If they’re serious, switch to white or highly reflective roofing on every building, and use nothing but concrete for roads and parking lots. This will only serve to minimize the “heat island” effect of highly urbanized areas. It will not change the climate. If they want a different climate, they need to move.
Ignoring all the global warming nonsense, there is no arguing with the fact that heat absorbing materials like roofs and asphalt create urban heat islands.
There are various white roof coatings that are deemed “cool roofs” which are effective. Most roofs are not structurally strong enough to put grass or sod or seedums on them. My preference is a 90% reflective aluminized mesh fabric used to shade greenhouses that could be attached to the parapets of commercial roofs and place everything on the roof like air conditioning equipment as well as the surface itself in perpetual shade. It is cheaper than the coatings and easily replaced every five years or so. Cooling the roof significantly reduces the electricity bill for air conditioning in the summer months so there is a cost-savings rationale regardless of any “city-cooling” goal.
One thing that LA has going for it is its proximity to the ocean. It seems like a no brainer to put in a pipeline to pump cold seawater inland. Then it can be pumped through sprinkler systems along the streets to wet them down every morning. The streets get cleaned, the water flows back out to sea through the storm drains, and the asphalt stays cool through evaporation. Plants absorb the water vapor and reduced watering is needed. The evaporative cooler effect would extend well above street level. I’ll have to look into what damaging effects salt water has on asphalt. I know it isn’t great for concrete.
Beyond that, cities should build new salt water ponds surrounded by salt tolerant ice plants and other vegetation. The evaporation and transpiration through the plants is effective at cooling outdoor areas. Since the “riverbeds” in LA don’t actually carry drinkable or irrigation water, they are simply for storm water management, during summer they could be kept running with sea water. Of course, the environmentalists would object to killing off all the fresh water mosses and worms and frogs that currently thrive in them.