The intensity of heatwaves in Europe has also increased by 1 to 2 degrees Celsius since 1950, WWA said.
More than 35,000 people died during a European heatwave in 2003, and tens of thousands perished in Russia during extreme heat in 2010.
The threat is particularly severe in already sweltering places, from South Asia to the Gulf.
"It is critical that cities work with scientists and public health experts to develop heat action plans," said Robert Vautard, a researcher at the Laboratory of Climate and Environmental Sciences.
"Climate change is impacting communities right now and these plans save lives."
During Lucifer, emergency admissions to hospital in Italy increased by 15 percent, WWA said.
Europe's death toll from weather disasters, including heatwaves, wildfires and drought, could increase 50-fold by the end of the century, The Lancet Planetary Health journal said in August.
Governments in 2015 agreed to limit warming to "well below" 2 degrees Celsius above pre-industrial times, while pursuing efforts for a 1.5 Celsius limit.
- Thomson Reuters Foundation