Summer days are growing hotter in the world's big cities at a significantly faster pace than the average rise in world temperatures.
It is a trend that could mean more deadly urban heatwaves in years ahead, scientists have claimed.
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The trend is particularly pronounced in parts of Australia, Europe and, East Asia, a report released in the journal Earth's Future said.
In cities such as Paris, Houston, Moscow and Beijing, 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.
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 centres. 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 told the Thomson Reuters Foundation.
The study found that average global temperatures have risen an average of 0.2degC per decade over the last 50 years.
But in Paris, the increase per decade of the hottest temperatures recorded was nearly 1deg 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 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 reported.