The association of megafires and extreme droughts in California

Background/Question/Methods

Anomalously large wildfires (defined as exceeding 50,000 ha) have occurred repeatedly on California landscapes. Written records beginning in the 19^th century document more than a dozen such fire events. This study investigates the association of extreme droughts  with these huge fires and contrasts climatic conditions during these fire years with other years of more moderate burning. Written records include 19^th century newspaper accounts of fires and 20^th and 21^stcentury fire statistics available for both federal and state responsibility lands. For the last 115 years climate data were from NOAA and DRI websites and prior to that from western US tree ring studies.

Results/Conclusions

Megafires have been documented from most parts of the state although the majority have been concentrated in southern California.  Extended droughts measured by the Palmer Drought Severity Index  are significantly longer and more intense in the months and years prior to major fire events. Drought exacerbates fires by increasing the duration of low fuel moisture through the year and by increasing the level of dead fuels due to vegetation die-back. The majority of these huge fires have occurred in non-forested vegetation types and preliminary data suggests that drought-induced die-back of fine fuels has contributed to fire size by increasing the chances of spot fires igniting from wind-blown embers. Future global changes are likely to present different effects in the northern vs southern half of the state with respect to the incidence of large fire events.