Extreme heat already affects almost two billion urban residents around the world, according to a new study.

The new research is the first to examine in fine detail global trends in extreme heat exposure across urban areas. The study spanned more than 13,000 settlements over nearly three and a half decades.

The authors found that exposure to dangerous temperatures increased by 200% since the mid 1980s, with poor and marginalized people particularly at risk.

“Our study reveals that exposure to extreme heat in urban areas is much more widespread—and increasing in many more areas—than we had previously realized,” says co-author Kelly Caylor, director of the Earth Research Institute at the University of California, Santa Barbara. “Almost one in five people on Earth experienced increases in exposure to urban heat over the past 30 years.”

The study in the Proceedings of the National Academy of Sciences is only the first of many that will delve into the rising threat of extreme heat and its impacts on society and the environment.

Lead author Cascade Tuholske was initially curious how climate change could directly affect urban food security, especially among low-income households.

“Many of these people are not necessarily food insecure in terms of, say, a calorie deficit, but they spend a huge percentage of their income on food,” he explains. This leaves them vulnerable, especially since extreme heat often drastically reduces labor output, and in turn, income and food security.

As a result, understanding urban food security required figuring out how many people extreme heat affects.

“I went through the literature and realized that we had no baseline understanding of where extreme heat is impacting individuals in cities at fine scales,” says Tuholske, now a postdoctoral research scientist at Columbia University’s Earth Institute.

Read the full article about extreme heat by Harrison Tasoff at Futurity.