A helicopter whisks off a rooftop in downtown Los Angeles, climbs above a thin layer of haze and soars over barren mountains past the city’s edge. Soon, scars of climatic stress are evident to L.A. Mayor Eric Garcetti and Martin Adams, general manager and chief engineer of the city’s water and power department, as they peer out the windows. Trees torched years ago by wildfire. Flats parched by sun and little precipitation.