Multidecadal climate variability and climate interactions affect subalpine fire occurrence, western Colorado (USA)

Characterization of trends in ecological phenomena over periods of several decades or more provides insight into the ‘long now’ but these multidecadal trends are difficult to perceive in the absence of long-term historical analysis. This study investigates the influence of multidecadal climatic variability on subalpine forest fire occurrence in western Colorado during the 1600-2003 AD period. Interannual and multidecadal relationships between fire occurrence and the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) were examined, in addition to the effects of phase interactions among these oscillations, using a novel statistical approach. Fires occurred during short-term periods of significant drought and extreme cool (negative) phases of ENSO and PDO and during positive departures from mean AMO index. At longer time scales, fires exhibited 20-yr periods of synchrony with the cool phase of the PDO, and 80-yr periods of synchrony with extreme warm (positive) phases of the AMO. Years of combined positive AMO and negative ENSO and PDO phases represent ‘triple whammies’ that significantly increased the occurrence of drought-induced fires. Fires were synchronous with this phase combination over 0-30 yr periods and distinctly asynchronous with the opposite phase combination. Overall, because fires are synchronous at supra-annual to multidecadal time scales with warm AMO events, particularly when combined with cool ENSO and PDO phases, suggests that we may be entering a qualitatively different fire regime in the next few decades due to the recent shift in 1998 to a likely long-term warm AMO phase. Although there remains uncertainty regarding the effects of CO₂-induced warming at regional scales, given the multidecadal persistence of the AMO there is mounting evidence that the recent shift to the positive phase will promote higher fire frequencies in the region.