Human ignitions + WUI/housing growth

  1. Humans drive most ignitions and expand the fire season (“fire niche”).
    Balch et al. (PNAS, 2017) analyzed >1.5 million wildfire records and found human ignitions account for the large majority of fires and substantially extend the season and geography of fire occurrence.

  2. WUI (housing growth) increases ignitions and the number of homes at risk.
    Radeloff et al. (PNAS, 2018) links rapid WUI growth largely to new housing and explicitly notes that WUI growth “often results in more wildfire ignitions,” increasing exposure.

  3. California-specific: human activities alter fire regimes; WUI ignitions matter.
    Syphard et al. (Ecological Applications, 2007) (“Human influence on California fire regimes”) is a classic cite for the claim that human ignitions at the WUI materially alter fire regimes and risk—i.e., it’s not just “biophysical/climate variables.”

  4. Population density ↔ human ignitions ↔ fire frequency (with threshold dynamics).
    Syphard et al. (Conservation Biology, 2009) discusses the relationship between population density and ignition/frequency (often rising with density up to a threshold, then sometimes declining where fuels are fragmented/suppressed). This is helpful because it’s nuanced—but still supports “more people → more ignitions.”

  5. Large fires: humans ignite many (and often under different conditions).
    Nagy et al. (Fire, 2018) finds humans ignite many more large fires than lightning and are a dominant source of large fires in multiple regions—useful when agencies try to hand-wave human causation.

  6. Ignition prevention is key because humans dominate ignition sources.
    Keeley & Syphard (2018) review historical ignition sources in California and emphasize humans as a dominant ignition source and the logic of prevention.

Development drives ignitions/exposure

  • WUI growth around public lands (National Forests as hotspots for WUI growth). This supports cumulative risk arguments tied to permitting and growth pressure.

  • Global WUI expansion & human ignition prevalence (Science Advances, 2024) provides broad support that WUI fires frequently originate from human ignitions.

  • Cross-boundary fires often start from human ignitions on private lands (Scientific Reports, 2022)—useful for “private development patterns create public fire burdens.”

  • WUI/housing growth and wildfire ignitions (Ecological Applications, 2022) reinforces the general point in a more current synthesis.