Expert Commentary

The context of the Yosemite Rim Fire: Research on the growing wildfire problem

2013 review of contextual research information relating to the Yosemite wildfire of August 2013. Included are representative studies in this area.

The 2013 “Rim Fire” in Yosemite National Park, which at one point threatened San Francisco’s water supply, is among the largest in California’s history. Wildfires are, of course, a naturally occurring phenomenon, and they can have benefits for ecosystems. But certain changing patterns — more human development and land use change, climate change, more intense insect outbreaks, and altered treatment of vegetation and natural landscapes — are creating the conditions for larger and more frequent wildfires, scientists say.

Increasing periods of drought in the United States and more intense heat waves are direct contributing factors. And more intense fires are expected even as forestry and fire experts try to perfect mitigation techniques, such as fuel treatments, to minimize the conditions that can propel erratic wildfires.

But specific weather conditions can, obviously, influence the occurrence of these events. For example, because July 2013 was generally a wet month across the United States, the “number of fires during July was the least on record, while the acreage burned was the third least in the 14-year period of record,” according to NOAA’s National Climate Data Center.


Still, the research consensus is strong that there will be greater trouble with massive fires in the future. The preliminary 2013 federal National Climate Assessment report notes the following: “Given strong relationships between climate and fire, even when modified by land use and management, projected climate changes suggest that western forests in the United States will be increasingly affected by large and intense fires that occur more frequently.” Likewise, a 2013 study published in a journal of the American Meteorological Society finds that “future climate conditions would likely result in more dry and less stable days in August which is the most active month for wildfires in the western U.S. As a result, the potential risk of erratic large wildfires could increase significantly.”

Studies of particular regions, particularly in the American West, suggest even more substantial problems lie ahead. Take, for example, the findings of two papers published in recent years in the Proceedings of the National Academy of Sciences (PNAS). In “Forest Responses to Increasing Aridity and Warmth in the Southwestern United States,” published in Dec. 2010, researchers note that at least 2.7% of southwestern forest and woodland area “experienced substantial mortality due to wildfires from 1984 to 2006.” Further, they state, “if temperature and aridity rise as they are projected to, southwestern trees will experience substantially reduced growth during this century.” Another 2010 paper, “Continued Warming Could Transform Greater Yellowstone Fire regimes by mid-21st Century,” also finds that the “conditions associated with extreme fire seasons are expected to become much more frequent, with fire occurrence and area burned exceeding that observed in the historical record or reconstructed from paleoproxy records for the past 10,000 y. Even in years without extreme fire events, average annual area burned is projected to increase, and years with no large fires — common until recently — are projected to become increasingly rare.”

Further, a 2013 study from Harvard researchers, “Ensemble Projections of Wildfire Activity and Carbonaceous Aerosol Concentrations over the Western United States in the Mid-21st Century,” finds that the wildfire season may grow in length by as much as three weeks by mid-century and that fires may burn substantially more area and create larger volumes of smoke pollution.

The following is a selection of other research papers in this area that give a sense of the looming problems both here in the United States and abroad:



“Will Future Climate Favor More Erratic Wildfires in the Western United States?”
Luo, Lifeng; et al.  Journal of Applied Meteorology and Climatology, 2013. doi: 10.1175/JAMC-D-12-0317.1.

Abstract: “Wildfires that occurred over the western US during August 2012 were fewer in number but larger in size, as compared to all other Augusts in the 21st century. This unique characteristic, along with the tremendous property damage and potential loss of life due to large wildfires with erratic behavior, raised the question whether future climate will favor rapid wildfire growth so that similar wildfire activities may become more frequent as climate changes. This study addresses this question by examining differences in the climatological distribution of the Haines Index (HI) between the current and projected future climate over the western US. The HI, ranging from 2 to 6, was designed to characterize dry, unstable air in the lower atmosphere which may contribute to erratic or extreme fire behavior. A shift in HI distribution from low values (2, 3) to higher values (5, 6) would indicate an increased risk for rapid wildfire growth and spread. Haines Index distributions are calculated from simulations of current (1971-2000) and future (2041-2070) climate using multiple regional climate models (RCMs) in the North American Regional Climate Change Assessment Program (NARCCAP). Despite some differences among the projections, the simulations indicate that there may be not only more days but also more consecutive days with HI ≥ 5 during August in the future. This suggests that future atmospheric environments will be more conducive to erratic wildfires in the mountainous regions of the western US.”


