Papers by Anthony Westerling
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'.
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.
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.
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 %.
Large wildfire occurrence and burned area are modeled using hydroclimate and landsurface characte... more Large wildfire occurrence and burned area are modeled using hydroclimate and landsurface characteristics under a range of future climate and development scenarios. The range of uncertainty for future wildfire regimes is analyzed over two emissions pathways (the Special Report on Emissions Scenarios [SRES] A2 and B1 scenarios); three global climate models (Centre National de Recherches Météorologiques CM3, Geophysical Fluid Dynamics Laboratory CM2.1 and National Center for Atmospheric Research PCM1); three scenarios for future population growth and development footprint; and two thresholds for defining the wildland-urban interface relative to housing density. Results were assessed for three 30-year time periods centered on 2020, 2050, and 2085, relative to a 30-year reference period centered on 1975. Increases in wildfire burned area are anticipated for most scenarios, although the range of outcomes is large and increases with time. The increase in wildfire burned area associated with the higher emissions pathway (SRES A2) is substantial, with increases statewide ranging from 36% to 74% by 2085, and increases exceeding 100% in much of the forested areas of Northern California in every SRES A2 scenario by 2085.
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.
… science & technology, 2008
Journal of Geophysical …, 2009
1] We investigate the impact of climate change on wildfire activity and carbonaceous aerosol conc... 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.
Journal of …, 2008
We conducted surveys of fire and fuels managers at local, regional, and national levels to gain i... more We conducted surveys of fire and fuels managers at local, regional, and national levels to gain insights into decision processes and information flows in wildfire management. Survey results in the form of fire managers' decision calendars show how climate information needs vary seasonally, over space, and through the organizational network, and help determine optimal points for introducing climate information and forecasts into decision processes. We identified opportunities to use climate information in fire management, including seasonal to interannual climate forecasts at all organizational levels, to improve the targeting of fuels treatments and prescribed burns, the positioning and movement of initial attack resources, and staffing and budgeting decisions. Longer-term (5-10 years) outlooks also could be useful at the national level in setting budget and research priorities. We discuss these opportunities and examine the kinds of organizational changes that could facilitate effective use of existing climate information and climate forecast capabilities.
… Journal of Wildland Fire, 2002
A statistical forecast methodology exploits large-scale patterns in monthly U.S. Climatological D... more A statistical forecast methodology exploits large-scale patterns in monthly U.S. Climatological Division Palmer Drought Severity Index (PDSI) values over a wide region and several seasons to predict area burned in western U.S. wildfires by ecosystem province a season in advance. The forecast model, which is based on canonical correlations, indicates that a few characteristic patterns determine predicted wildfire season area burned. Strong negative associations between anomalous soil moisture (inferred from PDSI) immediately prior to the fire season and area burned dominate in most higher elevation forested provinces, while strong positive associations between anomalous soil moisture a year prior to the fire season and area burned dominate in desert
Proceedings of the …, 2011
Climate change is likely to alter wildfire regimes, but the magnitude and timing of potential cli... more Climate change is likely to alter wildfire regimes, but the magnitude 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 developed a suite of statistical models that related monthly climate data 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 substantial 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-climatevegetation relationships in Greater Yellowstone by midcentury because fire frequency and extent would be inconsistent with persistence 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 subalpine forests.
Western US Forest managers face more wildfires than ever before, and it is increasingly imperativ... more Western US Forest managers face more wildfires than ever before, and it is increasingly imperative to anticipate the consequences of this trend. Large fires in the northern Rocky Mountains have increased in association with warmer temperatures, earlier snowmelt, and longer fire seasons (1), and this trend is likely to continue with global warming (2). Increased wildfire occurrence is already a concern shared by managers from many federal land-management agencies (3). However, new analyses for the western US suggest that future climate could diverge even more rapidly from past climate than previously suggested. Current model projections suggest end-of-century hydroclimatic conditions like those of 1988 (the year of the well-known Yellowstone Fires) may represent close to the average year rather than an extreme year. The consequences of a shift of this magnitude for the fire regime, post-fire succession and carbon (C) balance of western forest ecosystems are well beyond what scientists have explored to date, and may fundamentally change the potential of western forests to sequester atmospheric C. We link hydroclimatic extremes (spring and summer temperature and cumulative water-year moisture deficit) to extreme fire years in northern Rockies forests, using large forest fire histories and 1/8-degree gridded historical hydrologic simulations (1950 - 2005) (4) forced with historical gridded temperature and precipitation (5). The frequency of extremes in hydroclimate associated with historic severe fire years in the northern Rocky Mountains is compared to those projected under a range of climate change projections, using global climate model runs for the A2 and B1 emissions pathways for three global climate models (NCAR PCM1, GFDL CM2.1, CNRM CM3). Coarse-scale climatic variables are downscaled to a 1/8 degree grid and used to force hydrologic simulations (6, 7). We will present preliminary results using these hydrologic simulations to model spatially explicit annual wildfire occurrence historically and under the above-cited future climate scenarios, and discuss how these results are being integrated with process-based ecosystem models and field data to model changes in carbon flux across the Greater Yellowstone Ecosystem landscape (8). 1. Westerling, Hidalgo, Cayan, Swetnam, Science 313, 940 (2006). 2. Tymstra, Flannigan, Armitage, Logan, Int’l J. Wildland Fire 16, 153 (2007). 3. U. S. G. A. O. GAO. (2007). 4. Liang, Lettenmaier, Wood, Burges. J. Geophys. Res. 99(D7), 14,415 (1994). 5. Maurer, Wood, Adam, Lettenmaier, Nijssen. J. Climate 15:3237 (2002). 6. Cayan, Maurer, Dettinger, Tyree, Hayhoe. Climatic Change 87(Suppl. 1) 21 (2008). 7. Hidalgo, Dettinger Cayan, CEC Report CEC-500-2007-123 (2008). 8. We acknowledge support from the Joint Fire Science Program (Project ID 09-3-01-47), the NOAA RISA program for California, and the US Forest Service.
