Fire Dangers Increase Exponentially with Just a Small Rise in Temperature

“I don’t mean to scare anybody,” Andrew Pierce said after he had probably done exactly that.

Pierce’s new research, which he shared at July’s Hawai`i Conservation Conference in Waikiki, suggests that climate change has resulted in fires now that can spread 11 times faster and are 30 times more intense than those in the 1950s. And that’s just from a two-degree Celsius increase in temperature.

“Even if we think in our brains, ‘It’s two degrees warmer,’ … we’re going to have exponential feedbacks into fire behavior,” said Pierce, a post-doctoral researcher at the University of Hawai`i.

Over the past decade, states across the nation – Texas, New Mexico, Georgia, Arizona, Utah, Florida and California – have had some of the largest, if not the largest fires, in their history. The fire trend is due, in large part, to climatic factors, something Pierce has studied extensively in California. More recently, he worked under Creighton Litton of UH’s Department of Natural Resources and Environmental Management, and with Susan Cordell and Christian Giardina of the U.S. Forest Service’s Institute of Pacific Islands Forestry, to determine the potential impact of climate change on fires in Hawai`i.

While waiting for a projection from the Pacific Islands Climate Change Cooperative (PICCC) on what the climate here will be like in 40 to 50 years, Pierce and his team looked to the past and found that climate change has already made the islands more vulnerable to wildfire.

Hot! Hot! Hot!

Wind, temperature, and humidity all affect fire behavior. Using data collected at the Honolulu International Airport – one of the few locations in Hawai`i with reliable records going back several decades – Pierce noted all of the average daily and monthly high temperatures from 1950-1959 and compared them to records from 2000-2009. He also followed changes in relative humidity and wind speed.

“The 2000s look quite a bit different” from the 1950s, he said. He found that the average temperature in the 2000s was higher than the extreme high temperatures of the 1950s.

Humidity levels also varied. In the 2000s, the driest months were significantly drier than they were in the 1950s, but the average months are similar to what they were in the 1950s, he said. Wind speeds were higher in the 2000s, but only marginally so.

Pierce made what was perhaps his most dramatic finding when he compared the number of days where temperatures reached 88 degrees F or higher. In the 1950s, there were only 61 days where the daily high temperature was 88 degrees. In the 2000s, there were 752 days, an increase of more than 1,000 percent. When he included days where high temperatures exceeded 88 degrees, the difference was even greater: 69 days in the 1950s compared to 1,104 days in the 2000s.

Pierce, who has recently moved to Washington, D.C., said he wasn’t surprised by the differences in temperature and humidity because “what I found is essentially the same as what [other researchers] found.” But, he added, “I was surprised by how those seemingly small changes in weather variables produced large changes in fuel moisture levels and the exponentially larger changes in potential fire behavior.”


The drier and hotter weather has translated into drier landscapes, where “fuels ignite more quickly and release heat faster,” Pierce said.

Without any data from actual fires in the 1950s and 2000s, Pierce used the Honolulu airport weather data and a fire behavior model to calculate potential fire characteristics from those periods for fires burning under the top 10% most extreme conditions. Model results showed that fires in the 1950s would have spread at a rate of 0.1 meters/second, while those in the 2000s spread at a rate of 1.1 meters/second. Fireline intensity, an index of the severity of a fire, went from 274 kW/meter to 8,674 kW/meter, and flame length increased fivefold between the two decades, he said.

Those results do not bode well for Hawai`i’s wild landscapes, native or not.

“The general idea is that air temperatures will be higher with climate change,” Pierce said. And if fires become more frequent and intense as a result, forest restoration will also become more difficult, according to experts. At the 2012 conservation conference, Pierce reported on earlier research showing that repeated fire can drive out a lot of native species, shrubs, in particular. What’s more, he found that dryland areas on Hawai`i island where ungulates had been fenced out to protect native species had higher fuel loads than unfenced areas. Research presented at this year’s conference by the University of Maryland’s Kealoha Kinney also suggests that recurrent fire may reduce the amount of soil phosphorus that is available to plants in dryland systems and thus could alter the pathway of primary succession.

A presentation by state Department of Land and Natural Resources’ Division of Forestry and Wildlife (DLNR-DOFAW) forester Sheri Mann provided a glimpse of what fires in today’s climate can do.

In the summer of 2012, three large-scale fires ravaged 3,000 acres on Kaua`i’s west side. The DLNR spent nearly a million dollars controlling them, and even so, they burned more than a thousand acres in the Na Pali-Kona and Pu`u Ka Pele forest reserves and the Kuia Natural Area Reserve, including 149 acres of critical habitat for federally listed species.

