In the early 2000s, as climate denial infected political institutions around the world like a malevolent plague, an Australian epidemiologist named Anthony McMichael took on a curious and morbid scientific question: How many people are being killed by climate change? McMichael’s research team tallied how many lives were lost in the year 2000 due to diarrheal diseases, malnutrition, malaria, cardiovascular disease (a proxy for heat-related diseases) and flooding, worldwide. The researchers then used computer modeling to analyze the percentage of those deaths attributable to climate change. climate change, they estimatedwas responsible for 166,000 lives lost that year.
The world has changed a lot since then. Climate denial is no longer the world’s de facto climate policy, largely because the impact of rising temperatures has become impossible to ignore. The field of climate research has grown rapidly, and the science behind how climate change affects everything ultra-rare species of frogs after the velocity of baseballs to the intensity of heat waves, droughts, floodsand hurricanes became amazingly precise. But the research determining how many people are currently being killed by the climate crisis has remained remarkably stagnant. While a small handful of studies have attempted to quantify the effect of climate change on mortality decades into the futurethe McMichael standard, an ambitious holdover from the early 2000s, is still the only estimate of its kind.
This week a climate and health researcher released a commentary in the journal Nature Medicine that takes the McMichael standard to its logical conclusion. By the end of this year, climate change will have killed about 4 million people worldwide since the turn of the century, wrote Colin Carlson, a global change biologist and assistant professor at Georgetown University, in comments provided exclusively to Grist. That’s more than the population of Los Angeles or Berlin, “more than every other non-COVID public health emergency the World Health Organization has ever declared combined,” said Carlson, who also runs an institute focused on the prediction and prevention of pandemics.
And 4 million lives lost to climate change, a breathtakingly high number, is still an underestimate—probably a big one. The McMichael standard does not include deaths linked to climate-driven surges of the many non-malaria diseases spread by mosquitoes, such as dengue and West Nile virus. It does not include deaths caused by deadly bacteria, fungal spores, ticks and other diseases or carriers of diseases that shift in extent and width as the planet warms. It does not examine the impact of wildfires and wildfire smoke on long life. It does not look at the mental health effects of extreme heat and extreme weather and the related increase in suicides which has been documented in recent years. “When we did it, we already knew it was conservative,” said Diarmid Campbell-Lendrum, a co-author of McMichael’s 2003 study who is now head of the climate change and health unit at the World Health Organization.
The list of potential impacts that would need to be assessed to get a complete picture of the climate death toll is long and so far no researcher has attempted to do a complete accounting. “Climate change is killing a lot of people, nobody’s counting it, and nobody’s moving in the direction of counting it,” Carlson said. “If it was anything but climate change, we would be treating it in very different terms.”
Wael Al-Delaimy, a multidisciplinary epidemiologist at the University of California, San Diego, agreed that 4 million deaths since 2000 “is definitely an underestimate.” A significant lack of mortality data in low- and middle-income countries is one of the biggest obstacles standing in the way of a proper update of the McMichael standard. “The biggest challenge is that deaths are not well documented and measured around the world, and low- and middle-income countries suffer the most because they are not prepared, and there are no real epidemiological studies that try to link this to climate change,” did Al. – Delaimy said.
The lack of epidemiological data limits the methods researchers use to calculate climate-linked mortality in the first place.
Researchers who want to investigate how many deaths from a particular disaster are due to climate change usually use a method called attribution science. To understand the effects of climate change on mortality, scientists will use statistical methods and computer models to determine how climate change has affected the drivers of a discrete event, such as a heat wave. Then they will quantify the portion of heat-related deaths that can be attributed to climate change-related factors, using observed mortality data. As Al-Delaimy noted, mortality data is not always available. Attribution science, in the context of climate-related deaths, is a tool that is useful, specialized, and—according to experts like Carlson—limited by patchy data.
McMichael did not rely on the science of attribution to reach his conclusions, in part because the technique was still in its infancy when he did his mortality work. Instead, he used existing climate models to approximate how climate change affects specific diseases on a global scale. His research team figured out how diarrheal diseases, malnutrition and the other factors they chose to include were affected by warming — for example, they estimated a 5 percent increase in cases of diarrhea for every degree Celsius change in temperature — and then their calculations based on those findings. “To be honest, no one has been arrogant enough to ask that question before — what is the total burden of disease from climate change? — because it’s obviously a very big and difficult question,” Campbell-Lendrum said.
Carlson thinks the way forward builds on this work. Success depends on predictive computer modeling, he said: research that can simulate disease distribution and climate conditions and make predictions about how these patterns might change in the future. Predictive modeling does not require researchers to track mortality data that count every single person who died in a particular extreme weather event. The answer to the question of how many people have died from climate change, Carlson said, can be answered by developing a predictive modeling-based protocol for how researchers measure climate change-related deaths. This year, he aims to get the world’s leading climate and health experts together to build exactly such a system. Having researchers “bake to the same recipe,” he said, could eventually produce an updated, more accurate climate mortality estimate.
Developing something resembling a universal climate death protocol won’t be simple, but it could achieve what McMichael set out to do in the 2000s: give the public a rough understanding of the full climate death toll, not 50 years into the future. but as it happens now. “If you don’t know how big the challenge is, you can justify not investing in the challenge,” says Kristie L. Ebi, a climate and health researcher at the University of Washington. Mortality data drives policy, and more policy is needed to protect the public from what’s coming — and what’s already here.
In the summer of 2022 — a cooler summer than the summer of 2023, which is on track eclipsed by the summer of 2024 — causing extreme heat in Europe more than 60,000 deaths between the end of May and the beginning of September. Since early 2023, clouds of mosquitoes, spurred by unusual flooding and an intensifying monsoon season, spread dengue fever over large parts of the world, infecting nearly 5 million people and causing more than 5,000 deaths. Last year’s extreme weather killed 492 people in the US – one of the countries best equipped to deal with the fallout of extreme weather.
A deadly trend is underway. As McMichael put it in an open letter published just weeks before his death in 2014, “our mismanagement of the world’s climate and environment is weakening the foundations of health and longevity.” And yet, a very small fraction of the 4 million deaths caused by climate change so far, Carlson wrote in his commentary, “will be recognized by the victims’ families, or recognized by national governments as the result of climate change. ” What would happen if people knew the true extent of the risk? Carlson sets out to find out.
