How deadly could climate change be? Last fall, in an idiosyncratic corner of the internet where I happen to spend a lot of time, an argument broke out about how to quantify and characterize the mortality impact of global warming. An activist named Roger Hallam — a founder of Extinction Rebellion who now helps lead the harder-line group Just Stop Oil — had told the BBC that, if global temperatures reach two degrees Celsius above the preindustrial average, “mainly richer humans will be responsible for killing roughly one billion mainly poorer humans.”
Hallam was quoting from a somewhat obscure paper, published by an engineer and a musicologist and focused less on climate impacts than on climate justice. The claim was quickly picked apart by experts: “An oft-quoted adage within the climate-modeler community is that garbage in equals garbage out,” the climate advocate Mark Lynas wrote. “Getting the science right will strengthen rather than weaken the case for climate activism, both in the public mind and in court.”
These are inarguable principles, and I don’t think it’s right to suggest that reaching two degrees of warming (which now looks very likely) will mean a billion people dead. Certainly that isn’t scientific consensus. But it did make me wonder: How big would the number have to be to strike you as really big? And how small to seem acceptable?
I ask because many more rigorous estimates, while lower, are still quite shocking. Some calculations run easily into the tens of millions. If you include premature deaths from the air pollution produced by the burning of fossil fuels, you may well get estimates stretching into the hundreds of millions. These are all speculations, of course. Estimating climate mortality involves a huge range of calculations and projections, all of which are shrouded by large clouds of uncertainty — it’s literally a climate-scale puzzle, with billions of human variables and many more political and environmental ones, and settling on a number also requires separating the additional impact of warming from the ongoing mortality produced by social and environmental systems running continuously in the background today.
In a recent commentary for Nature Medicine, the Georgetown University biologist Colin Carlson used a decades-old formula to calculate that warming had already killed four million people globally since 2000 just from malnutrition, floods, diarrhea, malaria and cardiovascular disease. As Carlson notes, this means that, since the turn of the millennium, deaths from climate change have already exceeded those from all World Health Organization global-health emergencies other than Covid-19 combined. “Vanishingly few of these deaths will have been recognized by the victims’ families, or acknowledged by national governments, as the consequence of climate change,” he says.
Going forward, most estimates suggest the impact should grow along with global temperature. According to one 2014 projection by the W.H.O., climate change is most likely to cause 250,000 deaths annually from 2030 to 2050. According to research by the Climate Impact Lab, a moderate emissions trajectory, most likely leading to about two degrees of warming by the end of the century, would produce by that time about 40 million additional deaths.
