Urban landscapes intensify the effects of dangerously hot and humid days. But scientists only broadly understand the interplay between escalating urban population growth and extreme heat exposure.

A new study in the journal PNAS helps address that gap, and the news isn't good.

Fine-grained temperature and humidity data for more than 13,000 cities worldwide show extreme heat exposure tripled from 1983 to 2016.

"It shows the magnitude of how extreme heat and population change are converging in urban areas across the planet," said lead author Cascade Tuholske of Columbia University.

Those three decades have seen hundreds of millions of people move from rural areas to cities, which now house more than half the world's population. Half of those city-dwellers now experience the effects of extreme heat, which raises rates of sickness and death. 

By itself, urban population growth explained two-thirds of the exposure spike, whereas warming contributed only a third, although proportions varied by city and region.

In many urban areas, the tally of dangerous days far exceeded previous estimates.

"In official extreme hazard databases, there are two documented extreme heat events for the continent of Africa since 1900, which we know is not true," said Tuholske.

Researchers pulled data from a combination of infrared satellite imagery and ground instrument readings.

Tucson, Douglas and Sierra Vista were among the Arizona cities affected, as were Globe and Camp Verde. Extreme heat appears to affect the drier Sun Belt cities less, at least for now, though Tuholske says it still poses a real and growing anger to Arizona.

"But I actually think that there is some good news, because with dry heat, I think we do have some more options for adaptations and policies to reduce the impact."

Roughly 40 sizable cities in the United States, located mainly in Texas and around the Gulf Coast, experienced greater heat exposure.

Globally, the roster of the hardest-hit included Dhaka, Bangladesh; Shanghai and Guangzhou, China; Yangon, Myanmar; Bangkok; Dubai; Hanoi; Khartoum; and a number of cities in Pakistan, India and the Arabian Peninsula. Higher heat impacts in those cities were driven mainly by population growth.

Elsewhere, warming climate played an as-big or bigger role in driving dangerous heat levels. Such cities included Baghdad; Cairo; Kuwait City; Lagos, Nigeria; Kolkata, India; Mumbai, India; and other big cities in India and Bangladesh.

In Europe, where urban populations have remained fairly steady, exposure upticks stemmed mainly from global warming.

In many parts of the developing world, urbanization is viewed as supporting growing economies. But studies like this one suggest the loss of productivity due to intolerable temperatures might tip the balance the other way, negatively affecting people's ability to work and driving down economic output.

Despite the bad news, Tuholske says the study should not be seen as a forecast of gloom and doom.

"This is a map of where people need help, and we have so many resources to help those people," he said.