more long term, I need to consider climate change, fires, floods, and landslides.
You're lumping a lot of disparate things together, and you're also not considering the difference between different areas in Southern California, which is a big place.
Re climate change, today's temperatures in So Cal range from a high of 73 F in coastal areas to 96 F in Riverside. This difference is pretty much a standard thing, on average, every day of the summer. In terms of quality of life, climate change is not a comparable concern in these two areas. There are reasons that the Inland Empire has always had cheaper housing -- hot weather is one of them, along with the fact that it doesn't have as many jobs, so many people who live there end up with awful commutes.
Floods and landslides are purely local issues in So Cal. There were some big floods in 1938, but this led to the construction of big debris-control dams throughout the front range of the San Gabriels. Today, flooding is not a realistic concern except for a very tiny percentage of the population that lives in specific places at the urban-wilderness interface, e.g., a dozen houses at the mouth of Little Santa Anita Canyon. Flooding in So Cal is not a common, chronic, predictable problem like it is in places like riverfront properties in the midwest where people simply built in places where they should never have built.
Landslides are a big issue if you have a house built on a steep hillside. Houses built on this type of terrain are only a small fraction of the housing stock. If you're concerned, don't buy a house on a hill, or get a geological study before you buy.
Fires are more of a concern. They usually start in wilderness areas but then sometimes spread to populated ones. You buy insurance, and if the order comes to evacuate, you do it.
How are informed people making decisions in the face of a hot market and the risk of climate change
Here you're mixing together two different time scales. The hot market probably does make this a really bad time to buy a house if you can avoid it. Yes, you might be wise to rent for a few years for this reason.
Climate change isn't going to render So Cal uninhabitable on short time scales. If you're young and expect to stay where you are for 50 years, then you might want to settle in a coastal part of So Cal where this is less of a concern, not an inland area.
A lot of the problems you refer to (wildfires, flooding, landslides) are results of the fact that So Cal is bounded by the ocean on one side but by beautiful mountains on the other. I can roll out of bed in Fullerton before dawn on a weekend and be hiking in the alpine zone by noon. This is a great problem to have. If you want to cut your individual risk, don't live at the urban-wilderness interface itself, but just go to the wilderness for recreation.
You write that:
I want to own for a long time
But in a comment, you say you're concerned about:
risk of climate change leading to homes not gaining value, or even losing value, in the long run.
There have been downturns in the So Cal real estate market, such as in 1995 and 2008. These were temporary. Once you've settled in a certain area with the intention of living there indefinitely (as you say you do), the value of the real estate you're living on is meaningless Monopoly money.
Real estate is simply very expensive in coastal areas of the US that are desirable to live in and where there are a lot of high-paying jobs. The reason it's expensive is because a lot of people would like to live in these areas. You have to make your own decision about whether you want to pay $900k for a house in So Cal that would be $150k in small-town Ohio. Which lifestyle do you want? Do you have a job offer in Ohio that looks similarly appealing?