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.

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.