This story is part of the Grist series Vital Signs, exploring the ways climate change affects your health. This reporting initiative is made possible thanks to support from the Wellcome Trust.

As world leaders prepare to meet for the 30th annual United Nations climate change conference, or COP30, in northern Brazil later this month, a new report has found that climate change is already killing millions of people every year. The “Countdown on Health and Climate Change,” which is compiled by researchers around the world, has been published every year since 2015 by the British medical journal The Lancet.

The missives have grown increasingly dire over that decade. In 2020, the report warned that climate change threatened to “undermine the past 50 years of gains in public health.” Five years later, the same document suggests that this erosion is well underway.

“Climate change is increasingly destabilising the planetary systems and environmental conditions on which human life depends,” the countdown’s authors wrote.

Extreme heat now kills one person every minute, according to the report, noting that the rate of heat-related deaths has risen 23 percent since the 1990s — a trend the authors attribute in large part to planetary warming caused by the burning of fossil fuels. The vast majority of the heatwave days endured worldwide between 2020 and 2024 would not have occurred in the absence of climate change.

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

In sum, the report paints a picture of populations ill-equipped to cope with the shifting environmental parameters of a changing climate. “This Lancet report is a devastating global health audit,” said Harjeet Singh, founding director of a nature advocacy group called the Satat Sampada Climate Foundation, who was not involved in the assessment. “Our fossil fuel addiction is killing us by the millions.” 

Identifying the extent to which any isolated factor affects human health is no easy feat. A person’s wellbeing is linked to myriad behavioral, environmental, and social threads. In the past few decades, researchers all over the world have sought to isolate the role climate change plays in amplifying existing health trends and spurring the movement of disease. The process is not unlike how climate scientists seek to understand how the qualities of a particular hurricane or drought can be attributed to above-average sea surface or land temperatures. 

But because humans are so variable, attributing health impacts to planetary warming is an imperfect science. Reporting from the frontlines of climate hot spots in the world’s richest countries shows that illness and death that could be linked to climate change — heat stroke in Arizona emergency rooms during record-breaking heatwaves, for example — are rarely recorded as such. Conversely, top-down efforts to calculate the extent to which climate change may be influencing the spread of diseases such as tick-borne Lyme potentially overstate the effects of climate change by inadvertently underweighing the effects of urban sprawl, outdoor recreation, and other factors that put people in contact with ticks. 

The difficulty of separating signal from noise is what makes The Lancet’s annual report so important; it’s one of the only global efforts to make sense of the wide landscape of research at the intersection of climate and health. It tracks how 20 “health indicators” such as air pollution, food insecurity, days of extreme precipitation and drought, among others, have changed over the preceding 12 months. The 2025 report found that 13 of the 20 indicators tracked have grown more severe. 

As world leaders arrive in Brazil next month, much of the momentum for coordinated worldwide action to reduce emissions appears to be waning. Fossil fuel giants such as BP and ExxonMobil have reneged on their climate commitments. At President Donald Trump’s direction, the United States, the world’s biggest historical emitter of greenhouse gases, has begun the process of withdrawing from the Paris agreement and the World Health Organization.

“Paradoxically, as the need for decisive health-protective action grows, some world leaders are disregarding the growing body of scientific evidence on health and climate change,” the report’s authors wrote in a thinly veiled critique of Trump. “There is no time left for further delay.”