Wildfires in the Western US – Scientists Predict Flammable Future with Warming Climate

Image: A wildfire in southern Montana, August 2007, captured from space by astronauts docked to the International Space Station. Credit: NASA Johnson Space Center via FlickerNASA Goddard Photo and Video.

In March I ran a post about a NASA wildfire model that predicts increased fire activity in the Western U.S., along with other parts of the world, in future as a result of rising global temperatures. This feature (published at New West) expands on the earlier post, with insights from NASA scientist Olga Pechony (who built the NASA model with colleague Drew Shindell) and Anthony Westerling, an expert on wildfires in the West, based at the University of California’s Sierra Nevada Research Institute.

[Feature first published at New West]…

A NASA global wildfire model does not cast happy projections for the forests of the West in future. As global temperatures increase and the West becomes drier, fire activity in the region could increase by 30 percent to 60 percent from present day levels by the turn of the century, according to NASA scientist Olga Pechony, who designed the model with colleague Drew Shindell.

At the same time, Pechony and Shindell expect that the wetter, eastern half of the country will experience a drop in wildfires as warmer temperatures lead to more humid conditions there.

Increased wildfire activity would continue a trend that has been playing out over the past 30 years due to warmer and drier conditions in the West making fuel for wildfires more flammable, Peter Hildebrand, director of the earth sciences directorate at NASA’s Goddard Space Flight Center, said at the Conference on World Affairs at the University of Colorado in April.

As the earth heats up circulation systems are changing and the winter storm track is being pushed further north. This results in less precipitation, higher temperatures and more evaporation in the Rocky Mountain West, Hildebrand explained.

“I want you to think a little bit of fire as a metaphor for the many things that climate change holds for us,” Hildebrand told the crowd in Boulder.

In terms of adapting to increased fire activity, it’s important for people living in fire-prone areas to think about home construction, the vegetation around their homes, and the location of homes, he said.

Projecting Into the Future

The NASA model simulates worldwide wildfire and climate conditions dating back to the year 850. The model projects fire activity across the world as far ahead as 2100. The projections are based on three of the International Panel on Climate Change’s scenarios for predicted future greenhouse gas emissions, population growth, economic development and temperature rise. Under all three scenarios, rapidly rising temperatures and regional drying result in an increase in global fire activity after about 2050, according to Pechony and Shindell’s model.

Wildfire activity does not increase all over the planet, however. Along with the Western U.S., the model predicts elevated fire activity in southern Europe, India, central Asia, Siberia, southern Africa and Australia. On the other hand, the model predicts that Northern Europe, equatorial Africa and parts of South America will experience a drop in fire activity, along with the Eastern U.S.

To test the model’s accuracy Shindell and Pechony compared the model’s results to records from charcoal layers harvested from lake sediments around the world, which researchers have used to reconstruct historical fire activity dating back 2,000 years.

“The model really corresponds pretty well to what we see in the charcoal record, so that was quite a relief and a success,” said Pechony in a telephone interview.

At the same time, Pechony and Shindell used current satellite data to check the model’s results against present day fire activity. By using available precipitation, temperature, relative humidity and vegetation density data, they could estimate the flammability and availability of wildfire fuels, Pechony explained.

When looking at sources of fire ignitions, satellites provide lightning data from the 1990s onward. However, humans’ role in starting and extinguishing wildfires is the “most unknown” variable, Pechony said. To estimate the effects of humans on wildfire activity, Pechony and Shindell used land-use and population density reconstructions from the History Database of the Global Environment.

Using this data, they calculated global fire activity between 2005 and 2008 and checked the results against global fire data from two satellites over the same period. The model results and satellite data corresponded well, Pechony explained.

They also checked seasonal changes in global fire distribution and found the model’s results to be very close to the satellites’ records, she said. Using another satellite that gives 20 years’ worth of global average fire activity, Pechony and Shindell found that the model also reproduced interannual variations in fire activity, Pechony said.

Pechony and Shindell identify different drivers of global fire activity over time. From 850 up to the time of the Industrial Revolution, precipitation levels largely dictated fire levels around the world. But from the industrial age onward, rapid population growth led to humans becoming the main drivers of fire activity, according to their report. During this time people cleared tracts of forested land to make space for farmland and cattle pastures, which cut back on the amount of fuel that could burn. Around 1900, Pechony and Shindell identify a sharp drop in wildfire even though global temperatures were increasing and precipitation was on the decline, which they put down to humans successfully suppressing more fires.

