Urban planners could save lives by planting more trees. A new study -- the largest of its kind and the first to look at premature death caused by higher temperatures in cities -- found that increasing the tree cover to 30% in 93 European cities could have prevented 2,644 heat-related deaths over a summer period. The study authors looked at mortality and temperature data between June and August 2015, in a total population of 57 million, modeling the temperature with and without the urban heat island effect. Increasing tree cover to 40% would cool cities by a half of a degree Celsius and prevent 3,727 deaths. The European cities that most stand to benefit are those in Southern and Eastern Europe, which saw the highest temperatures and the highest heat-related mortality rates. Average city tree coverage is currently 14.9%.