“Not only was this one of the largest fire seasons on Kaua`i since the 1960s, but the fire severity was characterized as severe for much of the burned area. These high-fire severities across numerous watersheds and steep gulches above pristine near shore coral reefs have created a significant management challenge for [DOFAW],” Mann stated in her conference abstract.

The intense flames burned just about everything in their path, leaving vast areas of exposed earth. Although DOFAW started remediating the forested areas last year, Gov. Neil Abercrombie issued in February an emergency executive order allowing the DLNR to immediately expand remediation activities to include ground cover planting and dead tree removal, among other things.

“Immediate action is needed to stabilize and quickly revegetate denuded slopes and minimize the long-term ecological impacts to the mauka forests and marine ecosystems from heavy rains, downed woody debris and runoff,” according to a press release describing the order.

Mann agrees with Pierce’s assessment that fires today can be far more intense than they used to be.

“I would say that the fire intensity is much higher, as well as the fire frequency. In that area there hadn’t been a fire in 60 years,” she told Environment Hawai`i. “There’s no doubt it’s connected to climate change,” she continued, adding that species composition also affects fire intensity.

“A lot of the invasives are a lot more flammable,” she said.

Initially, the DLNR estimated it would cost $8 million to restore and remediate the burned timber management areas alone. Those areas consisted mainly of eucalyptus and pine, with some scattered native species, Mann said.

“We were asked to cut that in half,” Mann said, so it “will be in the neighborhood of $4 million to do what needs to be done. So far, we’ve received less than $1 million. We’ve been promised a million.”

Even so, her agency was able to remove hazardous trees, plant some ground cover, and create fire breaks early on.

“We got lucky because the big rains came really late so we were able to get some vegetation on the ground. The erosion potential was extremely high,” she said.

Future Work

At the time Pierce gave his presentation, Giardina said that they had hoped to be able to show the results of similar modeling, using PICCC’s climate change projections, for the Army’s 80,000 hectare Pohakuloa Training Area (PTA) on Hawai`i island, which has a history of burning. With an estimate of what the climate at PTA will be like during the decade 2050-2060, they planned to “illustrate how incremental increases in basic meteorological variables can have non-linear effects on the frequency of hot and dry days, as well as potential intensity of and size of wildfires,” Pierce’s abstract states. It notes that PTA, where the U.S. military conducts live-fire training, is also “where 15 native threatened and endangered species stand to be severely and negatively impacted by warming and drying trends and resulting fires forecasted for the region.”

However, Giardina says his team is just now receiving the data needed for the PTA work.

“We originally wanted to do current vs. future, but it turned out to be really interesting to go back and see how fire behavior may have already changed dramatically,” he says.

According to University of Hawai`i climatologist Thomas Giambelluca, who also spoke at the conference, the leeward side of the island of Hawai`i may feel climate change impacts sooner than some of the northern islands, such as Kaua`i and O`ahu.

“I’ve noticed it’s been very dry in the southern islands. That means that our wintertime systems are not migrating as far south as they were in the past,” and, in fact, they appear to be moving north, he said. “Whether that is connected with climate change, I’m not sure, but it makes sense that it is.”

Because the windward sides of the islands don’t rely as heavily on those winter storm systems for moisture, they aren’t likely to suffer as much as the leeward sides, Giambelluca said. PTA is located in the saddle of Hawai`i island, between Mauna Loa and Mauna Kea.

Management Tools

While reversing the current climate trends appears to be near impossible, government agencies, scientific institutions, and non-profit organizations have been working together to provide Hawai`i island, at least, with tools to help identify – in real time – areas most vulnerable to burning and to collate fire history data collected by various agencies.

In his 2012 presentation, Pierce said there’s usually a two-week lag between a hot spell and vegetation die-off. Dead vegetation is “correlated with [low] fuel moisture, which is key to the start and spread of fires,” he said.

Several years ago, Greg Asner of the Carnegie Institution for Science developed a web tool that allows visitors to view satellite images showing areas of live vegetation, dead vegetation, and bare ground.

In addition, the Hawai‘i Wildfire Management Organization (HWMO), a non-profit organization, has helped develop a map identifying the fuel loads across the island. It has also spearheaded the creation of a wildfire history map. Among other things, the map, which is the first of its kind in Hawai`i, will help agencies secure resources for mitigation and post-fire restoration, according to HWMO executive director Elizabeth Pickett, a conference presenter.

According to Mann, HWMO is working toward developing a statewide map.

Coupled with climate change information expected from the PICCC, the map will be a useful tool for fire pre-emption, management, and post-fire remediation, Mann said.

Having data on historic fires, as well as current day information on changes in precipitation, should give managers the ability to predict fire hazards and do pre-fire planning, she said.

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