“Long-term Perspective on Wildfires in the Western USA”
Marlona, Jennifer R.; et al. Proceedings of the National Academy of Sciences, February 2012. doi: 10.1073/pnas.1112839109.

Findings: Burning in the American West declined slightly over the past 3,000 years. Peaks in burning occurred between 950 and 1250 CE and then again during the 1800s CE. The rise in fires at 1000 CE occurred when temperatures high and drought area were widespread. Another increase in fires occurred at around 1400 CE, when drought conditions increased rapidly. Humans began to have a significant impact on fires in the 1800s. During expansion of Anglo-American settlements in the 19th century, evidence of burnings increased. In the 20th century this is reversed, due in part to changing practices nationally in fire outbreak management. Since then, “fire activity has strongly diverged from the trend predicted by climate alone and current levels of fire activity are clearly out of equilibrium with contemporary climate conditions.” Burning is currently at its lowest in history. The previous minimum was between 1440 and 1700 CE, and was due to a decrease in droughts and temperatures, which were at a 1,500-year minimum. Prior to the current era, fire reached its lowest historical level at 1500 CE, which corresponded with the collapse of several Native American populations.


“Ensemble Projections of Wildfire Activity and Carbonaceous Aerosol Concentrations over the Western United States in the Mid-21st Century”
Yue, Xu; Mickley, Loretta J.; Logan, Jennifer A.; Kaplan, Jed O. Atmospheric Environment, October 2013, Vol. 77.

Abstract: “We estimate future wildfire activity over the western United States during the mid-21st century (2046–2065), based on results from 15 climate models following the A1B scenario. We develop fire prediction models by regressing meteorological variables from the current and previous years together with fire indexes onto observed regional area burned. The regressions explain 0.25–0.60 of the variance in observed annual area burned during 1980–2004, depending on the ecoregion. We also parameterize daily area burned with temperature, precipitation, and relative humidity. This approach explains ∼0.5 of the variance in observed area burned over forest ecoregions but shows no predictive capability in the semi-arid regions of Nevada and California. By applying the meteorological fields from 15 climate models to our fire prediction models, we quantify the robustness of our wildfire projections at midcentury. We calculate increases of 24–124% in area burned using regressions and 63–169% with the parameterization. Our projections are most robust in the southwestern desert, where all GCMs predict significant (p < 0.05) meteorological changes. For forested ecoregions, more GCMs predict significant increases in future area burned with the parameterization than with the regressions, because the latter approach is sensitive to hydrological variables that show large inter-model variability in the climate projections. The parameterization predicts that the fire season lengthens by 23 days in the warmer and drier climate at midcentury. Using a chemical transport model, we find that wildfire emissions will increase summertime surface organic carbon aerosol over the western United States by 46–70% and black carbon by 20–27% at midcentury, relative to the present day. The pollution is most enhanced during extreme episodes: above the 84th percentile of concentrations, OC increases by ∼90% and BC by ∼50%, while visibility decreases from 130 km to 100 km in 32 Federal Class 1 areas in Rocky Mountains Forest.


“Ecological Effects of Large Fires on U.S. landscapes: Benefit or Catastrophe?”
Keane, Robert E., et al. International Journal of Wildland Fire, 2008, 17, 696–71.

Abstract: “The perception is that today’s large fires are an ecological catastrophe because they burn vast areas with high intensities and severities. However, little is known of the ecological impacts of large fires on both historical and contemporary landscapes. The present paper presents a review of the current knowledge of the effects of large fires in the United States by important ecosystems written by regional experts. The ecosystems are (1) ponderosa pine–Douglas-fir, (2) sagebrush–grasslands, (3) piñon–juniper, (4) chaparral, (5) mixed-conifer, and (6) spruce–fir. This review found that large fires were common on most historical western US landscapes and they will continue to be common today with exceptions. Sagebrush ecosystems are currently experiencing larger, more severe, and more frequent large fires compared to historical conditions due to exotic cheatgrass invasions. Historical large fires in south-west ponderosa pine forest created a mixed severity mosaic dominated by non-lethal surface fires while today’s large fires are mostly high severity crown fires. While large fires play an important role in landscape ecology for most regions, their importance is much less in the dry piñon–juniper forests and sagebrush–grasslands. Fire management must address the role of large fires in maintaining the health of many US fire-dominated ecosystems.”


“Historic and Future Extent of Wildfires in the Southern Rockies Ecoregion, USA”
Litschert, Sandra E.; Brown, Thomas C.; Theobald, David M. Forest Ecology and Management, April 2012, Vol. 269, 124-133. doi: 10.1016/j.foreco.2011.12.024.

