The American Southwest has been experiencing extreme summertime heat this week. But some cities, like Phoenix, are also "urban heat islands."
That's a phenomenon where temperatures go up in areas covered in heat-retaining asphalt and concrete, creating health dangers for residents.
Officials in Phoenix and other cities are tackling the issue by planting trees, capturing rainwater and promoting lighter-colored asphalt and other reflective construction materials.
But climate researchers say much more is needed to counter the urban heat island effect causing a warming trend in the world's major cities, from Dallas to Mumbai to Nanjing, China.
Dr. Brian Stone of the Georgia Institute of Technology's Urban Climate Lab says cities need to take urban warming into consideration when making land-use decisions. And hundreds of thousands of trees should be planted to cool off urban centers.