Wildfires and Warming – It’s complicated

Last Sunday, Tony Davis had a story in the Arizona Daily Star with the apocalyptic headline: “Destructive Southwest wildfires bear ‘really clear’ sign of warming,” complete with a hockey stick-like graph of wildfires in the print edition. The story is actually more balanced than the headline would suggest. However, throughout the story Tony mentions “human-induced warming” without citing any supporting evidence. Well Tony, the relationship between warming and wildfires is not quite so “really clear” because the picture is complicated by many factors. Two graphs below, which I constructed from data from the National Interagency Fire Center, would initially seem to support Tony’s contention, but as we will see, the data reflect just a regional effect during a short time interval that does not give a true picture.

Fire frequency

fire acres burned

We see from these graphs, that in the early 1980s, the frequency of fires in the U.S. dropped dramatically and remained low ever since. At the same time, the number of acres burned has increased dramatically, thereby giving a “hockey stick” appearance. This indicates that the biomass available for burning increased, possible due to warming or possibly due to a change in forest management under the Endangered Species Act. As we will see below, in some parts of the world, warming decreases fire frequency and intensity. The graphs above, show a relatively short time interval. The graph below, based on analysis of charcoal trapped in sediments, shows a longer perspective of cyclical wildfire regimes (source: http://www.pnas.org/content/109/9/E535).

Fire western US biomass burned

The folks at CO2Science.org provide a summary of long-term global research on the frequency and intensity of wildfires. I will summarize their summary. In Canada over the last several thousand years, fire frequency and intensity decreased with a warming climate. “[D]endroecological studies show that both frequency and size of fire decreased during the 20th century in both west and east Canadian coniferous forests possibly due to a drop in drought frequency and an increase in long-term annual precipitation.” At Lake Tahoe in Nevada, fire frequency increased with warming and decreased with cooling, but “current fire episode frequency on the west shore of Lake Tahoe is at one of its lowest points in at least the last 14,000 years.” In Colorado, “fires occurred during short-term periods of significant drought and extreme cool (negative) phases of ENSO (El Nino) and [the Pacific Decadal Oscillation (PDO)] and during positive departures from [the mean Atlantic Multidecadal Oscillation (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.” These oscillations are solar-driven and impact both temperature and precipitation. For southern Arizona grasslands, increases in fire frequency are coincident with the onset of ENSO (El Nino Southern Oscillation). In Finland, fire frequency decreased during warming and “the climatic change that triggered the increase in fire frequency was cooling and a shift to a more continental climate.” In Siberia, fire frequency has decreased significantly since the 18th Century. In Turkey, “climatically-induced variation in biomass availability was the main factor controlling the timing of regional fire activity during the Last Glacial-Interglacial climatic transition, and again during Mid-Holocene times, with fire frequency and magnitude increasing during wetter climatic phases.” In Australia, fire activity diminished with increased warming. The CO2Science conclusion: “Consequently, considering all of the above findings, although one can readily identify specific parts of the planet that have experienced both significant increases and decreases in land area burned over the last several decades, for the globe as a whole there has been absolutely no relationship between rising temperatures and total area burned over this latter period, when climate alarmists claim the world warmed at a rate and to a degree that were unprecedented over the past several millennia. And as a result, there is little support for the model-based contention that future CO2-induced global warming (if it occurs at all) will have any effect on global fire trends.” The data above deals mainly with natural, lightning-caused fires. Further complicating the issue are fires caused by humans, either by accident or arson (for arson fires caused as a diversion to illegal entry across the Mexican border, see: Consequences of our unsecured border). Is the sign still so “really clear”?

Copyrighted by Jonathan DuHamel. Reprint is permitted provided that credit of authorship is provided and linked back to the source.