In July and August 2016, the Roaring Lion fire devoured more than 8,000 acres of forest, along with over 60 homes and outbuildings in eastern Montana's Bitterroot Range. Here, the fire burns through dense conifers on 31 July 2016. Photo: Mike Daniels

10 October 2016 (University of Idaho) – Human-caused climate change has nearly doubled the amount of land burned in western U.S. forest fires over the past three decades, according to a new study by researchers at the University of Idaho and Columbia University.

The researchers estimate that human-caused climate change caused an additional 16,000 square miles of western forest lands to burn between 1984 and 2015. That’s about the size of the Bitterroot, Clearwater, Kootenai, Panhandle and Nez Perce National forests combined, or more than 30 times the size of the city of Los Angeles.

“We’re no longer waiting for human-caused climate change to leave its fingerprint on wildfire across the western U.S. It’s already here,” said John Abatzoglou, the study’s lead author and an associate professor of geography in UI’s College of Science. “Over the last several decades we’ve seen longer fire seasons, larger fires and more area burned — and those observations led us to ask, ‘Why?’ What we found was that human-caused climate change played a resounding role in observed increases in forest fire activity.”

The study, which was published today in the online early edition of the Proceedings of the National Academy of Sciences, is the first to quantify the degree to which human-caused climate change has increased wildfire activity in western U.S. forests. The study examined forested regions of the lower 48 states from the Rockies to the Pacific.

“Knowing that human-caused warming is responsible for approximately half of the western U.S. forest fire area in the past few decades, and understanding that this effect is becoming increasingly dominant, helps us better anticipate continued changes in forest fire activity in the coming decades,” said co-author Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory. “This knowledge will allow us to make more educated fire and land management decisions.”

The United States has seen dramatic fire seasons in recent years. In 2015, 10.1 million acres burned across the country — the largest since the National Interagency Fire Center began documenting wildland fire area in 1983. Federal firefighting costs for the 2015 fire season hit a record-high $2.1 billion.

The uptick in fire activity over the past decade has led to some calling recent wildfire seasons the “new normal.” Abatzoglou and Williams goal was to parse out how much of the observed increase in the extent of forest fires was due to natural climate variability and other factors, and how much was rooted in human-caused climate change. They chose to focus on forest fires, since grassland fires respond differently to climate variability.

Abatzoglou likened the many factors that lead to forest fires to instruments in an orchestra: When any instrument plays louder, the volume of the whole orchestra increases. His and Williams’ research finds that both human-caused climate change and natural variability have been playing with increasing volume in recent decades, leading to record-breaking fire seasons.

The history of fire suppression and forest management practices in parts of the West has also played an important role in the increase. Fewer fires have led to overgrowth in many forests, so there is now more fuel available to burn when conditions get dry.

"The legacy of fire suppression has probably amplified the effect of climate change on forest fire. Opposite to how climate change would have zero effect on forest fire activity if there was zero fuel to burn, climate change probably has had an amplified effect of forest fire activity due to an artificially high number of trees in many forested areas due to past fire suppression,” Williams said.

Increased temperature and reduced relative humidity facilitate fire activity by drying out timber. Such conditions allow fires to start and spread and make them more difficult to suppress.

“Wildfire is a function of several processes, some natural and some human. But what we know is on a year-to-year basis, warm, dry summers enable large fire seasons,” Abatzoglou said. “There is a remarkable relationship between the extent of forested area burned and fuel dryness that allows us to implicate climate as the preeminent driver of annual variability in forest fire over the past three decades.”

Abatzoglou and Williams complied data from eight measures of aridity, or dryness, in western U.S. forests. They then used climate model experiments to identify how human-caused climate change influenced these aridity measures.

They found that spring and summer temperatures warmed by 2 to 2.5 degrees Fahrenheit (1 to 1.5 degrees Celsius) since 1950, which is in agreement with the warming that climate models attribute to human-caused climate change. This warming led to significant drying within western U.S. forests, accounting for approximately 55 percent of the documented increases in forest aridity from 1979 to 2015. The remaining 45 percent was due to natural climate variations that caused reductions in humidity and precipitation in some regions.

This influence has been particularly notable since the turn of the century: Abatzoglou and Williams estimate that since 2000, human-caused climate change led to a 75-percent increase in the extent of forested lands with elevated aridity and contributed about nine additional days per year with exceptionally dry fuels and high fire potential.

