Papers by Leroy Westerling
International Journal of Wildland Fire, 2008
The National Fire Danger Rating System indices deduced from a regional simulation weather model w... more The National Fire Danger Rating System indices deduced from a regional simulation weather model were used to estimate probabilities and numbers of large fire events on monthly and 1-degree grid scales. The weather model simulations and forecasts are ongoing experimental products from the Experimental Climate Prediction Center at the Scripps Institution of Oceanography. The monthly average Fosberg Fire Weather Index, deduced from the weather simulation, along with the monthly average Keetch-Byram Drought Index and Energy Release Component, were found to be more strongly associated with large fire events on a monthly scale than any of the other stand-alone fire weather or danger indices. These selected indices were used in the spatially explicit probability model to estimate the number of large fire events. Historic probabilities were also estimated using spatially smoothed historic frequencies of large fire events. It was shown that the probability model using four fire danger indices outperformed the historic model, an indication that these indices have some skill. Geographical maps of the estimated monthly wildland fire probabilities, developed using a combination of four indices, were produced for each year and were found to give reasonable matches to actual fire events. This method paves a feasible way to assess the skill of climate forecast outputs, from a dynamical meteorological model, in forecasting the probability of wildland fire severity with known precision.
Climate change is likely to alter wildfire regimes, but the magni- tude and timing of potential c... more Climate change is likely to alter wildfire regimes, but the magni- tude and timing of potential climate-driven changes in regional fire regimes are not well understood. We considered how the occurrence, size, and spatial location of large fires might respond to climate projections in the Greater Yellowstone ecosystem (GYE) (Wyoming), a large wildland ecosystem dominated by conifer forests and characterized by infrequent, high-severity fire. We de- veloped a suite of statistical models that related monthly climate data (1972–1999) to the occurrence and size of fires >200 ha in the northern Rocky Mountains; these models were cross-validated and then used with downscaled (∼12 km × 12 km) climate projections from three global climate models to predict fire occurrence and area burned in the GYE through 2099. All models predicted sub- stantial increases in fire by midcentury, with fire rotation (the time to burn an area equal to the landscape area) reduced to <30 y from the historical 100–300 y for most of the GYE. Years without large fires were common historically but are expected to become rare as annual area burned and the frequency of regionally synchronous fires increase. Our findings suggest a shift to novel fire–climate– vegetation relationships in Greater Yellowstone by midcentury be- cause fire frequency and extent would be inconsistent with persis- tence of the current suite of conifer species. The predicted new fire regime would transform the flora, fauna, and ecosystem processes in this landscape and may indicate similar changes for other sub- alpine forests.
Global climate models predict relative humidity (RH) in the western US will decrease at a rate of... more Global climate models predict relative humidity (RH) in the western US will decrease at a rate of about 0.1– 0.6 percentage points per decade, albeit with seasonal differences (most drying in spring and summer), geographical variability (greater declines in the interior), stronger reductions for greater anthropogenic radiative forcing, and notable spread among the models. Although atmospheric moisture content increases, this is more than compensated for by higher air temperatures, leading to declining RH. Fine-scale hydrological simulations driven by the global model results should reproduce these trends. It is shown that the MT-CLIM meteorological algorithms used by the Variable Infiltration Capacity (VIC) hydrological model, when driven by daily T min , T max , and precipitation (a configuration used in numerous published studies), do not preserve the original global model's humidity trends. Trends are biased positive in the interior western US, so that strong RH decreases are changed to weak decreases, and weak decreases are changed to increases. This happens because the MT-CLIM algorithms VIC incorporates infer an overly large positive trend in atmospheric moisture content in this region, likely due to an underestimate of the effect of increasing aridity on RH. The result could downplay the effects of decreasing RH on plants and wildfire. RH trends along the coast have a weak negative bias due to neglect of the ocean's moderating influence. A numerical experiment where the values of T dew are altered to compensate for the RH error suggests that eliminating the atmospheric moisture bias could, in and of itself, decrease runoff up to 14 % in high-altitude regions east of the Sierra Nevada and Cascades, and reduce estimated Colorado River runoff at Lees Ferry up to 4 % by the end of the century. It could also increase the probability of large fires in the northern and central US Rocky Mountains by 13 to 60 %.
