In the early 2000s, as climate denialism was infecting political institutions around the world like a malevolent plague, an Australian epidemiologist named Anthony McMichael took on a peculiar and morbid scientific question: How many people were being killed by climate change? McMichael’s research team tallied up how many lives had been lost to diarrheal disease, malnutrition, malaria, cardiovascular disease (a proxy for heat-related illness), and flooding, worldwide, in the year 2000. The researchers then used computer modeling to parse out the percentage of those deaths that were attributable to climate change. Climate change, they estimated, was responsible for 166,000 lives lost that year.

The world has changed a great deal since. Climate denialism is no longer the world’s de facto climate policy, in large part because the impacts of rising temperatures have become impossible to ignore. The field of climate research has grown apace, and the science behind how climate change affects everything from ultra-rare species of frogs to the velocity of baseballs to the intensity of heatwavesdroughtsfloods, and hurricanes has become astonishingly precise. But the research assessing how many people are currently being killed by the climate crisis has remained conspicuously stagnant. While a small handful of studies have attempted to quantify the effect of climate change on mortality decades into the future, the McMichael standard, an ambitious relic of the early 2000s, is still the only estimate of its kind.

This week, a climate and health researcher published a commentary in the journal Nature Medicine that takes the McMichael standard to its logical conclusion. By the end of this year, Colin Carlson, a global change biologist and assistant professor at Georgetown University, wrote in the commentary provided exclusively to Grist, climate change will have killed roughly 4 million people globally since the turn of the century. 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 predicting and preventing pandemics.

And 4 million lives lost due to climate change, a breathtakingly high number, is still an underestimate — probably a big one. The McMichael standard doesn’t include deaths linked to climate-driven surges of the many non-malarial diseases spread by mosquitoes, like dengue and West Nile virus. It doesn’t incorporate deaths caused by deadly bacteria, fungal spores, ticks, and other diseases or carriers of disease that are shifting in range and breadth as the planet warms. It doesn’t examine the impacts of wildfires and wildfire smoke on longevity. It doesn’t look at the mental health consequences of extreme heat and extreme weather and the related increase in suicides that have been documented in recent years. “At the time we were doing it, we already knew it was conservative,” said Diarmid Campbell-Lendrum, a coauthor of McMichael’s 2003 study who is now the head of the climate change and health unit at the World Health Organization.

Read the full article about climate change killing millions by Zoya Teirstein at Grist.