Abstract: “Wildfires play a formative role in the processes that have created the ecosystems of the Southern Rockies Ecoregion (SRE). The extent of wildfires is influenced mainly by precipitation and temperature, which control biomass growth and fuel moisture. Forecasts of climate change in the SRE show an increase in temperatures, bringing warmer springs with earlier runoff and longer fire seasons…. Our summary of historical wildfire records from the national forests of the SRE from 1930 to 2006 revealed an order of magnitude increase in the annual number of fires recorded over the full time period and in the number of large fires since 1970. We developed a model of percent burned area in the SRE for the period 1970–2006 using temperature and precipitation variables (R2 = 0.51, p = 1.7E-05). We applied this model to predict percent burned area using data from two downscaled global circulation models (GCMs), for the Intergovernmental Panel on Climate Change Special Report Emissions Scenarios A2 (projects high increases in temperature) and B1 (projects lower temperature increases), for the time period 2010–2070. The results showed increasing trends in median burned areas for all scenarios and GCM combinations with higher increases for the B1 scenario. The results suggest that precipitation increases could at least partially compensate for the effect of temperature increases on burned area but the strength of this ameliorating effect of precipitation will remain uncertain until the GCMs are further developed.”


“Valuing Mortality Impacts of Smoke Exposure from Major Southern California Wildfires”
Kochia, Ikuho; Champ, Patricia A.; Loomis, John B.; Donovan, Geoffrey H. Journal of Forest Economics, January 2012, Vol. 18, Issue 1, 61-75.

Abstract: “While the mortality impacts of urban air pollution have been well addressed in the literature, very little is known about the mortality impacts and associated social cost from wildfire-smoke exposure (0035 and 0070). In an attempt to address this knowledge gap, we estimate the social cost associated with excess mortality due to smoke exposure during the 2003 southern California wildfires. Accounting for confounding factors such as seasonality and fluctuation of daily mortality levels, we identify 133 excess cardiorespiratory-related deaths caused by wildfire-smoke exposure. The mean estimated total mortality-related cost associated with the 2003 southern California wildfire event is approximately one billion U.S. dollars. Accounting for mortality costs associated with wildfire-smoke exposure allows for a better understanding of the tradeoffs associated with fuel treatment programs and suppression costs.”


“Land Management Practices Associated with House Loss in Wildfires”
Gibbons P.; van Bommel L.;  Gill, A.M.;  Cary, G.J.;  Driscoll, D.A.; et al. PLoS One, 2012, 7(1), e29212. doi: 10.1371/journal.pone.0029212.

Abstract: “Losses to life and property from [wildfires] are forecast to increase because of population growth in peri-urban areas and climate change. In response, there have been moves to increase fuel reduction—clearing, prescribed burning, biomass removal and grazing—to afford greater protection to peri-urban communities in fire-prone regions. But how effective are these measures? … Significant fuel variables in a logistic regression model we selected to predict house loss were (in order of decreasing effect): (1) the cover of trees and shrubs within 40 m of houses, (2) whether trees and shrubs within 40 m of houses was predominantly remnant or planted, (3) the upwind distance from houses to groups of trees or shrubs, (4) the upwind distance from houses to public forested land (irrespective of whether it was managed for nature conservation or logging), (5) the upwind distance from houses to prescribed burning within 5 years, and (6) the number of buildings or structures within 40 m of houses. All fuel treatments were more effective if undertaken closer to houses. For example, 15% fewer houses were destroyed if prescribed burning occurred at the observed minimum distance from houses (0.5 km) rather than the observed mean distance from houses (8.5 km). Our results imply that a shift in emphasis away from broad-scale fuel-reduction to intensive fuel treatments close to property will more effectively mitigate impacts from wildfires on peri-urban communities.”


“Influence of Social Capital on Community Preparedness for Wildfires”
Bihari, Menka; Ryan, Robert. Landscape and Urban Planning, June 2012, Vol. 106, Issue 3, 253–261.

Abstract: “The increased concern about wildfire risk creates the need to better understand the factors affecting community preparedness. Social capital may be one key factor for facilitating risk reduction. We examined how place attachment and past experience with wildfires influences a community’s social capital, which in turn affects the adoption of defensible space actions and improves wildfire awareness. A survey instrument was developed to identify variables affecting social capital, and measure self-reported change in attitudes and actions toward mitigation in six fire-prone communities across the USA. Findings indicate that place attachment and previous involvement in natural resource planning significantly affect social capital and community cohesion, encourage defensible space actions as well as build support for preparedness in the wildland–urban interface (WUI) communities by improving residents’ awareness of wildfire risk. Results suggest that planners and resource managers can take advantage of these factors to increase citizen participation for shaping and improving collaborative hazard mitigation and resource management. By identifying and monitoring the variables that affect social capital and preparedness, planning and natural resource management agencies can better direct programs to reduce fire danger and ensure safety in wildland–urban interface communities.”