The National Fire Danger Rating System indices deduced from a regional simulation weather model w... more The National Fire Danger Rating System indices deduced from a regional simulation weather model were used to estimate probabilities and numbers of large fire events on monthly and 1-degree grid scales. The weather model simulations and forecasts are ongoing experimental products from the Experimental Climate Prediction Center at the Scripps Institution of Oceanography. The monthly average Fosberg Fire Weather Index, deduced from the weather simulation , along with the monthly average Keetch–Byram Drought Index and Energy Release Component, were found to be more strongly associated with large fire events on a monthly scale than any of the other stand-alone fire weather or danger indices. These selected indices were used in the spatially explicit probability model to estimate the number of large fire events. Historic probabilities were also estimated using spatially smoothed historic frequencies of large fire events. It was shown that the probability model using four fire danger indices outperformed the historic model, an indication that these indices have some skill. Geographical maps of the estimated monthly wildland fire probabilities, developed using a combination of four indices, were produced for each year and were found to give reasonable matches to actual fire events. This method paves a feasible way to assess the skill of climate forecast outputs, from a dynamical meteorological model, in forecasting the probability of wildland fire severity with known precision.
In 2002. there were 88,458 fires reported on federal lands. These fires burned 6,937,584 acres an... more In 2002. there were 88,458 fires reported on federal lands. These fires burned 6,937,584 acres and 2,381 structures costing taxpayers $1.6 billion for fire suppression. On average, 4,215,089 acres of federal lands burn annually. Forecasting wildland fire risk (occurrence and size) is important to fire managers who desire to know the risks of severe events well in advance of their happening. In this talk, we discuss the estimation of a probability model for forecasting fire risk one month in advance. The model uses 25 years of historic fire data on federal lands in addition to weather and climatological variables.
The ability to forecast the number and location of large wildfire events (with specified confiden... more The ability to forecast the number and location of large wildfire events (with specified confidence bounds) is important to fire managers attempting to allocate and distribute suppression efforts during severe fire seasons. This paper describes the development of a statistical model for assessing the forecasting skills of fire-danger predictors and producing 1-month-ahead wildfire-danger probabilities in the western United States. The method is based on logistic regression techniques with spline functions to accommodate nonlinear relationships between fire-danger predictors and probability of large fire events. Estimates were based on 25 yr of historic fire occurrence data (1980–2004). The model using the predictors monthly average temperature, and lagged Palmer drought severity index demonstrated significant improvement in forecasting skill over historic frequencies (persistence forecasts) of large fire events. The statistical models were particularly amenable to model evaluation and production of probability-based fire-danger maps with prespecified precisions. For example, during the 25 yr of the study for the month of July, an area greater than 400 ha burned in 3% of locations where the model forecast was low; 11% of locations where the forecast was moderate; and 76% of locations where the forecast was extreme. The statistical techniques may be used to assess the skill of forecast fire-danger indices developed at other temporal or spatial scales.
Because forest fires emit substantial NOx and hydrocarbonssknown contributors to O3 productionswe... more Because forest fires emit substantial NOx and hydrocarbonssknown contributors to O3 productionswe hypothesize that interannual variation in western U.S. O3 is related to the burned area. To evaluate this hypothesis we used a gridded database of western U.S. summer burned area
(BA) and biomass consumed (BC) by fires between 101-125° W. The fire data were compared with daytime summer O3 mixing ratios from nine rural Clean Air Status and Trends Network (CASTNET) and National Park Service (NPS) sites. Large fire years exhibited widespread enhanced O3. The summer BA was significantly correlated with O3 at all sites. For each 1 million acres burned in the western U.S. during summer, we estimate that the daytime mean O3 was enhanced across the region
by 2.0 ppbv. For mean and maximum fire years, O3 was enhanced by an average of 3.5 and 8.8 ppbv, respectively. At most
sites O3 was significantly correlated with fires in the surrounding 5 × 5° and 10 × 10° regions, but not with fires in the
nearest 1 × 1° region, reflecting the balance between O3 production and destruction in a high NOx environment. BC was a slightly better predictor of O3, compared with BA. The relationship between O3 and temperature was examined at two sites (Yellowstone and Rocky Mountain National Parks). At these two sites, high fire years were significantly warmer than low fire years; however, daytime seasonal mean temperature and O3 were not significantly correlated. This indicates that the presence of fire is a more important predictor for O3 than is temperature.
