EXCLUSIVE!! Over the last 40 years, the number of days with extreme heat has increased in cities.

Cascade Tuholske is a postdoctoral research scientist at Columbia University’s Center for International Earth Science Information Network (CIESIN); Chris Funk is the director of the University of California Santa Barbara’s Climate Hazards Center; and Kathryn Grace is an associate professor of Geography, Environment, and Society at the University of Minnesota. This article first appeared on…