It’s not only economic inequality that’s increased in the city of Phoenix, Arizona. In the last 40 years, environmental inequality has increased, too.

The availability of foliage and vegetation — and the cooling effects they provide — now tracks with income. In 1970, there was no relationship.

“Historically there were more trees, more equitably distributed,” said University of California, Riverside ecologist Darrel Jenerette.

In a study published in Ecological Applications in October, researchers led by Jenerette and urban ecologist Chris Martin of Arizona State University crunched census data...