Climate and Fire Interactions
How do temperature, relative humidity, and vapor pressure deficit relate to fire frequency, rotation, and return interval?
The FireBGCv2 model in the Jemez mountains projected an increase in fire frequency and high-severity fire due to the predicted increases in temperature and moisture deficit.
The authors found that mixed-conifer ecosystems are drought-limited, not fuel limited; therefore, they do not require prior wet years to build up fuels before burning, but instead will burn when fuel moistures are low.
The author estimate an annual mean lightning-strike frequency increase of 12% per °C. The authors suggest this could increase the frequency of wildfire across North America.
Under all projections of future climate change, the authors found that ponderosa pine basal area was reduced significantly, and under A2 scenarios (continued increase in carbon emissions), ponderosa pine were eliminated from the site. 10- to 20-year prescribed fire intervals in areas that burned at high severity were too frequent to maintain basal areas of ponderosa pine within the historical range of variation under the most extreme climate scenarios. Prescribed fire was, however, effective at maintaining low-severity sites initially dominated by smaller diameter trees.
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.
The authors found that future fire probabilities increased with increasing temperature; however predictions for each of the climate models diverged for the southwestern U.S. The CGCM data resulted in decreased future fire probability (0 to ?30%) while the GFDL data resulted in increased fire probability (near 0 to greater than 40%). The authors suggest this discrepancy is due to the limitations in predicting precipitation and moisture conditions and their effect on fuel production.
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 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.
CGM models in this study project winter precipitation to continue to decreases across the warm deserts in the study area and an increase in the annual mean temperature by 2.5-3°C by the middle of the 21st century. This future climate change is likely to increase the potential for megafires across the Southwest by increasing the frequency of weather conditions conducive to extreme fire activity, especially an increase in temperature and decrease in corresponding precipitation and humidity indicators. Along with the predicted changes in temperature and precipitation, periods of extreme fire danger are expected to increase by one to three weeks across these areas leading to the occurrence of chronic fire seasons every fifth year to every other year.
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.
Charcoal concentration in the sediment records at both bogs varied considerably over 15,000 years suggesting that climate affected both vegetation and fire regimes. Around approximately 8,000 years before present, sharp increases in charcoal concentrations indicate increased warming, however, the authors were unable to interpret the fire history due to confounding factors, specifically the low sedimentation rate during these periods.
The interannual variability in wildfire frequency had a strong relationship with regional spring and summer temperature. The authors suggest that as temperatures continue to warm, wildfires will become larger and more frequent.
The authors summarized findings on high-severity fire regimes in subalpine forests and found that climatic variation is the predominant influence on fire frequency and severity in this ecosystem type and suggest that fuel reduction treatments would move stand structure away from its historical range of variability. For low severity fire regimes in low-elevation ponderosa pine forest, the authors found that fire frequency and severity were driven by the spatial and temporal variation of fine fuels more so than climate.
The authors found that during historically cooler periods, forests burned frequently at low severity, which they suggest was driven by increases in understory vegetation growth. Historically warm periods were linked to severe drought and an increase in high severity fires that caused large debris-flow events and fire-related erosion.
Historically, fire was highly synchronous across the region and appeared to be linked to climate conditions.
The authors found that lightning activity and lightning-started fires are likely to increase in a warmer climate. They found a 44% increase in the likelihood of lightning-caused fires for a 2 X CO2 climate scenario.