While human-caused climate change and natural climate variability have conspired to increase aridity in recent decades, natural variability could possibly revert and reduce aridity in coming decades, Abatzoglou said. However, unmitigated human-caused climate change will likely have a growing effect on fire seasons — like the drum section of the orchestra growing louder and louder.

“Is this the new normal? I’d say no, but we’re heading there,” Abatzoglou said. “We expect that the effects of human-caused climate change will intensify over the next several decades. So long as there are trees in our forests, climate change will continue to set the table for large fire seasons.”

“The impact of anthropogenic climate change on wildfire across western US forests” is available upon request from the authors or from PNAS through EurekAlert.

The study was funded by the National Science Foundation, the National Aeronautics and Space Administration’s Terrestrial Ecology Program and Columbia University’s Center for Climate and Life.


John Abatzoglou
University of Idaho Department of Geography

Park Williams
Columbia University Lamont-Doherty Earth Observatory

Tara Roberts
University of Idaho Communications

Kevin Krajick
Senior editor, science news, The Earth Institute/Lamont-Doherty Earth Observatory

Study: Human-Caused Climate Change Has Doubled Western U.S. Forest Fire Area

10 October 2016 (LDEO) – A new study says that human-induced climate change has doubled the area affected by forest fires in the U.S. West over the last 30 years. According to the study, since 1984 heightened temperatures and resulting aridity have caused fires to spread across an additional 16,000 square miles than they otherwise would have—an area larger than the states of Massachusetts and Connecticut combined. The authors warn that further warming will increase fire exponentially in coming decades. The study appears today in the journal Proceedings of the National Academy of Sciences.

“No matter how hard we try, the fires are going to keep getting bigger, and the reason is really clear,” said study coauthor Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory. “Climate is really running the show in terms of what burns. We should be getting ready for bigger fire years than those familiar to previous generations.”

Fires in western forests began increasing abruptly in the 1980s, as measured by area burned, the number of large fires, and length of the fire season. The increases have continued, and recently scientists and public officials have in part blamed human-influenced climate change. The new study is perhaps the first to quantify that assertion. “A lot of people are throwing around the words climate change and fire—specifically, last year fire chiefs and the governor of California started calling this the ‘new normal,’ ” said lead author John Abatzoglou, a professor of geography at the University of Idaho. “We wanted to put some numbers on it.”

Warmth drives fire by drying out the land. Warmer air can hold more moisture, and the air ends up sucking it out of plants, trees, dead vegetation on the ground, and soil. Average temperatures in forested parts of the U.S. West have gone up about 2.5 degrees F since 1970, and are expected to keep rising. The resulting drying effect is evident in the rise of more fires. Williams published a study last year showing how climate-driven removal of moisture from land worsened the recent California drought, which was accompanied by widespread fires.

The overall increase in fire since the 1980s is about twice what the researchers attribute to climate change; the rest is due to other factors, they say. One has been a long-term natural climate oscillation over the Pacific Ocean that has steered storms away from the western United States. Another: firefighting itself. By constantly putting out fires, authorities have allowed areas they “saved” to build up more dry fuel, which later ignites, causing ever more catastrophic blazes, the researchers say. The costs of fire fighting have risen sharply in step; last year the federal government alone spent more than $2.1 billion. “We’re seeing the consequence of very successful fire suppression, except now it’s not that successful anymore,” said Abatzoglou.

The authors teased out the effects of climate warming from other factors by looking at eight different systems for rating forest aridity; these included the Palmer Drought Severity Index, the MacArthur Forest Fire Danger Index and the Canadian Forest Fire Danger Rating System. They then compared such measurements with observations of actual fires and large-scale climate models that estimate manmade warming. The crunched data showed that 55 percent of the increase in fuel aridity expected to lead to fires could be attributed to human-influenced climate change. Climate’s role in increasing such aridity has grown since 2000, the researchers say, and will continue to do so.

Williams and Abatzoglou say they do not account for some factors that could be offshoots of climate warming, and thus they may be understating the effect. These include millions of trees killed in recent years by beetles that prefer warmer weather, and declines in spring soil moisture brought on by earlier snowmelt. There is also evidence that lightning—the usual initial spark—may increase with warming.

The study does not cover western grasslands. These have seen more fires too, but there is little evidence that climate plays a role there, said Abatzoglou; rather, the spread of highly flammable invasive grasses appears to be the main driver.