Western US Forest managers face more wildfires than ever before, and it is increasingly imperativ... more Western US Forest managers face more wildfires than ever before, and it is increasingly imperative to anticipate the consequences of this trend. Large fires in the northern Rocky Mountains have increased in association with warmer temperatures, earlier snowmelt, and longer fire seasons (1), and this trend is likely to continue with global warming (2). Increased wildfire occurrence is already a concern shared by managers from many federal land-management agencies (3). However, new analyses for the western US suggest that future climate could diverge even more rapidly from past climate than previously suggested. Current model projections suggest end-of-century hydroclimatic conditions like those of 1988 (the year of the well-known Yellowstone Fires) may represent close to the average year rather than an extreme year. The consequences of a shift of this magnitude for the fire regime, post-fire succession and carbon (C) balance of western forest ecosystems are well beyond what scientists have explored to date, and may fundamentally change the potential of western forests to sequester atmospheric C. We link hydroclimatic extremes (spring and summer temperature and cumulative water-year moisture deficit) to extreme fire years in northern Rockies forests, using large forest fire histories and 1/8-degree gridded historical hydrologic simulations (1950 - 2005) (4) forced with historical gridded temperature and precipitation (5). The frequency of extremes in hydroclimate associated with historic severe fire years in the northern Rocky Mountains is compared to those projected under a range of climate change projections, using global climate model runs for the A2 and B1 emissions pathways for three global climate models (NCAR PCM1, GFDL CM2.1, CNRM CM3). Coarse-scale climatic variables are downscaled to a 1/8 degree grid and used to force hydrologic simulations (6, 7). We will present preliminary results using these hydrologic simulations to model spatially explicit annual wildfire occurrence historically and under the above-cited future climate scenarios, and discuss how these results are being integrated with process-based ecosystem models and field data to model changes in carbon flux across the Greater Yellowstone Ecosystem landscape (8). 1. Westerling, Hidalgo, Cayan, Swetnam, Science 313, 940 (2006). 2. Tymstra, Flannigan, Armitage, Logan, Int’l J. Wildland Fire 16, 153 (2007). 3. U. S. G. A. O. GAO. (2007). 4. Liang, Lettenmaier, Wood, Burges. J. Geophys. Res. 99(D7), 14,415 (1994). 5. Maurer, Wood, Adam, Lettenmaier, Nijssen. J. Climate 15:3237 (2002). 6. Cayan, Maurer, Dettinger, Tyree, Hayhoe. Climatic Change 87(Suppl. 1) 21 (2008). 7. Hidalgo, Dettinger Cayan, CEC Report CEC-500-2007-123 (2008). 8. We acknowledge support from the Joint Fire Science Program (Project ID 09-3-01-47), the NOAA RISA program for California, and the US Forest Service.
