Fire (Relationship to Global Warming) — Summary

Fire (Relationship to Global Warming) — Summary 

10 September 2007

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In the Gospel According to Gore, i.e., 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,” and to support this claim he presents a bar graph which, as he describes it, “shows the steady increase in major wildfires in North and South America over the last five decades,” noting that “the same pattern is found on every other continent as well.”

How correct are these claims?

The first one – that there has been “a steady increase in major wildfires in North and South America over the last five decades” – can readily be shown to be false from Gore’s own graph (p. 229), where the decade of the 1960s is seen to have experienced the same number of wildfires as the decade of the 1950s. There is then an increase from the 1960s to the 1970s (over one decade), from the 1970s to the 1980s (over a second decade), and from the 1980s to the 1990s (over a third decade). Hence, there was an increase in major North and South American wildfires over only the last three decades of the 20th century, not the last five decades, as Gore contends.

So what about the rest of the world? Is the same pattern really found on every other continent? For starters, we can assure everyone that whether we’re talking five decades or three decades, Antarctica has not been an imitator of North and South America in this regard! As for the remainder of the world, and for various sub-regions of North America, we briefly review below the results of several analyses that have investigated the subject in detail, concluding with a recent satellite study that evaluated the globe as a whole.

Carcaillet et al. (2001) developed high-resolution charcoal records from laminated sediment cores extracted from three small kettle lakes located within the mixed-boreal and coniferous-boreal forest region of eastern Canada, after which they determined whether vegetation change or climate change was the primary determinant of changes in fire frequency, comparing their fire history with hydro-climatic reconstructions derived from δ18O and lake-level data. Throughout the Climatic Optimum of the mid-Holocene, between about 7000 and 3000 years ago, when it was significantly warmer than it is today, they report that “fire intervals were double those in the last 2000 years,” meaning fires were only half as frequent throughout the earlier warmer period as they were during the subsequent cooler period. They also determined that “vegetation does not control the long-term fire regime in the boreal forest,” but that “climate appears to be the main process triggering fire.” In addition, they report that “dendroecological studies show that both frequency and size of fire decreased during the 20th century in both west (e.g. Van Wagner, 1978; Johnson et al., 1990; Larsen, 1997; Weir et al., 2000) and east Canadian coniferous forests (e.g. Cwynar, 1997; Foster, 1983; Bergeron, 1991; Bergeron et al., 2001), possibly due to a drop in drought frequency and an increase in long-term annual precipitation (Bergeron and Archambault, 1993).” These several findings thus led them to conclude that a “future warmer climate is likely to be less favorable for fire ignition and spread in the east Canadian boreal forest than over the last 2 millennia,” which is good news for Canada and similar parts of the world.

Pitkanen et al. (2003) constructed a Holocene fire history of dry heath forests in eastern Finland on the basis of charcoal layer data obtained from two small mire basins and fire scars on living and dead pine trees. This work revealed a “decrease in fires during climatic warming in the Atlantic chronozone (about 9000-6000 cal. yr. BP),” prompting them to conclude that “the very low fire frequency during the Atlantic chronozone despite climatic warming with higher summer temperatures [our italics], is contrary to assumptions about possible implications of the present climatic warming due to greenhouse gasses.”

Thereafter, the researchers observed an increase in fire frequency at the transition between the Atlantic and Subboreal chronozones around 6000 cal. yr. BP, noting that “the climatic change that triggered the increase in fire frequency was cooling and a shift to a more continental climate.” In addition, they report that the data of Bergeron and Archambault (1993) and Carcaillet et al. (2001) from Canada suggest much the same thing, i.e., less boreal forest fires during periods of greater warmth. Consequently, “as regards the concern that fire frequency will increase in [the] near future owing to global warming,” the researchers say their data “suggest that fires from ‘natural’ causes (lightening) are not likely to increase significantly in eastern Finland and in geographically and climatically related areas.”

Back in Canada, citing a number of climate-alarmist predictions of what could happen there, Girardin et al. (2006) introduced their study of the subject by writing that “human-induced climate change could lead to an increase in forest fire activity in Ontario, owing to the increased frequency and severity of drought years, increased climatic variability and incidence of extreme climatic events, and increased spring and fall temperatures,” noting that “climate change therefore could cause longer fire seasons (Wotton and Flannigan, 1993), with greater fire activity and greater incidence of extreme fire activity years (Colombo et al., 1998; Parker et al., 2000).” Consequently, to see if to see if any of these negative prognostications might have recently come to pass, they reconstructed a history of area burned within the province of Ontario for the period AD 1781-1982 from 25 tree-ring width chronologies obtained from various sites throughout the province, spurred on, perhaps, by the increase in area burned within Ontario that is known to have occurred from 1970 through 1981 (Podur et al., 2002).

So what did they find?

