Lancet Countdown 2025: Climate Change Drives Record Health Threats Worldwide

How Does Extreme Heat Affect Human Health?

The Lancet Countdown on health and climate change has documented that heat-related mortality among adults aged 65 and older has reached record levels, with deaths more than doubling compared to the early 2000s baseline. Extreme heat places enormous strain on the cardiovascular system as the body works to dissipate heat through vasodilation and increased cardiac output. For individuals with pre-existing heart disease, hypertension, or diabetes, this additional physiological burden can trigger heart attacks, strokes, and acute kidney injury from dehydration.

Heat exposure also impairs cognitive function, increases aggression, and worsens mental health outcomes. Studies have found correlations between extreme heat events and increased emergency department visits for psychiatric conditions, substance abuse, and suicide. Outdoor workers, including agricultural laborers, construction workers, and delivery personnel, face heightened occupational heat stress, with the Lancet Countdown estimating hundreds of billions of potential labor hours lost globally due to heat exposure, concentrated in the agricultural and construction sectors of low- and middle-income countries.

Urban populations face compounded risks due to the heat island effect, where concrete, asphalt, and limited green space can make cities several degrees hotter than surrounding rural areas. The Lancet Countdown has found that urban heat exposure has increased dramatically since 2000, with cities in South Asia, the Middle East, and sub-Saharan Africa experiencing the most extreme conditions. Adaptation strategies including urban greening, cool roofs, early warning systems, and designated cooling centers are critical but remain inadequately funded globally.

Are Vector-Borne Diseases Spreading Due to Climate Change?

The Lancet Countdown has found that climate suitability for dengue transmission by the Aedes aegypti mosquito has increased substantially compared to a 1950s baseline, with the range expanding into previously temperate regions of southern Europe, the southern United States, and higher-altitude areas of East Africa and Latin America. Dengue cases have surged globally, with the WHO reporting record-breaking numbers in 2024, far exceeding previous years. Climate-driven factors including warmer temperatures, which accelerate mosquito development and viral replication, and altered precipitation patterns play a significant role in this expansion.

Malaria transmission is similarly affected, with the Lancet Countdown documenting increased suitability for Plasmodium falciparum transmission at higher altitudes in the East African highlands, threatening populations with little prior exposure and limited immunity. The months suitable for malaria transmission have expanded in multiple endemic regions. Additionally, tick-borne diseases such as Lyme disease and tick-borne encephalitis are spreading northward in Europe and North America as warmer winters allow tick populations to survive and reproduce in previously inhospitable areas.

The intersection of climate change and vector-borne disease requires integrated public health responses. Surveillance systems need upgrading to detect outbreaks in newly affected areas, healthcare workers require training in diagnosing unfamiliar tropical diseases, and vaccine development must be accelerated. The Lancet Countdown emphasizes that current health system preparedness is insufficient, with many countries in newly affected zones lacking the diagnostic capacity, vector control infrastructure, and clinical expertise to manage emerging outbreaks effectively.

What Is the Impact of Climate Change on Air Quality and Food Security?

Wildfire smoke has become a major and growing source of air pollution, with the Lancet Countdown reporting a substantial increase in population exposure to wildfire-generated PM2.5 particulate matter over the past two decades. The 2023 Canadian wildfire season blanketed much of eastern North America in hazardous smoke, and similar events in Greece, Chile, and Australia have exposed hundreds of millions to dangerous air quality levels. Wildfire smoke contains fine particulate matter, volatile organic compounds, and toxic gases that penetrate deep into the lungs and bloodstream, increasing risks of respiratory disease, cardiovascular events, and adverse pregnancy outcomes.

Ground-level ozone, formed when pollutants from fossil fuel combustion react with sunlight, is also worsening as temperatures rise. Research indicates that ozone-related mortality remains a significant global health burden, with hundreds of thousands of deaths attributable to ambient ozone exposure annually. Combined exposure to PM2.5 and ozone from both fossil fuel combustion and climate-amplified wildfires creates compounding health risks, particularly for individuals with asthma, COPD, and cardiovascular disease.

Climate change is simultaneously undermining food security. Rising temperatures, changing precipitation patterns, and increased frequency of extreme weather events are reducing crop yields for staple foods including wheat, rice, and maize. The Lancet Countdown has found that heat stress during critical growing periods has reduced yield potential for major crops across multiple breadbasket regions. Beyond quantity, elevated atmospheric CO2 reduces the nutritional content of crops, lowering concentrations of protein, iron, and zinc in staple grains. The WHO has projected that climate change will cause approximately 250,000 additional deaths per year between 2030 and 2050 from malnutrition, malaria, diarrhea, and heat stress combined.

What Actions Can Reduce Climate-Related Health Risks?

The Lancet Countdown emphasizes that health co-benefits from climate action represent one of the strongest arguments for rapid decarbonization. Phasing out fossil fuels would reduce ambient air pollution, which currently causes an estimated 4.2 million premature deaths annually according to the WHO. Transitioning to active transportation such as cycling and walking reduces cardiovascular disease, obesity, and diabetes while cutting emissions. Shifts toward plant-rich diets lower both greenhouse gas emissions from livestock and rates of diet-related chronic diseases including colorectal cancer and heart disease.

Health system resilience is critical for adaptation. The Lancet Countdown calls for climate-resilient healthcare infrastructure that can withstand extreme weather events, reliable energy supplies for hospitals and cold chains, and integration of climate data into disease surveillance systems. Heat-health action plans with early warning systems have been shown to significantly reduce heat-related mortality where effectively implemented, but need expansion to all at-risk regions. According to the WHO, many countries still lack comprehensive national heat-health action plans.

The Lancet Countdown has found that global health spending on climate adaptation remains woefully inadequate. Only a small fraction of global climate adaptation finance is directed specifically to health system strengthening. The report's authors call for dramatically increased investment in climate-health research, surveillance infrastructure, and health workforce training, arguing that the cost of inaction far exceeds the cost of prevention. The Global Commission on Adaptation and other bodies have estimated that investments in climate adaptation yield substantial returns in avoided healthcare costs and economic losses.