And while humans have been in the driving seat when it comes to global fire activity since the Industrial period, by 2050 Pechony and Shindell predict that rapidly warming temperatures will play the major role in dictating fire activity around the world.

At some point temperatures rise so rapidly and the environment becomes so flammable that fire suppression efforts are no longer sufficient to curb trends in fire activity that should have been increasing but have been dampened by human activity, Pechony explained.

However, Pechony does not discount the possibility that future fire management and developments in firefighting techniques could improve, allowing humans to override the impacts of temperature rise. “This is something we can’t know or predict or rely on,” she said.

Northern Rockies Show Greatest Increase in Wildfire Activity in Recent Decades

While large wildfire activity in the West has increased since the mid-1980s when compared to the 1970s and first half of the 1980s, the greatest spike in wildfire activity in recent decades has taken place in mid-elevation forests in the Northern Rockies, according to a study by Anthony Westerling and his colleagues. Westerling is an associate professor at the University of California’s Sierra Nevada Research Institute.

Using federal fire records, Westerling and his co-authors found that large wildfires (those larger than 400 hectares) burned more than six and a half times more forest in the West between 1987 and 2003 than from 1970 to 1986. At the same time, large fires blazed about four times more frequently between 1987 and 2003 than they did in the previous 17 years. Fires in the forests of the Northern Rockies, a stretch of the Rockies running from the Canadian border to the Utah-Idaho border and lying within parts of Idaho, Montana, Wyoming, Oregon and Washington, accounted for 60 percent of the increase in large fires, according to the study. During this time average spring and summer temperatures increased by 0.87 Celsius.*

At the same time, the average length of the wildfire season increased by 78 days between 1970 to 1986 and 1987 to 2003, according to the study.

Since the West is fairly arid, the densest forests are found in the coolest, wettest places, which are in higher elevation areas that get snow, Westerling explained. These dense forests foster high-severity, stand-replacing fires (those that kill off most trees so that new ones replace them), he said.

In the mountain areas Westerling studies topography – the lay of the land – and its role in generating precipitation when air masses move over mountain ridges, plays an important role in determining the location of forests and fire regimes, he explained. Since the NASA model covers the whole world at a coarse scale, it’s probably unable to capture these topographic effects on climate and fire regimes, he said.

“When I look at the Western U.S. – I’m looking, you know, in the last, say, 40 years – there’s been a very dramatic response to temperature already,” Westerling said.

Most of the increase in wildfire activity has happened in higher elevation forest areas where a short summer dry season is extended when snow melts earlier in spring, which increases the flammability of fuels, he explained.

The biggest increase in fire area burned has been in parts of the Western U.S. where fire management has not had the same effect as it has elsewhere, Westerling said.

Commenting on the NASA model, Westerling said: “I think the broad outline of what they’re saying is true, in the sense that as you warm things up temperature is going to overwhelm other effects like management and precipitation.”

“The only quibble I would have – I would say temperature is already a dominant driver for the fire regime in a large part of the Western United States,” he said.

Westerling’s paper, published in Science, was not intended to show that the increase in fire activity in the West was evidence of climate change, he said.

“We already know from other peer-reviewed research, including things covered in the International Panel on Climate Change’s reports, that we expect temperature to increase because of human-caused changes in the atmospheric constituents. And we have observed changes in temperature that have been conclusively attributed to human alteration of the climate system in the peer-reviewed literature and we expect that to continue in the future,” he said.

Understanding how current fire regimes respond to temperature changes allows researchers to use models that project forward to see how today’s fire regimes would respond under future climate conditions, Westerling explained.

In a 2009 paper (PDF), which Westerling co-authored, model projections show that warmer temperatures cause the average area burned each year in the West to spike by 54 percent by the 2050s compared to present day levels. Rocky Mountain forests are hardest hit, with 175 percent more area set to burn, according to the paper.

*The original post erroneously put this figure at 33.5 degrees Farenheit. Error corrected on June 14, 2011.


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