The three researchers report that “while in recent decades area burned has increased, it remained below the level recorded prior to 1850 and particularly below levels recorded in the 1910s and 1920s,” noting further that “the most recent increase in area burned in the province of Ontario was preceded by the period of lowest fire activity ever estimated for the past 200 years (1940s-1960s).” Consequently, although they say that, according to theory, “one should expect greater area burned in a changing climate,” especially one that is driven by anthropogenic-induced increases in atmospheric CO2 concentration, their findings revealed no support for this contention. In fact, they revealed just the opposite.

A contrary example, where warming does appear to enhance fire occurrence, is provided by Pierce et al. (2004), who dated fire-related sediment deposits in alluvial fans in central Idaho, USA, in a research program designed to reconstruct Holocene fire history in xeric ponderosa pine forests and to look for links to past climate change. This endeavor focused on tributary alluvial fans of the South Fork Payette (SFP) River area, where fans receive sediment from small but steep basins in weathered batholith granitic rocks that are conducive to post-fire erosion. Altogether, they obtained 133 AMS 14C-derived dates from 33 stratigraphic sites in 32 different alluvial fans. In addition, they compared their findings with those of Meyer et al. (1995), who had earlier reconstructed a similar fire history for nearby Yellowstone National Park in Wyoming, USA.

Pierce et al.’s work revealed, in their words, that “intervals of stand-replacing fires and large debris-flow events are largely coincident in SFP ponderosa pine forests and Yellowstone, most notably during the ‘Medieval Climatic Anomaly’ (MCA), ~1,050-650 cal. yr BP.” What is more, they note that “in the western USA, the MCA included widespread, severe miltidecadal droughts (Stine, 1998; Woodhouse and Overpeck, 1998), with increased fire activity across diverse northwestern conifer forests (Meyer et al., 1995; Rollins et al., 2002).”

Following the Medieval Warm Period and its frequent large-event fires was the Little Ice Age, when, as Pierce et al. describe it, “colder conditions maintained high canopy moisture, inhibiting stand-replacing fires in both Yellowstone lodgepole pine forests and SFP ponderosa pine forests (Meyer et al., 1995; Rollins et al., 2002; Whitlock et al., 2003).” Subsequently, however, they report that “over the twentieth century, fire size and severity have increased in most ponderosa pine forests,” which they suggest may be largely due to “the rapidity and magnitude of twentieth-century global climate change.”

With respect to their central thesis, which appears to be well supported by both the SFP and Yellowstone data, we agree with Pierce et al. that both the size and severity of large-event stand-replacing fires tend to increase with increasing temperature in the part of the world and for the specific forests they studied; and we note that the Yellowstone data also depict a sharp drop in large-event fire frequency and severity during the earlier Dark Ages Cold Period, which followed on the heels of the preceding peak in such fires that was concomitant with the still earlier Roman Warm Period.

Also working in the United States, and coming to much the same general conclusion, were Westerling et al. (2006), who compiled a comprehensive database of large wildfires in western United States forests since 1970 and compared it to hydro-climatic and land-surface data. Their findings are succinctly summarized by Running (2006) in an accompanying Perspective, wherein he writes that “since 1986, longer warmer summers have resulted in a fourfold increase of major wildfires and a sixfold increase in the area of forest burned, compared to the period from 1970 to 1986,” noting also that “the length of the active wildfire season in the western United States has increased by 78 days, and that the average burn duration of large fires has increased from 7.5 to 37.1 days.” In addition, he notes that “four critical factors – earlier snowmelt [by one to four weeks], higher summer temperatures [by about 0.9°C], longer fire season, and expanded vulnerable area of high-elevation forests – are combining to produce the observed increase in wildfire activity.”

So what is the case for the world as a whole, i.e., what is the net result of the often opposite wildfire responses to warming that are typical of different parts of the planet?

This question was recently explored by Riano et al. (2007), who conducted “an analysis of the spatial and temporal patterns of global burned area with the Daily Tile US National Oceanic and Atmospheric Administration-Advanced Very High-Resolution Radiometer Pathfinder 8 km Land dataset between 1981 and 2000.” For several areas of the world, this effort revealed there were indeed significant upward trends in land area burned. Some parts of Eurasia and western North America, for example, had annual upward trends as high as 24.2 pixels per year, where a pixel represents an area of 64 km2. These increases in burned area, however, were offset by equivalent decreases in burned area in tropical southeast Asia and Central America. Consequently, in the words of Riano et al., “there was no significant global annual upward or downward trend in burned area.” In fact, they say “there was also no significant upward or downward global trend in the burned area for any individual month.” In addition, they say that “latitude was not determinative, as divergent fire patterns were encountered for various land cover areas at the same latitude.”

Consequently, 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 two to three decades of the 20th century, as we have done in the materials reviewed above, for the globe as a whole there was absolutely no relationship between global warming and total area burned over this latter period, when climate alarmists claim the world warmed at a rate and to a degree that were both unprecedented over the past several millennia. As a result, it should be abundantly clear there is simply no truth to the contention of Al Gore that the pattern of increasing wildfires over the last three decades of the 20th century, which he plots for North and South America, “is found on every other continent as well.” To reprise a portion of a favorite quote of his (An Inconvenient Truth, p. 20-21), it just ain’t so.


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