Climate and Fire Interactions
How do temperature, relative humidity and vapor pressure deficit relate to annual area burned and fire size?
The interannual variation of each of the eight fuel aridity metrics, reference potential evapotranspiration (ETo), VPD, climatic water deficit (CWD), Palmer drought severity index (PDSI), fire weather index (FWI) from the Canadian forest fire danger rating system, energy release component (ERC) from the US national fire danger rating system, McArthur forest fire danger index (FFDI), and the Keetch–Byram drought index (KBDI), were all significantly correlated to annual area burned of forested lands from 1984-2015. Furthermore, the authors observed a 3.3-fold difference in the area burned between 1984 to 1999 and 2000 to 2015.
The authors found that fire season length has significantly increased across approximately 25% of the Earth’s vegetated area resulting in a doubling of the global burnable area. Areas with the greatest increase in fire season length occurred where there were the greatest changes in especially temperature, but also humidity, length of rain-free periods, and wind speeds.
The authors found that very large fire potential is projected to increase across much of the U.S. with the greatest increases expected across the intermountain West due to overall warming and diminished moisture. The increase in potential is due to both an increase in the frequency and the length of the seasonal window when conditions are conducive to extreme fire activity.
March through August VPD, a measure of the ability of the atmosphere to extract moisture from surface vegetation, is more strongly correlated with area burned than temperature alone for the Southwest region.
Temperature and humidity were only marginally important. Wind speed percentile was, however, the strongest climate/weather predictor of fire severity (and third strongest predictor overall) for daily areas burned when considering only the largest daily areas burned (99.5% percentile or > 600 ha) while temperature and humidity were not important in this model at all. The authors suggest that the factors influencing fire severity are not necessarily the same as those that control fire extent.
During the 2011 fire season, which was characterized by record-breaking area burned in the southwest, temperature was not anomalously warm, however low precipitation led to exceptionally low atmospheric moisture content and subsequent record-breaking VPD. The authors’ model projections show that the climate conditions like those exhibited in 2011 are likely to occur more frequently in coming decades as temperatures are predicted to increase. The authors suggest increasing trends in VPD could lead to increasingly common catastrophic wildfires if fuels are not limiting.
The authors found that fire activity/area burned and fire severity across the western US increased with increasing actual evapotranspiration (AET) which represented fuel amount in the study. However, area burned increased with increasing water deficit (fuel moisture) to a threshold before fire activity subsequently decreased. This suggests that fire activity is both fuel and moisture limited. Regional areas across the West that have higher fuel moistures and lower water deficits rarely burn because fuel is not dry enough to ignite, whereas areas with low fuel moisture and high moisture deficits are not productive enough to support fire spread.
The authors found positive trends in the number of large fires in most ecoregions of the western U.S. including the Arizona-New Mexico Mountains with an average increase of 0.6 large fires per year. The ecoregions with trends toward increased fire activity also concurrently displayed increasing trends in drought severity over the period of the analysis. While the authors acknowledge that it is difficult to directly correlate human-caused climate change and fire activity, they assert that regardless of cause, changes in fire activity are part of larger trends tied to higher temperature and drought that are likely to continue.
The authors found an increase in the future probability of very large fire (VLF, >50,000 ac) occurrence and frequency under both representative concentration pathways (RCP) 4.5 and 8.5 for all GACCs in the contiguous Western U.S. by 2060. Areas where fire is directly tied to hotter and drier conditions, or non-fuel limited ecosystems, were more likely to experience an increase in the likelihood of VLFs than areas that typically burn due to lagged effects of temperature and precipitation, typically fuel-limited ecosystems.
For the Southwest GACC specifically, temperature, precipitation, and drought indicators were strongly correlated to forested area burned. Unburned area within a fire perimeter did show a negative correlation to summer (June – Aug) temperature, however, the authors conclude that temperature is acting as an indirect proxy for fuel moisture stress and flammability. In general, biophysical variables that include a direct link to fuel moisture conditions performed better than any single individual variable, such as temperature or precipitation.
The authors found that anomalously warm conditions occurred during a period of intense fire activity in the summer of 1910. A strong La Niña pattern in the year of 1910 likely contributed to warming and drought during the spring and summer prior to the fires. Furthermore, similar anomalous conditions in 1998 and 2012 occurred during other significant fire years across areas of the western U.S.
The authors found that from 1930 to 2006, the number and extent of wildfires has increased significantly due to increases in human-ignited fires and warmer temperatures, especially in the last few decades. The more moderate scenario, B1 that projects lower temperature increases, resulted in the highest increases in median area burned over the A2 scenario, which projects the highest increases in temperature. The authors suggest that an increase in precipitation under the more extreme A2 scenario may moderate area burned to some extent.
The authors agreed with the consensus that increases in temperature will increase fire activity in the Southwest U.S. by the end of the century. Their results indicated fire intensity may not be significantly greater under the effects of climate change on an individual fire level, however, fire will likely occur more frequently and burn a larger total area.
The authors predicted an increase in fire occurrence, frequency, and size by midcentury in the Greater Yellowstone ecosystem due to projected increases in average spring and summer temperatures.
From 1984 to 2006, fire and bark beetle outbreaks have caused approximately 14-18% of the mortality in forested areas of the Southwest, and the area burned has increased substantially with a greater proportion burning at high severity since 1984. Recent elevated temperatures are thought to have led to both the increase in bark beetle outbreak and high severity fire.
The authors reconstruction of past fire activity suggests that until the late 18th century fire was driven mostly by climate. Specifically, they found that variation in global precipitation had the strongest influence on global fire activity. Post Industrial Revolution, anthropogenic activities had a stronger influence on global fire trends. Finally, predictions in future warming due to climate change suggest an imminent shift to temperature-driven global fire activity over the next century.
Fire activity declined significantly during the late 16th century during a period of anomalously low temperature, referred to as the Little Ice Age.
The authors found that globally, changes in temperature and precipitation may result in a rearrangement of fire probabilities where fire activity will increase in some areas and decrease in others under climate change.
For the mountainous ecoprovince of Arizona and New Mexico, temperature during the year-of-the fire was related to area burned in these areas as well as annual (water year) PDSI.
Convective instability associated with high temperatures may also lead to increased lightning occurrence and thus increased ignition rates.
The authors predicted an increase in wildfire area burned across most of the western U.S. based on a 1–3°C increase in temperature, with larger increases in some areas, Pacific Northwest and Rocky Mountains Forest ecoregions, than others, almost no increase predicted for Nevada Nevada Mountains/Semidesert and Eastern Rocky Mountains/Great Plains ecoregions. Along with predicted increases in area burned, the authors modeled a 40% increase in summertime organic carbon (OC) concentrations and an 18% increase in elemental carbon (EC) concentrations by 2050. Areas with the highest predicted future increases in carbonaceous aerosol concentrations are concomitant with the areas of greatest increases in wildfire.
Climate change projections suggest a shift toward hotter and drier conditions across the west resulting in an approximately 50 to 500% increase in area burned across the western U.S.
Increasingly warm temperatures have significantly increased annual area burned by at least six and a half times between the time periods of 1970 to 1986 and 1987 to 2003. The authors suggest that as temperatures continue to warm, wildfires will become larger and more frequent.
The authors’ modeling effort found that seasonal severity rating (SSR) may increase 10-50% by 2060 likely increasing area burned and fire severity across much of the U.S.
The authors found a significant correlation between annual area burned and fire intensity directly related to the weather variable frequency. Years where large area burned were associated with extreme fire weather and subsequent high intensity fire.