Between global warming and the urban heat island effect, many of the world’s cities are heating up. In fact, extreme heat already affects almost two billion urban residents worldwide, according to a new study led by former UC Santa Barbara graduate student Cascade Tuholske(link is external).

The paper, published in the Proceedings of the National Academy of Sciences(link is external), 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,” said co-author Kelly Caylor(link is external), director of the university’s Earth Research Institute and Tuholske’s doctoral advisor. “Almost one in five people on Earth experienced increases in exposure to urban heat over the past 30 years.” The new study is only the first of many that will delve into the rising threat of extreme heat and its impacts on society and the environment.