Climate Number: 510 Years
Better understanding how fluctuations in climate have affected regional wildfire frequency over the past few centuries may help to improve our ability to predict severe wildfire seasons. Some of the West’s older groves have experienced dozens of wildfires over the past few centuries. The trees that survived these fires recorded black scars in their annual tree rings, providing us with the ability to know what year a given area burned. Core samples (profiles of the annual rings taken from a part of the tree trunk) are used to figure out when and where of fires occurred in the past. Enough of these fire scars from a long enough period of time make it possible to investigate the possible links between large scale climate variability and fire in given regions of the West. A recent investigation using core samples from hundreds of trees across the West found that fire years are more common during drought years and the occurrence of drought is controlled by large-scale movements of water in the oceans. Specifically, different phases of the El Nino-Southern Oscillation, the periodic warming and cooling of the eastern tropical Pacific, and the Atlantic Multidecadal Oscillation, the periodic warming and cooling of the North Atlantic, influence the frequency of fire in the West. Fire in all regions of the West is more common when the North Atlantic is warmer. The Pacific Northwest has more fires when the eastern tropical Pacific is warmer and the Southwest has more fires when the eastern tropical Pacific is cooler than average. The oldest trees used in this study had fire records going back to 1400 CE, 510 years ago.
For Comparison: The Mongol Empire was at its peak 510 years ago, with its soldiers sacking cities as far west as Damascus. In Florence, Italy, the Medici family was building the banking empire that would revolutionize western finance.
Seasons: Winter, Spring, Summer
Sources: Kitzberger, T et al. “Contingent Pacific-Atlantic Ocean influence on multicentury wildfire synchrony over western North America.” Proceedings of the National Academy of Sciences 104 (2007): 543-548 and Trouet, V et al. “Fire-climate interactions in the American West since 1400 CE.” Geophysical Research Letters 37 (2010): L04702.
