development
During years with very strong El Niño episodes, the largest increases in annual heat-related mortality were estimated in South America (76.13%), Small Island Developing States (SIDS, 65.93%), sub-Saharan Africa (54.02%) and South and South-East Asia (43.11%). Overall, tropical regions experienced the largest increases in heat-related mortality, even with relatively modest temperature changes.
Differences were also observed according to the Human Development Index (HDI). Countries with a low or medium HDI, which account for 42.16% of the world's population, accounted for 67.65% of the estimated increase in heat-related deaths associated with a very strong El Niño episode. According to the study's results, the medium-HDI group had the largest absolute increase, with 61,863 additional annual heat-related deaths, representing 58% of the global increase.
Limitations and opportunities for prevention
This is the first study to provide global and regional estimates of heat- and cold-related mortality associated with El Niño variability and intensity. However, limitations need to be considered. Mortality records were unavailable for fitting the epidemiological models in one-third of the countries analysed, mostly in under-resourced countries. Estimates for most low-HDI countries and much of sub-Saharan Africa were therefore based on predictions from models built with data from other regions and local climatic and socioeconomic characteristics, and should be interpreted with caution. The research team also notes that the associations were estimated using retrospective data and cannot be used directly to predict mortality during the ongoing 2026–2027 El Niño episode.
El Niño is one of the most predictable patterns of climate variability, with reliable forecasts issued several seasons in advance. "Our findings identify the regions and seasons most affected by El Niño and, therefore, where health preparedness could be most valuable. This could support coordinated action plans involving health authorities, region-specific warnings and targeted interventions for populations at greatest risk," says Joan Ballester Claramunt, ISGlobal researcher and study coordinator.
Reference
Shartova, N., Liu, X., Janoš, T., Chen, ZY., Mishra, N., Ruiz-Cabrejos, J., Basagaña, X., Ballester, J. Global inequalities in temperature-related mortality of El Niño-Southern Oscillation. Nature Medicine, 2026. DOI: https://doi.org/10.1038/s41591-026-04793-w