“Emergency Health Risk Communication During the 2007 San Diego Wildfires: Comprehension, Compliance, and Recall”
Sugerman, David E.; et al. Journal of Health Communications: Health Perspectives, April 2012, 698-712. doi: 10.1080/10810730.2011.635777.

Abstract: “In October 2007, wildfires burned nearly 300,000 acres in San Diego County, California. Emergency risk communication messages were broadcast to reduce community exposure to air pollution caused by the fires. The objective of this investigation was to determine residents’ exposure to, understanding of, and compliance with these messages…. Most persons surveyed reported hearing fire-related health messages (87.9%) and nearly all (97.9%) understood the messages they heard. Respondents complied with most to all of the nontechnical health messages, including staying inside the home (58.7%), avoiding outdoor exercise (88.4%), keeping windows and doors closed (75.8%), and wetting ash before cleanup (75.6%). In contrast, few (<5%) recalled hearing technical messages to place air conditioners on recirculate, use High-Efficiency Particulate Air filters, or use N-95 respirators during ash cleanup, and less than 10% of all respondents followed these specific recommendations. The authors found that nontechnical message recall, understanding, and compliance were high during the wildfires, and reported recall and compliance with technical messages were much lower.”


“Review of Fuel Treatment Effectiveness in Forests and Rangelands and a Case Study from the 2007 Megafires in Central Idaho”
Hudak, Andrew T.; Rickert, Ian; Morgan, Penelope; Strand, Eva; Lewis, Sarah A.; Robichaud, Peter R.; Hoffman, Chad; Holden, Zachary A. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, 2011.

Findings: Analysis of the 2007 Central Idaho megafire confirms what previous scientific literature had hypothesized, that the most effective treatment was mechanical thinning of trees and brush followed by a controlled surface fire. Prescribed burn treatments alone are more limited in their effectiveness, and the benefits diminish over time: “It is difficult to kill most medium-sized trees and many small trees by fire alone. Multiple rounds of prescribed fire are more effective … than single entry treatments.” Overall, there appears to be no “magic formula” for all forest area types: “While thinning from below is a common treatment and thresholds in tree density, crown base height, crown bulk density, tree spacing, and other fuel composition descriptors exist for a given stand, there is no general prescription that will work in all or even most stands.”


“Ten Years After Wildfires: How Does Varying Tree Mortality Impact Fire Hazard and Forest Resiliency?”
Stevens-Rumanna, Camille S.; Siegb, Carolyn H.; Huntera, Molly E. Forest Ecology and Management, March 2012, Vol. 267, 199-208. doi: 10.1016/j.foreco.2011.12.003.

Abstract: “Severe wildfires across the western US have lead to concerns about heavy surface fuel loading and the potential for high-intensity reburning. Ponderosa pine forests, often overly dense from a century of fire suppression, are increasingly susceptible to large and severe wildfires especially given warmer and drier climate projections for the future. However, the majority of research on fuel dynamics after wildfires has focused on high-severity burned areas in more productive forest types. We sampled fuel loadings in 2009 and 2010 across a range of tree mortality on two high-severity wildfires that occurred in 2000…. Ten years after wildfire, low mortality (0–40%) plots resembled unburned plots in almost every fuels attribute. Basal area in low-mortality plots exceeded reconstructed historical ranges and fire hazard reduction targets by up to 130%. However, coarse woody debris loadings fell below a recommended “optimum” range and herbaceous fuels were sparse. Mid-mortality (41–80%) plots were characterized by more open stands and increased surface fuel loadings, basal area was close to target ranges and CWD loadings were within the recommended range. High mortality (81–100%) plots had few trees but CWD loadings exceeded recommended levels by up to 28%, and herbaceous fuels were adequate to carry a surface fire. These findings suggest that post-fire management should be targeted to the level of mortality. Low mortality and unburned areas should be targeted for reducing stand densities and promoting understory growth, to minimize crown fire hazard and increase site potential.”


“Wildfires in Bamboo-Dominated Amazonian Forest: Impacts on Above-Ground Biomass and Biodiversity”
Barlow, Jos; et al. PLoS One, 2012, 7(3), e33373. doi:10.1371/journal.pone.0033373.

