Global warming caused by human emissions of greenhouse gases is having clear effects in the physical world: more heat waves, heavier rainstorms and higher sea levels, to cite a few.
In recent years, though, social scientists have been wrestling with a murkier question: What will climate change mean for human welfare?
Forecasts in this realm are tricky, necessarily based on a long chain of assumptions. Scientific papers have predicted effects as varied as a greater spread of tropical diseases, fewer deaths from cold weather and more from hot weather, and even bumpier rides on airplanes.
Now comes another entry in this literature: a prediction that in a hotter world, people will get less sleep.
In a paper published online Friday by the journal Science Advances, Nick Obradovich and colleagues predicted more restless nights, especially in the summer, as global temperatures rise. They found that the poor, who are less likely to have air-conditioning or be able to run it, as well as the elderly, who have more difficulty regulating their body temperature, would be hit hard.
If global emissions are allowed to continue at a high level, the paper found, by 2050 every 100 people in the United States may endure an extra six nights of insufficient sleep per month. By 2099, that would double to 14 nights of tossing and turning, in their estimation.
Researchers have long known that being too hot or too cold at night can disturb anyone’s sleep, but nobody had thought to ask how that might affect people in a world grown hotter because of climate change.
Dr. Obradovich is a political scientist who researches both the politics of climate change and its likely human impacts, holding appointments at Harvard and the Massachusetts Institute of Technology. He started the research while completing a doctoral degree at the University of California, San Diego.
He got the idea for the study while enduring a 2015 heat wave in an apartment in San Diego with no air-conditioner in the bedroom.
”I wasn’t sleeping,” he recalled. “My friends weren’t sleeping. My colleagues weren’t sleeping. The levels of grumpiness were higher than normal.”
To calculate the effect of warmer temperatures in the future, he turned to data collected by the Centers for Disease Control and Prevention, which asks people in a survey to recall their sleep patterns in the previous month. Sure enough, he found a correlation between higher temperatures in particular cities and disturbed sleep as reported by their residents. To make forecasts, he drew on computer estimates of how hot particular places will get if greenhouse emissions continue at a high level.
Dr. Obradovich acknowledged that a survey about sleep over the previous month was subject to the vagaries of memory. More definitive research would involve putting lots of people in a sleep laboratory and manipulating the temperature to see what happened. “Those ideal data don’t exist and would be prohibitively expensive to collect,” he said.
A bigger weakness in the study, perhaps, is that it is impossible to know what human society will look like 100 years from now. How many people will be without air-conditioning in that world?
Jerome M. Siegel, head of a sleep laboratory at the University of California, Los Angeles, who was not involved in the study, said the assumptions and data limitations gave him pause.
“It’s sort of a nice exercise — yes, this is something that might affect people,” Dr. Siegel said. “But this would be way down on my list of things to worry about with climate change, even though I’m a sleep researcher.”