Whether the other blazes were set intentionally or by accident, experts say it's highly likely that humans are to blame. Two people were arrested north of San Diego on Thursday on suspicion of arson, though it's not clear if they are thought to be connected to this week's big fires. (Related: What's Behind Early Season Winds Fueling Southern California Wildfires?")
Unlike remote parts of the world where natural events like lightning strikes are prime sources of wildfires, in southern California, such fires are almost always started by people. Ninety-five percent have a human cause, according to Cal Fire, the state's firefighting agency.
The situation may worsen in the face of expected population growth. Metropolitan San Diego's population is expected to reach nearly 4.5 million by 2050, over a million more than today. (Pictures: San Diego Wildfires)