(Bloomberg) -- London’s “heat island” — high temperatures generated by built-up urban environments — kills more people than any other European city, as global warming intensifies the threat.

Summer heat caused the death of 664 people in the UK capital, or almost 10% of the total across 93 European cities, according to new research led by the Barcelona Institute for Global Health. Madrid had the second-highest toll, but with a higher mortality rate.

Climate change is driving more extreme weather, with record-breaking heat across London last summer crippling power grids and transport infrastructure. The study shows cities were on average 1.5 degrees Celsius hotter than the surrounding countryside as buildings, roads and pavements trap the sun’s heat. That elevated heat accounts for 4.3% of all premature deaths, ISGlobal said.

“Predictions based on current emissions reveal that heat-related illness and death will become a bigger burden to our health services over the next decades,” said ISGlobal researcher Tamara Iungman, one of the authors of the study.  

The study was based on data from 2015, with the Romanian city of Cluj-Napoca suffering the highest mortality rate.

Britain breached 40 degrees Celsius (104 degrees Fahrenheit) for the first time last year, with the Office for National Statistics estimating 2,227 excess deaths in the UK from the record breaking heat. 

Planting trees to cover 30% of the urban environment would cool cities and cut mortality rates by one-third, according to the study. Cities in southern and eastern Europe — where death rates are the highest — would benefit the most.

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