The United States spends billions of dollars every year fighting wildfires. A recent paper published in PNAS finds that human-started conflagrations account for 84 percent of all wildfires in the US in recent years. These human-started wildfires have tripled the length of the annual fire season and have dominated a geographic area that is seven times larger than the region affected by lightning-started fires. Overall, human-started fires were responsible for nearly half of all the land that was burned over the two-decade period of study.

The study analyzes wildfire data from 1992 to 2012, focusing only on wildfires that needed an agency response to manage or suppress and were a threat to ecosystems or infrastructure. The fire data came from the publicly available US Forest Service Fire Program Database, which includes US federal, state, and local records for both public and private lands. Some fires are burns set intentionally for agricultural purposes, but the researchers excluded those. They also excluded fires with an unknown cause and only concentrated on fires that were started either by humans or by lightning.

Despite these exclusions, there were still 1.5 million wildfires included in this analysis. Humans started 84 percent of them, primarily in the mountains of the western US. Meanwhile, 60 percent of the total land area of the continental US was affected by human-started wildfires, while only 8 percent of the land area was affected by lightning-started fires.

Historically, the wildfire season has been approximately 46 days during which conditions would allow lightning to start fires. In recent years, however, human-started fires have lengthened the fire season to 154 days. The authors report that this longer fire season is driven by fires that are started by humans earlier in the spring on the East Coast and fires started later in the summer on the West Coast. Human-started fires also appear to be evenly distributed throughout the year, with about a quarter of the fires occurring each season and small increases in the spring and summer. Additionally, the data shows that the most common day for human-started fires is the Fourth of July, a national holiday marked by fireworks.

By comparison, lightning-sparked fires did not typically fall outside the traditional fire season. According to the study, 78 percent of lightning-ignited fires started in the summer, which is historically the biggest season for fires (only 9 percent were sparked in the spring and 12 percent were sparked in the fall). The most common day for lightning-sparked fires was July 22, a date that appears to occur arbitrarily and is near midsummer.

We also start fires when the humidity and temperature are not necessarily at peak fire risk—human-started fires occur even when the fire risk doesn’t seem to be particularly high due to weather conditions. The average moisture conditions were significantly different for human-started fires and lightning-started fires.

The authors conclude that humans appear to play a serious role in increasing the frequency and severity of US wildfires. Though other literature shows that climate change has extended the fire season in other parts of the world, this data argues that there's another mechanism by which humans are responsible for changes in the US fire season. So, while climate change may influence the extent and severity of the burns, the ignition seems to come down to carelessness.

Nature Neuroscience, 2017. DOI: 10.1073/pnas.1617394114 (About DOIs)