11 In this study we have evaluated the role of wildfires on concentrations of fine particle 12
[1] We investigate the impact of climate change on wildfire activity and carbonaceous aerosol con... more [1] We investigate the impact of climate change on wildfire activity and carbonaceous aerosol concentrations in the western United States. We regress observed area burned onto observed meteorological fields and fire indices from the Canadian Fire Weather Index system and find that May–October mean temperature and fuel moisture explain 24–57% of the variance in annual area burned in this region. Applying meteorological fields calculated by a general circulation model (GCM) to our regression model, we show that increases in temperature cause annual mean area burned in the western United States to increase by 54% by the 2050s relative to the present day. Changes in area burned are ecosystem dependent, with the forests of the Pacific Northwest and Rocky Mountains experiencing the greatest increases of 78 and 175%, respectively. Increased area burned results in near doubling of wildfire carbonaceous aerosol emissions by midcentury. Using a chemical transport model driven by meteorology from the same GCM, we calculate that climate change will increase summertime organic carbon (OC) aerosol concentrations over the western United States by 40% and elemental carbon (EC) concentrations by 20% from 2000 to 2050. Most of this increase (75% for OC and 95% for EC) is caused by larger wildfire emissions with the rest caused by changes in meteorology and for OC by increased monoterpene emissions in a warmer climate. Such an increase in carbonaceous aerosol would have important consequences for western U.S. air quality and visibility.
and Bureau of Indian Affairs (BIA). Longer historical records for specific sites, especially some... more and Bureau of Indian Affairs (BIA). Longer historical records for specific sites, especially some USFSand NPS-managed areas, are available but limited in spatial representation. Results shown here indicate that, despite their foibles, an amalgamation of these datasets yields a spatial and temporal history that is of sufficient quality, spatial resolution, and duration to resolve regional characteristics of the wildfire season, and that these characteristics are related to modes of interannual climate variability. We report here results of our analysis of wildfire frequency and acres burned for the contiguous western United States based on these data. The next sections review the data, describe the seasonality of wildfire, and describe relationships between the PDSI and anomalous fire season severity. Finally, these relationships are used to predict fire season severity at lead times of one season to one year in advance of the fire season in regions with diverse fuels and climes.
Over 21,000 future California residential wildfire risk scenarios were developed on a monthly 1/8... more Over 21,000 future California residential wildfire risk scenarios were developed on a monthly 1/8° grid, using statistical wildfire models. We explore interactions between two global emissions scenarios, three climate models, six spatially explicit population growth scenarios derived from two growth models, and a range of parameters defining properties' vulnerability to loss. Scenarios are evaluated over two future time periods relative to historic baselines. We also explore effects of spatial resolutions for calculating household exposure to wildfire on changes in estimated future property losses. Our goal was not to produce one authoritative set of future risk scenarios but rather to understand what parameters are important for robustly characterizing effects of climate and growth on future residential property risks. By end of century, variation across development scenarios accounts for far more variability in statewide residential wildfire risks than does variation across climate scenarios. However, the most extreme increases in residential fire risks result from combining high-growth/high-sprawl scenarios with the most extreme climates considered here. Case studies for the Bay Area and the Sierra foothills demonstrate that, while land use decisions profoundly influence future residential wildfire risks, effects of diverse growth and land use strategies vary greatly around the state.
Changing climatic conditions are influencing large wildfire frequency, a globally widespread dist... more Changing climatic conditions are influencing large wildfire frequency, a globally widespread disturbance that affects both human and natural systems. Understanding how climate change, population growth, and development patterns will affect the area burned by and emissions from wildfires and how populations will in turn be exposed to emissions is critical for climate change adaptation and mitigation planning. We quantified the effects of a range of population growth and development patterns in California on emission projections from large wildfires under six future climate scenarios. Here we show that end-of-century wildfire emissions are projected to increase by 19−101% (median increase 56%) above the baseline period (1961−1990) in California for a medium-high temperature scenario, with the largest emissions increases concentrated in northern California. In contrast to other measures of wildfire impacts previously studied (e.g., structural loss), projected population growth and development patterns are unlikely to substantially influence the amount of projected statewide wildfire emissions. However, increases in wildfire emissions due to climate change may have detrimental impacts on air quality and, combined with a growing population, may result in increased population exposure to unhealthy air pollutants.
