LONDON - Scorching summer days are growing hotter in the world's big cities at a significantly faster pace than the average rise in world temperatures - a trend that could mean more deadly urban heatwaves in years ahead, scientists said.
In cities such as Paris, Houston, Moscow and Beijing, the level of heat on the hottest summer days is growing two or three times as quickly as general temperature rises linked to climate change over the last 50 years, said researchers at the University of California-Irvine.
The trend is particularly pronounced in Europe, East Asia and parts of Australia, they said in a report released this week in the journal Earth's Future.
Worsening heat extremes are already one of the world's biggest health threats, health officials say.
With more than half of the world's population now living in cities - and more than 65 percent of people expected to live there by 2050 - rising city heat extremes could put billions at risk, particularly the poor, the researchers said.
"There are more than a billion people living in extreme poverty, with many of them living in megacities and large urban centers. These are people struggling to survive," said Simon Michael Papalexiou, an environmental engineer and the lead author of the study.
Most such people have no access to air conditioning or other alternatives to protect themselves, he said in an interview with the Thomson Reuters Foundation.
With global income inequality on the rise, that suggests the number of urban poor without the resources to cope with extreme heat will continue to grow, Papalexiou said.
The study, which looked at data from 9,000 weather stations around the globe, found that average global temperatures have risen an average of 0.2 degrees Celsius per decade over the last 50 years.
But in Paris, the increase per decade of the hottest temperatures recorded was nearly 1 degree Celsius, researchers found, with cities such as Barcelona, Houston, Moscow and Beijing also seeing big hikes.
Such increases have contributed to heatwave disasters, including almost 70,000 deaths in Europe in 2003, and about 55,000 deaths in Russia in 2010, the report said.
But the bigger worry is what will happen in decades to come as more people crowd into big cities, the authors said. In the last two decades, the number of cities with at least 5 million people doubled, they said.
"This dramatic urbanisation, which has occurred more rapidly in the least-developed parts of the world, will aggravate the impacts of extreme weather events and increase the risk of heat-related fatalities in the future," the report noted.
Amir AghaKouchak, a civil engineer and co-author of the study, said growing urban heat risk will require city authorities to think about new measures save lives - something some are already putting in place.
"In France after that massive heatwave (in 2003), now all nursing homes or places where there are a lot of vulnerable people have to have at least a common room with air conditioning," he said. "That can be done and it's already happening in some places."
"But some countries don't have the resources to do that," he noted in an interview.
Papalexiou said that making cities greener and "smarter" - such as changing architectural styles and providing more green areas with trees and plants that can lower temperatures - also will be crucial to reducing the threat.
"It's the social, economic and political factors" that will largely determine how resilient cities of the future are to the growing heat risk, he said.
Ultimately, however, to control the risks, "you need to look at the big picture and more sustainable solutions, and that means reducing our (climate changing) emissions", AghaKouchak said.