To generate this layer, I first dissolved the CA wildfire polygons (CAL FIRE) and then intersected them with the state county boundaries (California State Board of Equalization). I used the calculate geometry tool to calculate the total wildfire area in US Survey Acres for each county in the intersected layer, then I used the summary statistics tool to generate a new table with this information. I joined this table with the county table to fill a new field with the wildfire area, then I removed the join. I added a new field where I calculated the percentage of the area burned by wildfires to the total area of each county using calculate field. I used graduated colors to represent this field.

The CAL FIRE dataset does not include San Francisco, so I had to account for this discrepancy while joining the tables.