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).

Some western states hit hard by lightning Sunday

lighting strike map
Lightning that occurred during the 30-hour period that ended at 6 a.m. MDT July 23, 2018. The lighter colors are the most recent. The map shows all strikes. Approximately 20% are usually cloud to ground. Click to enlarge.

In spite of the weather forecast, the lightning predicted for Southwest Oregon on Sunday didn’t really pan out, but portions of other western states received thousands of strikes. Firefighters may have some followup to do in Northeast Montana, Southwest Wyoming, Utah, Colorado, Nevada, the Sierras in California, and states in the Great Plains.

During the rest of this week Southern California will likely experience temperatures much higher than normal, possibly setting a few daily records. Weak hot and dry sundowner winds could occur near Santa Barbara bringing compressional heating and high fire danger.

Here’s more weather information from the National Interagency Fire Center, issued July 23:

A critical fire weather scenario will begin to set up as shards of monsoonal moisture begin to filter further into the Pacific Northwest and the Northern Rockies. These storms, while still isolated, will be dry and should trigger new starts as the stage begins to set for the next few days after [Monday], which should be pretty convective.

Abundant lightning predicted for Southern Oregon Sunday

The National Weather Service is describing the lightning that is in the forecast for south-central Oregon and extreme northern California as “abundant”. It should begin by early Sunday afternoon and continue into the evening.

Lightning is also predicted for southwest Idaho and northern Nevada. There could be wetting rain with the thunderstorms except in Nevada, where dry strikes could occur. All of these areas are under a Red Flag Warning Sunday.

Some land managers in Oregon have brought in additional firefighting resources to assist in initial attack of any new lightning-caused fires.

Southern California lightning

(Originally published at 1:33 p.m. MDT September 11, 2017)

Jeff Zimmerman took these photos September 10 and sent them to us today. Here is how he described the event.

From the deserts to the sea, a wonderful display of lightning. Off the coast near Avalon where hundreds of strikes were recorded with numerous strikes all the way to Tehachapi Mountains. Tejon Ranch and Highway 58 area was bathed in lightning too. A new fire was reported on the Los Padres National Forest, lightning strike near Chuchapate (Sawmill). Possibly a few more isolated storms today, followed by gusty NW winds later this week. The first Santa Ana may set up when snow comes to Montana and offshore flow begins later this week bringing critical fire weather with it. Attached are a few shots from the desert in Neenach, CA (map).

lightning Jeff Zimmerman

lightning Jeff Zimmerman

Thanks Jeff!

Thousands of lightning strikes in southern California

(Above: lightning strikes in the 24 hours before 1 p.m. PDT October 24, 2016.)

Thousands of lightning strikes have occurred in southern California between Sunday afternoon and Monday afternoon.