Climatologists call it the “urban heat island” effect - when a heatwave transforms a city into a furnace where citizens swelter, suffer or even die.
The phenomenon is a major worry in hot countries where global warming will drive peak temperatures even higher in coming decades.
Urban planners and researchers, gathering in a parallel conference to UN climate talks in Bonn, turned a spotlight on the problem.
But they also pointed to an emerging arsenal to fight it, from planting trees to designing breeze-channelling buildings.
The urban heat island derives from simple physics.
Cities store heat from the Sun and traffic in concrete and tarmac during the day, and release it at night.
But in a heatwave, an infernal cycle starts up.
The night is not long enough for all the heat to dissipate, so at dawn the new day is already warm... and can only get warmer.
Without cooling or respite, old or sick people are especially at risk. A heatwave that gripped Europe in 2003 killed more than 70,000 people, according to peer-reviewed research.
“Urban heat island is a big problem for us,” said Catarina Freitas, head of sustainable environment management for Almada, a city south of Lisbon.
The difference between central Almada and its outskirts can be a whopping 4C (7.2F), she explained.
“In the summer of 2013, we were caught in a severe heatwave. We had 1,400 more deaths during this period compared to previous years... We had a night-time temperature of 33 degrees - we couldn’t cool down our bodies, it was a nightmare.”
Michael Boswell, a professor at California Polytechnic State University, said Los Angeles, a sprawling car-dependent city of 4mn people and few parks, has also found itself vulnerable.
“Currently, downtown LA experiences six days a year that exceed 95 Fahrenheit or 35 C,” he said.
“By 2050 they anticipate 22 days - and by 2100 they anticipate 54 days.”
So what can we do to roll back urban heat?
“The first thing that you can do is plant trees, to provide shade,” said Ingrid Coninx at Wageningen University and Research Centre in the Netherlands.
Another quick and relatively cheap weapon is moving water: streams running through cities are not only a source of pleasure - they also absorb a remarkable amount of heat from the air, said Coninx.
Other ideas are “green” and “cool” roofs - planting shrubs and trees on the tops of buildings, or painting or covering roofs to reflect sunlight.
Masdar City, a brand-new city being built in the United Arab Emirates and portrayed as a champion of urban sustainability, is being closely watched for new ideas.
“They are seeking to reduce heat architecturally, through the shaping of buildings and through wind towers, which are big, clever tubes that capture wind up above the roof line of the city and push it down to street level,” said Boswell.
Computer models for mapping city temperatures using satellites and instruments at ground level, are progressing fast.
In the near future, such data could be incorporated in real-estate apps.