Western United States forest wildfire activity is widely thought to have increased in recent deca... more Western United States forest wildfire activity is widely thought to have increased in recent decades, yet neither the extent of recent changes nor the degree to which climate may be driving regional changes in wildfire has been systematically documented. Much of the public and scientific discussion of changes in western United States wildfire has focused instead on the effects of 19th-and 20th-century land-use history. We compiled a comprehensive database of large wildfires in western United States forests since 1970 and compared it with hydroclimatic and land-surface data. Here, we show that large wildfire activity increased suddenly and markedly in the mid-1980s, with higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons. The greatest increases occurred in mid-elevation, Northern Rockies forests, where land-use histories have relatively little effect on fire risks and are strongly associated with increased spring and summer temperatures and an earlier spring snowmelt.
Prior work shows western US forest wildfire activity increased abruptly in the mid-1980s. Large f... more Prior work shows western US forest wildfire activity increased abruptly in the mid-1980s. Large forest wildfires and areas burned in them have continued to increase over recent decades, with most of the increase in lightning-ignited fires. Northern US Rockies forests dominated early increases in wildfire activity, and still contributed 50% of the increase in large fires over the last decade. However, the percentage growth in wildfire activity in Pacific northwestern and southwestern US forests has rapidly increased over the last two decades. Wildfire numbers and burned area are also increasing in non-forest vegetation types. Wildfire activity appears strongly associated with warming and earlier spring snowmelt. Analysis of the drivers of forest wildfire sensitivity to changes in the timing of spring demonstrates that forests at elevations where the historical mean snow-free season ranged between two and four months, with relatively high cumulative warm-season actual evapotranspiration, have been most affected. Increases in large wildfires associated with earlier spring snowmelt scale exponentially with changes in moisture deficit, and moisture deficit changes can explain most of the spatial variability in forest wildfire regime response to the timing of spring. This article is part of the themed issue 'The interaction of fire and mankind'.
Statistical forecast models for fire season severity and fire suppression costs are developed for... more Statistical forecast models for fire season severity and fire suppression costs are developed for the Sierra Nevada and used to analyze the change in mean severity and suppression cost from present day conditions to climate change scenarios for 2030, 2060 and 2090. Statistical models estimate normalized area burned and fire suppression costs in constant 2000 dollars based on relationships between
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Papers by Leroy Westerling
(BA) and biomass consumed (BC) by fires between 101-125° W. The fire data were compared with daytime summer O3 mixing ratios from nine rural Clean Air Status and Trends Network (CASTNET) and National Park Service (NPS) sites. Large fire years exhibited widespread enhanced O3. The summer BA was significantly correlated with O3 at all sites. For each 1 million acres burned in the western U.S. during summer, we estimate that the daytime mean O3 was enhanced across the region
by 2.0 ppbv. For mean and maximum fire years, O3 was enhanced by an average of 3.5 and 8.8 ppbv, respectively. At most
sites O3 was significantly correlated with fires in the surrounding 5 × 5° and 10 × 10° regions, but not with fires in the
nearest 1 × 1° region, reflecting the balance between O3 production and destruction in a high NOx environment. BC was a slightly better predictor of O3, compared with BA. The relationship between O3 and temperature was examined at two sites (Yellowstone and Rocky Mountain National Parks). At these two sites, high fire years were significantly warmer than low fire years; however, daytime seasonal mean temperature and O3 were not significantly correlated. This indicates that the presence of fire is a more important predictor for O3 than is temperature.
(BA) and biomass consumed (BC) by fires between 101-125° W. The fire data were compared with daytime summer O3 mixing ratios from nine rural Clean Air Status and Trends Network (CASTNET) and National Park Service (NPS) sites. Large fire years exhibited widespread enhanced O3. The summer BA was significantly correlated with O3 at all sites. For each 1 million acres burned in the western U.S. during summer, we estimate that the daytime mean O3 was enhanced across the region
by 2.0 ppbv. For mean and maximum fire years, O3 was enhanced by an average of 3.5 and 8.8 ppbv, respectively. At most
sites O3 was significantly correlated with fires in the surrounding 5 × 5° and 10 × 10° regions, but not with fires in the
nearest 1 × 1° region, reflecting the balance between O3 production and destruction in a high NOx environment. BC was a slightly better predictor of O3, compared with BA. The relationship between O3 and temperature was examined at two sites (Yellowstone and Rocky Mountain National Parks). At these two sites, high fire years were significantly warmer than low fire years; however, daytime seasonal mean temperature and O3 were not significantly correlated. This indicates that the presence of fire is a more important predictor for O3 than is temperature.