How climate change is undermining the health of millions worldwide

This story is part of the Grist series. Vital Signslearning how climate change affects your health. This reporting initiative is made possible with the support of the Wellcome Trust.

As world leaders prepare to meet at the 30th annual UN climate change conference (COP30) in northern Brazil later this month, a new report says climate change is already killing millions of people every year. “Health and climate change countdown“, compiled by researchers from around the world, has been published every year since 2015 by the British medical journal The Lancet.

Over the course of this decade, the messages became increasingly dire. In 2020 the report warned that climate change threatens to “undermine the last 50 years of public health gains.” Five years later, the same document states that this erosion is already in full swing.

“Climate change is increasingly destabilizing the planetary systems and environmental conditions on which human life depends,” the countdown authors write.

According to the report, one person is currently dying every minute due to extreme heat. The rate of heat-related deaths has increased by 23 percent since the 1990s. The authors attribute this trend in large part to planetary warming caused by the burning of fossil fuels. The vast majority of heat waves experienced globally between 2020 and 2024 would not have occurred if not for climate change.

But it's not just the extreme heat: the risk of death due to… inhalation of hazardous particulate matter from wildfire smoke and the spread of infectious diseases such as mosquito-borne dengue fever are also on the rise. The number of deaths related to wildfire smoke inhalation in 2024 was 36 percent higher than the baseline level established from 2003 to 2012. More severe droughts and heatwaves caused by rising temperatures were also associated with 124 million cases of moderate or severe food insecurity in 2023, compared with the baseline average from 1981 to 2010.

Overall, the report paints a picture of a population ill-equipped to cope with changing environmental conditions resulting from climate change. “This Lancet report is a devastating global health audit,” said Harjeet Singh, founding director of an environmental group called the Satat Sampada Climate Foundation, who was not involved in the assessment. “Our dependence on fossil fuels is killing us by the millions.”

Determining the degree of influence of any isolated factor on human health is not an easy task. Human well-being is related to a variety of behavioural, environmental and social factors. Over the past few decades, researchers around the world sought to isolate The role of climate change in exacerbating existing health trends and driving the spread of disease. This process is not much different from how climate scientists strive to understand how the qualities of a particular hurricane or drought may be related to above-average sea or land surface temperatures.

But because humans are so changeable, attributing the health effects to planetary warming is an imperfect science. Reporting from the front lines of climate hotspots in the world's richest countries shows that illness and death that may be linked to climate change – such as heatstroke in Arizona emergency rooms during record heatwaves – rarely registered as such. Conversely, top-down efforts to calculate the extent to which climate change may be influencing the spread of diseases such as Lyme. potentially exaggerate the effects of climate change by unintentionally underestimating the effects of urban sprawl, outdoor recreation, and other factors that bring people into contact with ticks.

The difficulty of separating signal from noise is what makes The Lancet's annual report so important; it is one of the few global attempts to make sense of the wide range of research at the intersection of climate and health. It tracks how 20 “health indicators” such as air pollution, food insecurity, days with extreme rainfall and drought changed over the previous 12 months. The 2025 report found that 13 of the 20 indicators monitored had become more severe.

As world leaders descend on Brazil next month, momentum for coordinated global action to cut emissions appears to be waning. Fossil fuel giants such as BP and ExxonMobil abandoned their climate commitments. Under President Donald Trump's direction, the United States, the world's largest historical emitter greenhouse gasesthe process has begun withdrawal from the Paris Agreement And World Health Organization.

“It is ironic that as the need for decisive action to protect health grows, some world leaders are ignoring the growing body of scientific evidence on health and climate change,” the report’s authors wrote in a thinly veiled criticism of Trump. “There is no time for further delays.”