Reevaluation of the spring onset/fire association in the western U.S. using Phenological vs. Hydrological Models
An important aspect of climate variability and change is the exact timing of the transition from ... more An important aspect of climate variability and change is the exact timing of the transition from winter to spring, generally defined here as spring onset. Spring onset can have important hydrological and ecological consequences, including changes in the timing of snowmelt and snowmelt runoff, in timing of plant and animal phenologies and their interactions, in ecosystem fluxes, and in the probabilities of ecological disturbances such as fire and insect outbreaks. Spring onset can be variably defined, which can affect its use as a predictor. To evaluate changing fire probabilities in the western U.S., Westerling et al. (2006) used center of mass of annual streamflow (CT, after Stewart et al 2005) for snowmelt-dominated gauge records as a proxy for spring onset, and compared it with the number of forest wildfires greater than 400 ha annually around the western U.S. This study indicated a strong association between large wildfire occurrence across the West and CT, and a particular sensitivity to the timing of snowmelt in the Northern Rockies. Though the timing of snowmelt can affect fire occurrence in several ways, the use of CT as a proxy for spring onset biased the analysis towards higher elevations and latitudes. To skirt this bias, we undertook a similar analysis using Spring Indices (SI) developed from cloned lilac and honeysuckle phenological data and representing seasonally integrated changes in temperature (Schwartz et al. 2006). The SI models can be generated at any location that has daily maximum-minimum temperature time series, and allowed comparison of large fire occurrence in defined regions with a network of select weather stations across the West for which we computed SI. The SI/fire comparison showed strong associations between SI at weather stations, particularly those in the Central Rockies/Colorado Plateau and large fire frequency in the northern, central and southern Rockies, as well as in the Sierra Nevada, but less so in southern California and the Black Hills. Given large differences in fire seasonality, vegetation type, and the importance of snowpack, explanations for the spring onset/fire association could be inherently complex. Though they also have biases and shortcomings, phenological models such as SI may be particularly useful in predicting climate change impacts on fire and other phenomena, and could offer more precision and better lead time in fire forecasting. Schwartz, M.D., R. Ahas, A. Aasa 2006: "Onset of spring starting earlier across the Northern Hemisphere" Global Change Biology, 12: 343-351. Stewart, I.T., D.R. Cayan and M.D. Dettinger, 2005: Changes toward earlier streamflow timing across Western North America. Journal of Climate, 18, 1136-1155. Westerling, A.L., H.G. Hidalgo, D.R. Cayan, T.W. Swetnam 2006: "Warming and Earlier Spring Increases Western U.S. Forest Wildfire Activity" Science, 313: 940-943.
Reevaluation of the spring onset/fire association in the western U.S. using Phenological vs. Hydrological Models
An important aspect of climate variability and change is the exact timing of the transition from ... more An important aspect of climate variability and change is the exact timing of the transition from winter to spring, generally defined here as spring onset. Spring onset can have important hydrological and ecological consequences, including changes in the timing of snowmelt and snowmelt runoff, in timing of plant and animal phenologies and their interactions, in ecosystem fluxes, and in the probabilities of ecological disturbances such as fire and insect outbreaks. Spring onset can be variably defined, which can affect its use as a predictor. To evaluate changing fire probabilities in the western U.S., Westerling et al. (2006) used center of mass of annual streamflow (CT, after Stewart et al 2005) for snowmelt-dominated gauge records as a proxy for spring onset, and compared it with the number of forest wildfires greater than 400 ha annually around the western U.S. This study indicated a strong association between large wildfire occurrence across the West and CT, and a particular sensitivity to the timing of snowmelt in the Northern Rockies. Though the timing of snowmelt can affect fire occurrence in several ways, the use of CT as a proxy for spring onset biased the analysis towards higher elevations and latitudes. To skirt this bias, we undertook a similar analysis using Spring Indices (SI) developed from cloned lilac and honeysuckle phenological data and representing seasonally integrated changes in temperature (Schwartz et al. 2006). The SI models can be generated at any location that has daily maximum-minimum temperature time series, and allowed comparison of large fire occurrence in defined regions with a network of select weather stations across the West for which we computed SI. The SI/fire comparison showed strong associations between SI at weather stations, particularly those in the Central Rockies/Colorado Plateau and large fire frequency in the northern, central and southern Rockies, as well as in the Sierra Nevada, but less so in southern California and the Black Hills. Given large differences in fire seasonality, vegetation type, and the importance of snowpack, explanations for the spring onset/fire association could be inherently complex. Though they also have biases and shortcomings, phenological models such as SI may be particularly useful in predicting climate change impacts on fire and other phenomena, and could offer more precision and better lead time in fire forecasting. Schwartz, M.D., R. Ahas, A. Aasa 2006: "Onset of spring starting earlier across the Northern Hemisphere" Global Change Biology, 12: 343-351. Stewart, I.T., D.R. Cayan and M.D. Dettinger, 2005: Changes toward earlier streamflow timing across Western North America. Journal of Climate, 18, 1136-1155. Westerling, A.L., H.G. Hidalgo, D.R. Cayan, T.W. Swetnam 2006: "Warming and Earlier Spring Increases Western U.S. Forest Wildfire Activity" Science, 313: 940-943.
In the near future, the Sierra Nevada's climate is projected to experience a new form of climate ... more In the near future, the Sierra Nevada's climate is projected to experience a new form of climate change due to increasing concentrations of greenhouse gases in the global atmosphere from the burning of fossil fuels and other human activities. If the changes occur, they presumably will be added to the large interannual and longer-term climate variations in the recent and distant past that have been described in this chapter. The projected changes include much-discussed warming trends as well as important changes in precipitation, extreme weather, and other climatic conditions, all of which may be expected to affect Sierra Nevada rivers, watersheds, landscapes, and ecosystems.
O clima, é dicir, a temperatura e a precipitación, inflúen de maneira importante en que se produz... more O clima, é dicir, a temperatura e a precipitación, inflúen de maneira importante en que se produzan lumes naturais grandes en múltiples escalas de tempo, a través dos seus efectos sobre a dispoñibili-40 PREVENDO OS DESASTRES AMBIENTAIS: UNHA REFLEXIÓN CRÍTICA 5 Osmond e outros, 1990.
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Papers by Anthony Westerling