The root problem is we keep dumping more CO2 into the atmosphere. Any biologist can tell you about cultures that choke on their own waste products. But you can't solve that on a state or even country basis. It takes everybody, and humanity has way too many "I've got mine, so screw you" types.
Adequate fresh water supply for California? Easily solved on a water district, county, or state level. You may have noticed that California has a long coastline, and most of the population lives near the coast. Most of the rest of the population lives in the Central Valley along Highway 99. So you build desal plants along the coast, and put one in Suisun Bay to serve the Central Valley. You've just covered almost everybody in the state. Small towns further inland have reasonable water resources. Hardly anybody lives in Death Valley, and there's plenty of Eastern Sierra water for them if LA stops depending on it.
It would actually take a lot of pressure off Nevada, Arizona, and California's Southeastern farmers near El Centro, if California stopped shipping Colorado River water to Southern California. California has priority because of very old agreements, but that ends up screwing over Mexican farmers since there is very little river by the time it gets to them. Of course, they live near the Gulf of California, so as long as we are cranking out desal plants at commodity prices maybe we build a few for them.