Proceedings of  the National Academy of Sciences
PNAS _ January 9, 2007 _ vol. 104 _ no. 2 _ 543-548
Contingent Pacific-Atlantic Ocean influence on
multicentury wildfire synchrony over western
North America
Thomas Kitzberger*, Peter M. Brown , Emily K.
Heyerdahl , Thomas W. Swetnam , and Thomas T.
Veblen _

ABSTRACT : 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 Nino-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 scalesÅ .. Since 1550
CE, drought and forest fires covaried across the
West, but in a manner contingent on SST modes. Å .
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.

18 AUGUST 2006 VOL 313
Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity
A. L. Westerling, H. G. Hidalgo, D. R. Cayan, T. W. Swetnam

ABSTRACT : Much of the public and scientific
discussion of changes in western United States
wildfire has focused instead on the effects of
19th-and 20th-century land-use history. We
compiled a comprehensive database of large
wildfires in western United States forests since
1970 and compared it with hydroclimatic and
land-surface data. Here, we show that large
wildfire activity increased suddenly and markedly
in the mid-1980s, with higher large-wildfire
frequency, longer wildfire durations, and longer
wildfire seasons. The greatest increases occurred
in mid-elevation, Northern Rockies forests, where
land-use histories have relatively little effect
on fire risks and are strongly associated with
increased spring and summer temperatures and an
earlier spring snowmelt.

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