Even with drastic cuts to the emissions of greenhouse gases that are driving up Earth’s temperature, more than half of the world’s population could be exposed to deadly heat waves by century’s end.
If emissions continue on their current path, that proportion will jump to three-quarters of the world’s residents, due to both rising temperatures and humidity, a new study detailed Monday in the journal Nature Climate Change, finds.
That future is what study author Camilo Mora calls a choice between “bad and terrible,” but crucially, it is still a choice. If the world reduces emissions, it can reduce the impacts of warming as much as possible.
Societies will also have to adapt to rising temperatures and the higher risks to human health they bring, but it is the developing countries that are least able to adapt that are likely to be hit the hardest.
“The projections of future risk to human health are sobering,” Michael Wehner, a senior staff scientist at Lawrence Berkeley National Laboratory who wasn’t involved in the study, said in an email, citing the particular risk to developing nations. “I welcome this paper for highlighting this inequity and for its novel demonstration that changes in the combination of heat and humidity are critical to understanding the effect of global warming on human health.”
A shift to more, and more extreme, heat is one of the clearest outcomes of global warming: as overall temperatures rise, extreme heat becomes more likely, and extreme cold less likely. Several studies have shown that warming is already influencing such heat waves.
Mora, a climate scientist at the University of Hawaii at Mona, and his colleagues wanted to see how various greenhouse gas emissions scenarios would affect the future occurrence of deadly heat waves, but found there wasn’t a good sense of how common they are now or over what threshold heat waves became deadly.
They pored over 30,000 studies and found nearly 800 cases of deadly heat events reported in 164 cities across 36 countries since 1980. “That was kind of exciting and scary at the same time to find so many of them,” Mora said.
From those historical events, the team found that the thresholds for deadly events varied from place to place based on the combination of temperature and humidity levels. Heat waves could lead to excess deaths at lower temperatures when humidity was sky high, as was the case during a 2010 Moscow heat event that killed thousands.
How the number of days of deadly heat will change under different scenarios of greenhouse gas emissions?
Humidity exacerbates the effects of high temperatures because it shuts down the body’s natural cooling system, preventing the evaporation of sweat.