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Forest Fires in the Western United States
Reference
Schoennagel, T., Veblen, T.T., Kulakowski, D. and Holz, A. 2007. Multidecadal climate variability and climate interactions affect subalpine fire occurrence, western Colorado (USA). Ecology 88: 2891-2902.

Background
In his book An Inconvenient Truth, Al Gore writes that "wildfires are becoming much more common as hotter temperatures dry out the soil and the leaves," stating that there has been a "steady increase in major wildfires in North and South America over the last five decades" and "on every other continent as well," due to concurrent global warming. Although Gore's claim has convincingly been shown to be absolutely false -- see our review of Riano et al. (2007) -- there are many people who continue to promulgate this discredited scenario.

What was done
In a study that analyzed the relationship between climate change and wildfire occurrence in considerable detail, Schoennagel et al. investigated "climatic mechanisms influencing subalpine forest fire occurrence in western Colorado, which provide a key to the intuitive link between drought and large, high-severity fires that are keystone disturbance processes in many high-elevation forests in the western United States," focusing on three major climatic oscillations: the El Niņo Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO).

What was learned
The four University of Colorado geographers report that "fires occurred during short-term periods of significant drought and extreme cool (negative) phases of ENSO and PDO and during positive departures from [the] mean AMO index," while "at longer time scales, fires exhibited 20-year periods of synchrony with the cool phase of the PDO, and 80-year periods of synchrony with extreme warm (positive) phases of the AMO." In addition, they say that "years of combined positive AMO and negative ENSO and PDO phases represent 'triple whammies' that significantly increased the occurrence of drought-induced fires." On the other hand, they write that "drought and wildfire are associated with warm phases of ENSO and PDO in the Pacific Northwest and northern Rockies while the opposite occurs in the Southwest and southern Rockies," citing the findings of Westerling and Swetnam (2003), McCabe et al. (2004) and Schoennagel et al. (2005).

What it means
Climatic influences on the occurrence of western U.S. wildfires are definitely much more complex than what is suggested by simplistic climate-alarmist claims; and Schoennagel et al. thus conclude "there remains considerable uncertainty regarding the effects of CO2-induced warming at regional scales." Nevertheless, they report "there is mounting evidence that the recent shift to the positive phase of the AMO will promote higher fire frequencies" in the region of their study, i.e., high-elevation western U.S. forests; but the body of their work clearly suggests that such a consequence should certainly not be viewed as a response to CO2-induced global warming, either real or imagined.

References
McCabe, G.J., Palecki, M.A. and Betancourt, J.L. 2004. Pacific and Atlantic Ocean influences on multidecadal drought frequency in the United States. Proceedings of the National Academy of Sciences (USA) 101: 4136-4141.

Riano, D., Moreno Ruiz, J.A., Isidoro, D. and Ustin, S.L. 2007. Global spatial patterns and temporal trends of burned area between 1981 and 2000 using NOAA-NASA Pathfinder. Global Change Biology 13: 40-50.

Schoennagel, T., Veblen, T.T., Romme, W.H., Sibold, J.S. and Cook, E.R. 2005. ENSO and PDO variability affect drought-induced fire occurrence in Rocky Mountain subalpine forests. Ecological Applications 15: 2000-2014.

Westerling, A.L. and Swetnam, T.W. 2003. Interannual to decadal drought and wildfire in the western United States. EOS, Transactions, American Geophysical Union 84: 545-560.

Reviewed 19 March 2008