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PHYSICAL SCIENCES / BIOLOGICAL SCIENCES / GEOPHYSICS / ENVIRONMENTAL SCIENCES
Contingent Pacific–Atlantic Ocean influence on multicentury wildfire synchrony over western North America
*Consejo Nacional de Investigaciones Científicas y Técnicas de Argentina and Laboratorio Ecotono, Universidad Nacional del Comahue, Quintral 1250, 8400 Bariloche, Argentina; Rocky Mountain Tree-Ring Research, Inc., 2901 Moore Lane, Ft. Collins, CO 80526; U.S. Department of Agriculture Forest Service, Rocky Mountain Research Station, 5775 U.S. West Highway 10, Missoula, MT 59808; ¶Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ 85721; and ||Department of Geography, University of Colorado, Boulder, CO 80301
Edited by Christopher B. Field, Carnegie Institution of Washington, Stanford, CA, and approved November 8, 2006 (received for review July 19, 2006)
Widespread synchronous wildfires driven by climatic variation, such as those that swept western North America during 1996, 2000, and 2002, can result in major environmental and societal impacts. Understanding relationships between continental-scale patterns of drought and modes of sea surface temperatures (SSTs) such as El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) may explain how interannual to multidecadal variability in SSTs drives fire at continental scales. We used local wildfire chronologies reconstructed from fire scars on tree rings across western North America and independent reconstructions of SST developed from tree-ring widths at other sites to examine the relationships of multicentury patterns of climate and fire synchrony. From 33,039 annually resolved fire-scar dates at 238 sites (the largest paleofire record yet assembled), we examined forest fires at regional and subcontinental scales. Since 1550 CE, drought and forest fires covaried across the West, but in a manner contingent on SST modes. During certain phases of ENSO and PDO, fire was synchronous within broad subregions and sometimes asynchronous among those regions. In contrast, fires were most commonly synchronous across the West during warm phases of the AMO. ENSO and PDO were the main drivers of high-frequency variation in fire (interannual to decadal), whereas the AMO conditionally changed the strength and spatial influence of ENSO and PDO on wildfire occurrence at multidecadal scales. A current warming trend in AMO suggests that we may expect an increase in widespread, synchronous fires across the western U.S. in coming decades.
Atlantic Multidecadal Oscillation | El Niño Southern Oscillation | fire history network | ocean warming | Pacific Decadal Oscillation
The authors declare no conflict of interest.
This article is a PNAS direct submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0606078104/DC1.
To whom correspondence should be addressed. E-mail: kitzberger{at}gmail.com
© 2007 by The National Academy of Sciences of the USA
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