Angelenos high on the income ladder go everywhere in air-conditioned cars, leaving the city’s buses and baking sidewalks largely to those on the lower economic rungs. Citing the impact of climate change, Mr. Hawthorne said: “This city is noticeably less hospitable to pedestrians now than it was when I got here in 2004. So 15 years has changed this conversation.” Mr. Hawthorne has been leading the effort to bring shade to nearly 750 bus stops, utilizing data that overlays the hottest areas of the city with the locations of the busiest bus stops.

Researchers at U.C.L.A. have forecast that Los Angeles is likely to see a sharp increase in the number of days of extreme heat — defined as 95 degrees or higher . Downtown Los Angeles currently experiences about seven days of extreme heat per year, but that figure could rise to 22 by 2050 and to more than 50 days by the end of the century, according to forecasts. (Of course, temperatures don’t need to soar above 95 degrees for a lack of shade to be a burden on the city’s poor.)

Like Mr. Hawthorne, Rachel Malarich, whom Mr. Garcetti hired earlier this year as Los Angeles’s first forestry officer, is trying to bring shade to the city’s underserved communities, particularly in South Los Angeles and East Los Angeles, by planting more trees.

“These communities should have access to the same resources other communities have,” she said. “I don’t want a bunch of small trees. We need to find spaces for big trees.”