Comparing the numbers of human and lightning caused wildfires

During a 21-year period 84 percent of the wildfires in the United States were caused by humans, but the ratio varies greatly across the country.

lighting human caused wildfires

A study published by the National Academy of Sciences looked at the causes of wildland fires, human vs. lightning, and their occurrence geographically and seasonally. The researchers analyzed 1.5 million fire occurrence records from 1992 to 2012.

I was interested in reading the paper after having been attracted to the compelling graphics comparing the numbers of fires caused by humans and lighting, ecoregion by ecoregion over time.

The research was conducted by Jennifer K. Balch, Bethany A. Bradley, John T. Abatzogloue, R. Chelsea Nagy, Emily J. Fusco, and Adam L. Mahood.

lighting human caused wildfires
Frequency distributions of wildfires by ecoregions, ordered by decreasing human dominance. Click to enlarge.

You might have noticed a large short-lived spike in the number of human caused fires in several of the ecoregions around June-July. That represents ignitions caused by fireworks on the Fourth of July.

Below is an excerpt from the research:


“In conclusion, we demonstrate the remarkable influence that humans have on modern United States wildfire regimes through changes in the spatial and seasonal distribution of ignitions. Although considerable fire research in the United States has rightly focused on increased fire activity (e.g., larger fires and more area burned) because of climate change, we demonstrate that the expanded fire niche as a result of human-related ignitions is equally profound. Moreover, the convergence of warming trends and expanded ignition pressure from people is increasing the number of large human-caused wildfires. Currently, humans are extending the fire niche into conditions that are less conducive to fire activity, including regions and seasons with wetter fuels and higher biomass.

“Land-use practices, such as clearing and logging, may also be creating an abundance of drier fuels, potentially leading to larger fires even under historically wetter conditions. Additionally, projected climate warming is expected to lower fuel moisture and create more frequent weather conditions conducive to fire ignition and spread, and earlier springs attributed to climate change are leading to accelerated phenology. Although plant physiological responses to rising CO2 may reduce some drought stress, climate change will likely lead to faster desiccation of fuels and increased risk in areas where human ignitions are prevalent.”

(end of excerpt)


You can download the paper HERE (it is a large 13 Mb file).

Author: Bill Gabbert

After working full time in wildland fire for 33 years, he continues to learn, and strives to be a Student of Fire. Google+

3 thoughts on “Comparing the numbers of human and lightning caused wildfires”

  1. Gee. Maybe Smokey Bear’s successful wildfire campaigns during the 1950s thru 1990 needs some renewed emphasis and financial support by all federal state and local government fire protection agencies. “One less Spark” without Smokey Bear as the “spokes bear”—-really? “Only You….

  2. The number of wildfires by ignition source–human or lightning–is an interesting data set. What would be more of interest is the acres burned by ignition source.

  3. Interesting conclusion: “Land-use practices, such as clearing and logging, may also be creating an abundance of drier fuels, potentially leading to larger fires even under historically wetter conditions.

    They did not study logging effects in this paper, at least I didn’t see any analysis done for that land use, yet they conclude that logging may be creating an abundance of drier fuels. I think there is author bias here.

    The fact is that logging on federal lands has decreased dramatically in the last two decades however the bulk of large forest fires are on federal lands.

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