Incorporating anthropogenic influences into fire probability models : effects of human activity and climate change on fire activity in California

The costly interactions between humans and wildfires throughout California demonstrate the need to understand the relationships between them, especially in the face of a changing climate and expanding human communities. Although a number of statistical and process-based wildfire models exist for California, there is enormous uncertainty about the location and number of future fires, with previously published estimates of increases ranging from nine to fifty-three percent by the end of the century. Our goal is to assess the role of climate and anthropogenic influences on the state's fire regimes from 1975 to 2050. We develop an empirical model that integrates estimates of biophysical indicators relevant to plant communities and anthropogenic influences at each forecast time step. Historically, we find that anthropogenic influences account for up to fifty percent of explanatory power in the model. We also find that the total area burned is likely to increase, with burned area expected to increase by 2.2 and 5.0 percent by 2050 under climatic bookends (PCM and GFDL climate models, respectively). Our two climate models show considerable agreement, but due to potential shifts in rainfall patterns, substantial uncertainty remains for the semiarid inland deserts and coastal areas of the south. Given the strength of human-related variables in some regions, however, it is clear that comprehensive projections of future fire activity should include both anthropogenic and biophysical influences. Previous findings of substantially increased numbers of fires and burned area for California may be tied to omitted variable bias from the exclusion of human influences. The omission of anthropogenic variables in our model would overstate the importance of climatic ones by at least 24%. As such, the failure to include anthropogenic effects in many models likely overstates the response of wildfire to climatic change

Incorporating anthropogenic influences into fire probability models : effects of human activity and climate change on fire activity in California

The costly interactions between humans and wildfires throughout California demonstrate the need to understand the relationships between them, especially in the face of a changing climate and expanding human communities. Although a number of statistical and process-based wildfire models exist for California, there is enormous uncertainty about the location and number of future fires, with previously published estimates of increases ranging from nine to fifty-three percent by the end of the century. Our goal is to assess the role of climate and anthropogenic influences on the state's fire regimes from 1975 to 2050. We develop an empirical model that integrates estimates of biophysical indicators relevant to plant communities and anthropogenic influences at each forecast time step. Historically, we find that anthropogenic influences account for up to fifty percent of explanatory power in the model. We also find that the total area burned is likely to increase, with burned area expected to increase by 2.2 and 5.0 percent by 2050 under climatic bookends (PCM and GFDL climate models, respectively). Our two climate models show considerable agreement, but due to potential shifts in rainfall patterns, substantial uncertainty remains for the semiarid inland deserts and coastal areas of the south. Given the strength of human-related variables in some regions, however, it is clear that comprehensive projections of future fire activity should include both anthropogenic and biophysical influences. Previous findings of substantially increased numbers of fires and burned area for California may be tied to omitted variable bias from the exclusion of human influences. The omission of anthropogenic variables in our model would overstate the importance of climatic ones by at least 24%. As such, the failure to include anthropogenic effects in many models likely overstates the response of wildfire to climatic change

Description of the 10 variables considered to estimate fire occurrence and burned area.

<p>Description of the 10 variables considered to estimate fire occurrence and burned area.</p

Biophysical and anthropogenic determinants of wildfire.

<p>Interactions of wildfire requirements as regulated by biophysical and anthropogenic influences. Anthropogenic determinants are shaded in yellow and biophysical influences in orange.</p

Mean fire return interval (mFRI) estimate 1976–2000.

<p>Mean fire return interval (mFRI) estimate 1976–2000 (A), and its change from the historical period (1951–1975) (B). In (a) low mFRIs (red) designate short intervals between fire events, and high mFRI designate long intervals (green). In (B) positive values (green) indicate an increase in mFRI (i.e., decrease in the number of fires) between the estimation and previous period (1976–2000 less 1951–1975), whereas negative values (red) indicate a decreasing mFRI or an increase in the number of fires. (C) Varying constraint map of fire resource and conditions 1976–2000: Four categories of fire resources as proxied by AET, and fire conditions proxied by CWD. Color is determined by the unique combinations of resources and conditions. Where increasingly favorable fire conditions are indicated with deepening red hues, and increasingly favorable resource classes are indicated with deeper hues of blue.</p

Contribution to 25 year expected fire count by natural and human factors.

<p>Contribution of natural (A) and human (B) variables, ceteris paribus, to the expected fire count predicted during the 1976–2000 period. Natural variables are defined as all variants of AET, CWD, lighting, elevation and slope. Human variables include distance to population centers, neighboring maximum housing density, distance to populated places, and distance to campsites.</p

Change in MFRI 2001–2025, 2026–2050 PCM compared to 1976–2000.

<p>Change in MFRI for the PCM climate from the base period 1976–2000 to the first forecast period 2001–2025 (A), the change in MFRI from the base period 1976–2000 to the second forecast period 2026–2050 (B). Positive values indicate increasing MFRI or fewer fires, and the inverse for negative values.</p

Area burned (Km²) by modeling period and climate model.

<p>Grey bars depict the area burned over the historical (1951–1975) and estimation (1976–2001) periods, whereas red and green bars depict the expected area burned into the future under the Geophysical Fluid Laboratory (GFDL) and Parallel Climate Model (PCM), respectively.</p

Model agreement for change in MFRI 2001–2025 and 2026–2050 compared to 1976–2000.

<p>Model agreement between the GFDL and PCM A2 scenarios for the (A) 2001–2025 and (B) 2026–2050 periods. Green indicates areas where model estimations from both GFDL and PCM models indicate increasing MFRI, or fewer fires. Red indicates areas where model estimations from both indicate a decreasing MFRI, or more fires. Grey indicates areas of disagreement.</p