Abstract: “Fire has become an increasingly important disturbance event in south-western Amazonia. We conducted the first assessment of the ecological impacts of these wildfires in 2008, sampling forest structure and biodiversity along twelve 500 m transects in the Chico Mendes Extractive Reserve, Acre, Brazil…. Fire had limited influence upon either faunal or floral species richness or community structure responses, and stems <10 cm [diameter at breast height] were the only group to show highly significant (p = 0.001) community turnover in burned forests. Mean aboveground live biomass was statistically indistinguishable in the unburned and burned plots, although there was a significant increase in the total abundance of dead stems in burned plots. Comparisons with previous studies suggest that wildfires had much less effect upon forest structure and biodiversity in these south-western Amazonian forests than in central and eastern Amazonia, where most fire research has been undertaken to date.”


“Modeling the Potential for Prescribed Burning to Mitigate Carbon Emissions from Wildfires in Fire-prone Forests of Australia”
Bradstock, R.A.; et al. International Journal of Wildland Fire, July 2012, 21(6), 629-639. doi: 10.1071/WF11023.

Abstract: “Prescribed fire can potentially reduce carbon emissions from unplanned fires. This potential will differ among ecosystems owing to inherent differences in the efficacy of prescribed burning in reducing unplanned fire activity (or ‘leverage’, i.e. the reduction in area of unplanned fire per unit area of prescribed fire). In temperate eucalypt forests, prescribed burning leverage is relatively low and potential for mitigation of carbon emissions from unplanned fires via prescribed fire is potentially limited. Simulations of fire regimes accounting for non-linear patterns of fuel dynamics for three fuel types characteristic of eucalypt forests in south-eastern Australia supported this prediction. Estimated mean annual fuel consumption increased with diminishing leverage and increasing rate of prescribed burning, even though average fire intensity (prescribed and unplanned fires combined) decreased. The results indicated that use of prescribed burning in these temperate forests is unlikely to yield a net reduction in carbon emissions. Future increases in burning rates under climate change may increase emissions and reduce carbon sequestration.”


“Vegetation Limits the Impact of a Warm Climate on Boreal Wildfires”
Girardin, Martin P.; et al., New Phytologist,  September 2013, Vol. 199, Issue 4, 1001-1011. doi: 10.1111/nph.12322.

Abstract: “Strategic introduction of less-flammable broadleaf vegetation into landscapes was suggested as a management strategy for decreasing the risk of boreal wildfires projected under climatic change. However, the realization and strength of this offsetting effect in an actual environment remain to be demonstrated. Here we combined paleoecological data, global climate models and wildfire modelling to assess regional fire frequency (RegFF, i.e. the number of fires through time) in boreal forests as it relates to tree species composition and climate over millennial time-scales…. The modelling experiment indicates that the high fire risk brought about by a warmer and drier climate in the south during the mid-Holocene was offset by a higher broadleaf component. Our data highlight an important function for broadleaf vegetation in determining boreal RegFF in a warmer climate. We estimate that its feedback may be large enough to offset the projected climate change impacts on drought conditions.”


“Assessing the Potential of Wildfires as a Sustainable Bioenergy Opportunity”
Verón, Santiago R.; Jobbágy, Esteban G.; Di Bella, Carlos M.; Paruelo, José M.; Jackson, Robert B. Global Change Biology: Bioenergy, May 2012. doi: 10.1111/j.1757-1707.2012.01181.x.

Abstract: “As the environmental and economic consequences of fossil-fuel use become clear, land is increasingly targeted as a source of bioenergy. We explore the potential for generating electricity from biomass vulnerable to fires as an ecologic and socioeconomic opportunity that can reduce the risk of greenhouse gas generation from wildfires and help to create incentives to preserve natural and seminatural vegetation and prevent its conversion to agriculture, including biofuel crops. On the basis of a global analysis of the energy generation and spatial distribution of fires, we show that between 2003 and 2010, global fires consumed ~8300 ± 592 PJ yr−1 of energy, equivalent to ~36–44% of the global electricity consumption in 2008 and >100% national consumption in 57 countries. Forests/woodlands, cultivated areas, shrublands, and grasslands contributed 53%, 19%, 16%, and 3.5% of the global energy released by fires. Although many agroecological, socioeconomic, and engineering challenges need to be overcome before diverting the energy lost in fires into more useable forms, done cautiously it could reconcile habitat preservation with economic yields in natural systems.”


Keywords: biodiversity, disasters, wildfires, California

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