Using historic data, the team found that 30 percent of the world’s population sees at least 20 days each year that surpass the temperature and humidity thresholds for a deadly event at a given location.
Under even the most stringent cuts to emissions — cuts that are virtually unfeasible at present — that proportion would increase to half by 2100. If emissions aren’t curtailed at all and continue to increase, 75 percent would be under threat.
In particular, New York City would see 50 days over the threshold considered deadly, while Los Angeles would see 30, and Orlando and Houston would be above those conditions for the entire summer under the latter scenario. The increase in deadly heat days for various cities can be seen in an interactive map the researchers put together.
Mora called the results “mind blowing” and said they underscore the need for global action to curtail emissions of carbon dioxide, the main greenhouse gas. Such action is under threat with President Trump’s announcement this month that he will pull the U.S. out of the landmark Paris accord, though some cities and states are looking to fill the gap left by the federal government.
Mora said that even though deadly heat events will rise under more moderate warming, the need for action was akin jumping from a building. “You either jump from the 4th floor or you jump from the 8th floor,” he said.
The biggest increases in potentially deadly heat were found, somewhat surprisingly, in the tropics, where many developing countries are. While this region won’t see the biggest increase in temperature, conditions are already so hot and humid that a small rise can tip them over the edge.
Of course, just because more heat events will surpass the deadly threshold, they won’t necessarily cause excess deaths, as society can adapt to hotter conditions. This is more likely to occur in developed countries, which have an edge because of more widespread access to air conditioning, clean water and quality health care.
Developing countries, though, often lack the resources to fund adaptation efforts. Some such countries are looking for ways to reduce deaths and illness associated with extreme heat, though; several cities in India are instituting heat health action plans to better warn residents of the risks of heat waves and have shown success in reducing deaths.
Mora hopes this study and others like it will show policymakers that they need to take action to curb warming, and the sooner the better. The best way to deal with the rise in deadly heat “is to prevent it from happening,” he said. “Let’s try not to let the lion out of the cage.”