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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 these.

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.
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