Mike Flannigan, a fire researcher at the University of Alberta, said that previous studies have tried to understand the effects of climate on fires in parts of Canada, but that nothing had been done for the United States on this scale. “What’s great about this paper is that it quantifies this effect, and it does it on a national scale,” he said.

Worldwide, wildfires of all kinds have been increasing, often with a suspected climate connection. Many see a huge fire that leveled part of the northern city of Fort McMurray, Alberta, this May as the result of a warming trend that is drying out northern forests. Fires have even been spreading beyond, into the tundra, in places where blazes have not been seen for thousands of years. That said, fires are not expected to increase everywhere. “Increased fire in a lot of places agrees with the projections,” said Jeremy Littell, a research ecologist with the U.S. Geological Survey in Anchorage, Alaska. “But in many woodlands, the relationship between climate and fire is not as tidy.”

So far, this year has seen huge, though not record, fires. Over the summer, some 3 million acres burned across the United States, mostly in the West, from Washington state across to the Dakotas and down into Texas. Some scientists say the worst could be yet to come; in some places, the most dangerous conditions often occur from September to December, when desert winds interact with fuels that have been drying for five or six months.

The effects go beyond loss of trees and other vegetation. A 2012 study estimates that smoke from fires worldwide causes long-term health effects that kill some 340,000 people each year, mainly in sub-Saharan Africa and southeast Asia. Carbon released to the air adds to the burden of greenhouse gases already there, thus producing even more warming. Soot settling on snow and ice causes them to absorb more heat and melt faster.    

Many scientists studying the issue believe the growth in U.S. western fires will continue for many years. Williams and others say that eventually, so many western forests will burn, they will become too fragmented for fires to spread easily, and the growth in fire will cease. But, he says, “there’s no hint we’re even getting close to that yet. I’d expect increases to proceed exponentially for at least the next few decades.” In the meantime, he said, “It means getting out of fire’s way. I’d definitely be worried about living in a forested area with only one road in and one road out.”


Kevin Krajick
(212) 854-9729

Climate Change Has Doubled Western U.S. Forest Fire Area, Study Says

ABSTRACT: Increased forest fire activity across the western continental United States (US) in recent decades has likely been enabled by a number of factors, including the legacy of fire suppression and human settlement, natural climate variability, and human-caused climate change. We use modeled climate projections to estimate the contribution of anthropogenic climate change to observed increases in eight fuel aridity metrics and forest fire area across the western United States. Anthropogenic increases in temperature and vapor pressure deficit significantly enhanced fuel aridity across western US forests over the past several decades and, during 2000–2015, contributed to 75% more forested area experiencing high (>1 σ) fire-season fuel aridity and an average of nine additional days per year of high fire potential. Anthropogenic climate change accounted for ∼55% of observed increases in fuel aridity from 1979 to 2015 across western US forests, highlighting both anthropogenic climate change and natural climate variability as important contributors to increased wildfire potential in recent decades. We estimate that human-caused climate change contributed to an additional 4.2 million ha of forest fire area during 1984–2015, nearly doubling the forest fire area expected in its absence. Natural climate variability will continue to alternate between modulating and compounding anthropogenic increases in fuel aridity, but anthropogenic climate change has emerged as a driver of increased forest fire activity and should continue to do so while fuels are not limiting.

SIGNIFICANCE: Increased forest fire activity across the western United States in recent decades has contributed to widespread forest mortality, carbon emissions, periods of degraded air quality, and substantial fire suppression expenditures. Although numerous factors aided the recent rise in fire activity, observed warming and drying have significantly increased fire-season fuel aridity, fostering a more favorable fire environment across forested systems. We demonstrate that human-caused climate change caused over half of the documented increases in fuel aridity since the 1970s and doubled the cumulative forest fire area since 1984. This analysis suggests that anthropogenic climate change will continue to chronically enhance the potential for western US forest fire activity while fuels are not limiting.

Impact of anthropogenic climate change on wildfire across western US forests


  1. Dennis Mitchell said...

    ....but if we clear cut the forest then we could stop the fires and build more houses and shopping malls and gas stations with the lumber right on the land we just cleared. Creating jobs and more taxes so we could get a stronger military to keep the world safe from terrorism. (Sorry Trump must be wearing off on me!)  


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