More, bigger wildfires burning western U.S.: study

The trend of wildfires across the western United States becoming larger and more frequent over the last 30 years could continue as climate change causes temperatures to rise and drought to become more severe in the coming decades, notes a new study.

The number of wildfires of more than 1,000 acres in size in the region stretching from Nebraska to California increased by a rate of seven fires annually from 1984 to 2011, notes the study, Large wildfire trends in the western United States, 1984-2011.

The study was accepted for publication in Geophysical Research Letters, a journal published by the American Geophysical Union (AGU).

AGU reports that study authors used satellite data to measure areas burned by large fires since 1984, and then looked at climate variables, such as seasonal temperature and rainfall, during the same time.

The 2011 Las Conchas Fire in New Mexico – which burned 150,874 acres – was one of hundreds of fires looked at in the study, AGU notes in a statement issued last week. For all fires considered, total area burned increased at a rate of almost 90,000 acres a year (an area the size of Las Vegas), while individually, the largest wildfires grew at a rate of 350 acres a year.

“We looked at the probability that increases of this magnitude could be random, and in each case it was less than 1%,” says Philip Dennison, an associate professor of geography at the University of Utah and the paper’s lead author.

“Twenty-eight years is a pretty short period of record, and yet we are seeing statistically significant trends in different wildfire variables — it is striking,” Max Moritz, study co-author and a fire specialist at the University of California-Berkeley Cooperative Extension, says in the statement.

Among other things, the researchers found the following:

• most areas with increases in fire activity also experienced increases in drought severity over the same time period;
• there was an increase in both fire activity and drought over a range of different ecosystems across the region; and
• the trends suggest large-scale climate changes, rather than local factors, could be driving increases in fire activity.

“Most of these trends show strong correlations with drought-related conditions which, to a large degree, agree with what we expect from climate change projections,” Moritz says.

Jeremy Littell of the U.S. Geological Survey (USGS) at the Alaska Climate Science Center, a research ecologist not connected to the study, says the fire activity trends reported in the paper resemble what would be expected from rising temperatures caused by climate change.

Littell and Moritz say increases in fire activity in forested areas could be at least a partial response to decades of fire suppression. “It could be that our past fire suppression has caught up with us, and an increased area burned is a response of more continuous fuel sources,” Littell suggests in the statement. “It could also be a response to changes in climate, or both.”

Study authors found the rise in fire activity was the strongest in certain regions of the U.S.: across the Rocky Mountains, Sierra Nevada and Arizona- New Mexico mountains; the southwest desert in California, Nevada, Arizona, New Mexico and parts of Texas; and the southern plains across western Texas, Oklahoma, Kansas and eastern Colorado.

Dennison notes these are the same regions that would be expected to be most severely affected by changes in climate.