The climate and health picture begins with heat. Extreme heat killed an estimated 61,672 people across 35 European countries during the summer of 2022, the hottest summer on record for the continent, according to an ecological analysis of the Eurostat mortality database by Ballester and colleagues published in Nature Medicine in 2023. The figure sits between the 71,449 excess deaths of the 2003 heatwave, a rare weather event at the time, and the emerging pattern of the 2020s in which comparable summers have become routine. The 2025 report of the Lancet Countdown on health and climate change, led by Marina Romanello, tracks these effects across dozens of indicators and finds most of them moving in the wrong direction.
Heat is the most direct pathway from climate to mortality
Heat kills through several routes. Sustained high body temperatures strain the cardiovascular system, worsen kidney function, and can trigger heat stroke, in which core temperature exceeds the threshold at which the body can cool itself. Deaths concentrate among older adults, people with existing cardiovascular and respiratory disease, outdoor workers, and people in poorly ventilated housing. Only a small share of heat-related deaths are officially recorded as heatstroke; most register as cardiovascular or respiratory deaths on days when heat was the trigger.
The Ballester analysis found that Italy (18,010 deaths), Spain (11,324 deaths), and Germany (8,173 deaths) had the highest absolute mortality numbers in 2022, with Italy, Greece, Spain, and Portugal showing the highest per-capita rates. Ballester, the study’s lead author at the Barcelona Institute for Global Health, said in interviews that “we are probably not aware of how heat is a silent killer.” The comparison with 2003 matters. That summer’s toll was an exceptional shock that drove Europe to build heat prevention plans. The 2022 figure, only slightly lower despite two decades of adaptation, suggests those plans are being outpaced by the warming itself.
The disease map is redrawn as vectors move
Warmer temperatures and altered rainfall are expanding the geographic ranges of the mosquito species that carry dengue, chikungunya, Zika, and West Nile virus. Aedes aegypti and Aedes albopictus are now established in parts of Europe and North America where they previously could not survive winters. Autochthonous (locally acquired) dengue transmission has been documented in France, Italy, Croatia, and Spain over the past decade. West Nile virus is now endemic across much of Europe. The clinical consequences are individually small in most cases; the epidemiological consequences of shifting a mosquito’s range by a few hundred kilometers are large.
Wildfires and air pollution are converging
Longer, drier fire seasons produce more particulate matter, including PM2.5 (fine particles under 2.5 micrometers) that penetrates deep into the lungs and enters the bloodstream. Short-term exposure worsens asthma and increases hospitalizations for cardiovascular and respiratory disease. Long-term exposure has been linked to earlier cognitive decline and higher all-cause mortality. Fossil fuel combustion, still the dominant source of PM2.5 in most cities, adds to this baseline. The World Health Organization estimates that ambient air pollution causes about 4.2 million premature deaths per year globally.
Water, food, and mental health are also affected
Flooding contaminates water supplies and spreads waterborne pathogens. Drought stresses food production. Crop yields for staples such as wheat and maize show declines in extreme heat years, and rising atmospheric carbon dioxide reduces the protein and micronutrient content of some grains. Climate-related displacement, loss of home and livelihood, and the psychological weight of ecological anxiety all show measurable effects on mental health, particularly in young people and communities directly affected by disasters.
Many climate actions are also health interventions
The Lancet Countdown makes a case that has gained ground in policy circles: measures to cut greenhouse gas emissions often improve health independently of their climate effect. Reducing fossil fuel combustion in transport and power generation cuts urban air pollution. Shifting toward plant-forward diets reduces both cardiovascular disease risk and methane emissions from livestock. Redesigning cities around walking, cycling, and public transport improves fitness while lowering emissions. These co-benefits are direct enough that public health economists increasingly argue climate policy should be framed as health policy.
What the evidence cannot yet answer
Attribution studies can now link many individual weather events to climate change, but attributing specific deaths to specific degrees of warming remains uncertain. Adaptation reduces risk but the ceiling on how much adaptation can offset warming is not known. And climate-health impacts fall unevenly: heat-related mortality is highest in lower-income countries and among socially isolated older adults in higher-income ones, and neither the resources nor the policy attention have followed that gradient. The 2025 Lancet Countdown notes that most of the indicators it tracks are worsening; the emerging question is whether the pace of adaptation and mitigation can catch up.
References
- Ballester J, Quijal-Zamorano M, Méndez Turrubiates RF, et al. Heat-related mortality in Europe during the summer of 2022. Nature Medicine, 2023; 29: 1857-1866. DOI: 10.1038/s41591-023-02419-z
- Romanello M, Walawender M, Hsu SC, et al. The 2024 report of the Lancet Countdown on health and climate change: facing record-breaking threats from delayed action. The Lancet, 2024; 404: 1847-1896. DOI: 10.1016/S0140-6736(24)01822-1
- Robine JM, Cheung SLK, Le Roy S, et al. Death toll exceeded 70,000 in Europe during the summer of 2003. Comptes Rendus Biologies, 2008; 331: 171-178. DOI: 10.1016/j.crvi.2007.12.001
- Romanello M, et al. The 2025 report of the Lancet Countdown on health and climate change: climate change action offers a lifeline. The Lancet, 2025. DOI: 10.1016/S0140-6736(25)01919-1