Even if global warming is limited to 2 degrees Celsius (3.6 degrees Fahrenheit), dangerous heat waves will become much more common by the end of the century, according to a new study.
Further, many tropical regions will experience “dangerous” heat for about half the year.
Record-breaking heat waves have occurred recently from Delhi to the Pacific Northwest, and the number of these deadly events is expected to increase. The new research gives a range of heat impacts worldwide by the end of this century, depending on future emissions of greenhouse gases.
“The record-breaking heat events of recent summers will become much more common in places like North America and Europe,” says lead author Lucas Vargas Zeppetello, who did the research as a doctoral student at the University of Washington and is now a postdoctoral researcher at Harvard University.
“For many places close to the equator, by 2100 more than half the year will be a challenge to work outside, even if we begin to curb emissions. Our study shows a broad range of possible scenarios for 2100,” he adds. “This shows that the emissions choices we make now still matter for creating a habitable future.”
The study looks at a combination of air temperature and humidity known as the heat index that measures impact on the human body. A “dangerous” heat index is defined by the National Weather Service as 103 F (39.4 C). An “extremely dangerous” heat index is 124 F (51 C), deemed unsafe to humans for any amount of time.
“This study shows you the abyss, but it also shows you that we have some agency to prevent these scenarios from happening.”
“These standards were first created for people working indoors in places like boiler rooms—they were not thought of as conditions that would happen in outdoor, ambient environments. But we are seeing them now,” Vargas Zeppetello says.
The study finds that even if countries manage to meet the Paris Agreement goal of keeping warming to 2 degrees Celsius, crossing the “dangerous” threshold will be three to 10 times more common by 2100 in the US, Western Europe, China, and Japan. In that same scenario, dangerous days could double by 2100 in the tropics, covering half the year.
In a worst-case scenario in which emissions remain unchecked until 2100, “extremely dangerous” conditions, in which humans should not be outdoors for any amount of time, could become common in countries closer to the equator—notably in India and sub-Saharan Africa.
“It’s extremely frightening to think what would happen if 30 to 40 days a year were exceeding the extremely dangerous threshold,” Vargas Zeppetello says. “These are frightening scenarios that we still have the capacity to prevent. This study shows you the abyss, but it also shows you that we have some agency to prevent these scenarios from happening.”
The study uses a probability-based method to calculate the range of future conditions. Instead of using the four future emissions pathways included in the Intergovernmental Panel on Climate Change reports, the authors use a statistical approach that combines historical data with population projections, economic growth, and carbon intensity—the amount of carbon emitted for each dollar of economic activity—to predict the likely range of future CO2 concentrations.
The statistical approach “gives plausible ranges for carbon emissions and future temperature and has been estimated statistically from and validated against historical data,” says coauthor Adrian Raftery, a professor of statistics and of sociology at the University of Washington with an adjunct appointment in atmospheric sciences.
The authors translated the higher carbon dioxide levels into a range of global temperature increases, then looked at how that would affect global monthly weather patterns.
“The number of days with dangerous levels of heat in the mid-latitudes—including the southeastern and central US—will more than double by 2050,” says coauthor David Battisti, a professor of atmospheric sciences. “Even for the very low-end estimates of carbon emissions and climate response, by 2100 much of the tropics will experience ‘dangerous’ levels of heat stress for nearly half the year.”
The findings, published in the journal Communications Earth & Environment, underline the need to both reduce future greenhouse gas emissions and to protect populations, especially outdoor workers, against dangerous heat.
The National Institutes of Health, the James S. McDonnell Foundation, and the Tamaki Foundation funded the work.
Source: University of Washington