Feb. 27 (UPI) -- Wildfires are getting bigger, more frequent and occurring during a larger portion of the calendar year. And new research suggests humans are largely to blame.
Researchers looked at records for every wildfire that required firefighting between 1992 and 2012. Of the 1.5 million blazes, humans were responsible for sparking 84 percent. Humans were also to blame for nearly half, 44 percent, of the acreage burned over the 20 years.
Lightning was still responsible for more than half of the acreage burned, but researchers say humans are expanding the "fire niche" into places where lightning isn't capable of starting wildfires.
"Humans are expanding fires into more locations and environmental conditions than lightning is able to reach," researchers wrote in their new paper on the subject, published this week in the journal PNAS. "Human ignitions have expanded the fire niche into areas with historically low lightning strike density."
Researchers say wildfire mitigation efforts would be well-served to focus on limiting the human-led expansion of the fire niche. Such a task will be increasingly difficult as more people move to the Sun Belt and the suburbs continue to expand.
The growth of the wildland-urban interface -- the percentage of homes and people living in and on the edge of wild lands -- is a major challenge for forest managers and those tasked with fighting wildfires. The interface is predicted to double from 9 to 18 percent by 2030.
"It's generally pretty well known that people start a lot of fires; everything from campfires to burning yard waste to accidental fires in homes and other structures," Bethany Bradley, a researcher at the University of Massachusetts Amherst, said in a news release. "But in the past, I used to think of 'wildfire' as a process that was primarily natural and driven by lightning. This analysis made me realize that human ignitions have an extraordinary impact on national fire regimes."
Natural wildfires sparked by lightning are largely relegated to the summer season, when conditions are most likely to be hot and dry. Manmade fires have extended the fire season into the fall and spring.
Researchers say global warming is likely to increase the risk of wildfire in many parts of the American West, magnifying the detrimental effects of humans on national fire regimes.
"We saw significant increases in the numbers of large, human-started fires over time, especially in the spring," Bradley said. "I think that's interesting, and scary, because it suggests that as spring seasons get warmer and earlier due to climate change, human ignitions are putting us at increasing risk of some of the largest, most damaging wildfires."