Exceptionally hot days and rainfall extremes have become more common in urban areas in the last 40 years, while the number of cold spells has declined, said a study published on Friday.

Heatwaves have significantly increased in more than 200 cities across the world, with the largest number of hot spells occurring in the last decade.

“Over half of the world’s population now live in urban areas; hence, it is particularly important to understand how the climate and climate extremes … are changing these areas,” Vimal Mishra, lead author of the study, said in a statement.

The number of people moving to cities has been steadily growing and the U.N. predicts that 70 percent of the world population will be living in cities by 2050.

Last year tied with 2010 as the hottest on record, according to other data released earlier this week.

The statement linked a long-term trend to rising temperatures in recent decades to human emissions of greenhouse gases.

Increased heatwaves have a negative impact on the health of elderly people and children, sometimes resulting in deaths, while more frequent and severe droughts and floods compromise water supplies.

The urban poor, who often live in flood- and landslide-prone areas bear a disproportionate burden of the effects of extreme weather.

“Urban areas make up relatively small part of the global land area; however, they are the centre of wealth, so damage to urban infrastructure could result in potentially large economic losses,” said Mishra.

In addition to heat waves lasting for six consecutive days or more, more than half of the cities also recorded an increase in the number of individual extremely hot days.

At the same time urban areas saw a decline in cold spells, with 60 percent of cities experiencing a significant decline in extremely windy days.

The study, published in Environmental Research Letters journal, analysed 217 urban areas with population over 250,000 people.

This article is published in collaboration with Thomson Reuters Foundation. Publication does not imply endorsement of views by the World Economic Forum.

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Author: Magda Mis is a Production Editor at the Thomson Reuters Foundation

Image: A view of a partially dried-up pond at a village of Guangnan county. REUTERS/Stringer