THE WILDFIRES of 1910 Climatology of an Extreme Early Twentieth-Century Event and Comparison with More Recent Extremes

THE WILDFIRES OF 1910 Climatology of an Extreme Early Twentieth-Century Event and Comparison with More Recent Extremes

by Henry F. Diaz and Thomas W. Swetnam

The unusual U.S. climatic conditions of the historic year called "The Big Burn" were not matched until the devastating fire year of 2012.

he Great Fire of 1910 (also commonly referred Relatively dry conditions prevailed across to as the Big Blowup or the Big Burn) was a the western United States during the spring and wildfire that burned about three million acres summer months of that year, but the northern Rocky T 2 (12,000 km , approximately the size of Connecticut) Mountains were especially dry. Prior to the estab- in northeast Washington, northern Idaho (the lishment of the U.S. Forest Service (in 1905) land panhandle), and western Montana. The area burned use practices on public and private forestlands were included parts of the Bitterroot, Cabinet, Clearwater, commonly laissez-faire, resulting in overharvesting Coeur d’Alene, Flathead, Kaniksu, Kootenai, Lewis of timber and accumulated slash fuels in some areas. and Clark, Lolo, and St. Joe National Forests. The fire- During and following timbering activity, people set storm burned over two days (20 and 21 August 1910) fire to the slash to dispose of it. Accidental fires were and killed 87 people, including 78 firefighters. It is also common, especially from sparks along railways believed to be the largest, although not the deadliest, from wood-burning locomotives. The fledgling fire in recorded U.S. history. The “Big Blowup” in the Forest Service had a tiny force of rangers with the summer of 1910 was a singular event in the history responsibility for detecting and suppressing wildfires of the U.S. Forest Service, shaping fire management over enormous and remote areas. The national forests strategies and policies from that time to today (Pyne increased by 16 million acres in 1907 by executive 2008; Egan 2009). order of Theodore Roosevelt in the last days of his presidency (Egan 2009). Given the regional dryness, abundant slash fuels near frontier logging settle- AFFILIATIONS: Diaz—NOAA/ESRL, and CIRES, University of ments, ubiquitous ignitions from human sources, and Colorado, Boulder, Colorado; Swetnam—Laboratory of Tree- the lack of fire detection and firefighting capacity by Ring Research, The University of Arizona, Tucson, Arizona CORRESPONDING AUTHOR: Henry F. Diaz, CIRES, University the Forest Service, the conditions were ripe for the of Colorado, UCB 216, Boulder, CO 80309 Big Blowup. E-mail: [email protected] Although there have been various descriptions of the human and natural history of this episode (Pyne The abstract for this article can be found in this issue, following the table of contents. 2008; Egan 2009), and some climatological analyses DOI:10.1175/BAMS-D-12-00150.1 encompassing the 1910 year and this region (e.g.,

In final form 8 January 2013 Morgan et al. 2008), we show here that warm weather ©2013 American Meteorological Society conditions in 1910 were highly anomalous, and more so than previously reported or evaluated. Further, we

AMERICAN METEOROLOGICAL SOCIETY SEPTEMBER 2013 | 1361 Unauthenticated | Downloaded 10/04/21 08:13 AM UTC identify and evaluate analogous and different spatial climate conditions in other large regional fire years in the northern Rockies, including spring 2012 (as we write this paper), and we discuss the implications of these observations and patterns for upcoming fire seasons. Last, we also illustrate an example of extensive wildfire and drought synchrony across western North America during the eighteenth century using a recently compiled network of tree- ring-based reconstructions.

DATA SOURCES. Climate data were accessed from the National Oceanic and Atmospheric Administration (NOAA)’s National Climatic Data Center in Asheville, North Carolina (www.ncdc.noaa .gov/climate-monitoring/index .php) and the Earth System Research Laboratory of NOAA in Boulder, Colorado (www.esrl.noaa.gov/psd /psd1/). Data accessed included U.S. surface temperature and precipitation, Palmer drought severity Fig. 1. Map of the occurrence and extent of the 1910 wildfires across index (PDSI) data, and upper-level the western United States (after Plummer 1912). data from the National Centers

Fig. 2. (a) Map of surface temperature anomaly (°F, for Mar 1910 for the contiguous United States) and (b) time series of Mar mean temperature anomalies (°F) for the period of record. Red line highlights the value for 1910. Note that the monthly record stood for 102 years until Mar 2012. (Source: National Climatic Data Center, NOAA.)

1362 | SEPTEMBER 2013 Unauthenticated | Downloaded 10/04/21 08:13 AM UTC for Environmental Prediction (NCEP)–National from other studies showing seasonal temperature Center for Atmospheric Research (NCAR) reanalysis and drought associations with wildfire activity in (NRA) (Kistler et al. 2001) and the historical the western United States.) The anomaly field of the reanalysis (HRA) dataset (Compo et al. 2006, 2011). 500-mb geopotential height surface based on the Tree-ring width and fire scar data used to illus- HRA dataset is consistent with the record warmth trate drought and fire synchrony come from the recorded for the Lower 48 during that month (Fig. 3). North American Drought Atlas dataset (Cook The anomalous warmth persisted throughout the et al. 2004), and the International Multiproxy nominal spring season of March–May (Fig. 4), which Paleofire Database (www.ncdc.noaa.gov/paleo/impd was then followed by a rather dry summer, particu- /paleofire.html) and T. W. Swetnam et al. (2011, larly in the state of Idaho, with departures exceeding unpublished manuscript). minus two standard deviations (Fig. 5). We examined several sequences of daily weather ANALYSIS RESULTS. Plummer’s (1912) map of maps available from the NOAA Central Library site the 1910 fires across the western United States (Fig. 1) (http://docs.lib.noaa.gov/rescue/dwm/data_rescue_ illustrates the widespread occurrence of wildfires in daily_weather_maps.html). Daily warm anomalies that year over the whole region. The largest burned areas were in the northern Rockies and particularly in Idaho, where most fatalities occurred. March 1910 was an exceptionally warm month, as illustrated by the spatial pattern and magnitude of the temperature departure from the long-term average and the time series of area-weighted mean temperature over the contiguous United States (Fig. 2). The warmth during March 1910 was not exceeded in the climate record until 2012, and it was particularly unusual in the early part of the twentieth century, when generally cooler temperatures prevailed in the United States. These extreme warm conditions likely contributed to the extent and magnitude of the wildfires during that year. The extreme warm conditions during March 1910 set the stage for the great fires later that summer. (We will discuss this interpretation and likely mechanisms in more detail later in this paper, along with findings

Fig. 4. (a) As in Fig. 2a, but for the 3-month average of Fig. 3. Map of the anomalous 500-mb geopotential Mar–May 1910. (b) As in Fig. 2b, but for Mar–May aver- height field (m) for Mar 1910 (Compo et al. 2006). ages. Spring temperature record of 1910 was broken (Data source: HRA.) by a substantial margin in 2012.

AMERICAN METEOROLOGICAL SOCIETY SEPTEMBER 2013 | 1363 Unauthenticated | Downloaded 10/04/21 08:13 AM UTC Fig. 5. (left) Standardized precipitation anomalies for the summer season (Jun–Aug) of 1910. (right) PDSI for Aug 1910. Note drought index values of –3 and below in Idaho.

gives essentially the same picture. However, during the days when the wildfires were at their peak, around the third week of August, relative humidity was extremely low, with values in the areas most affected around 20% or lower (top two panels in Fig. 7). One other factor that appeared to be important in the extent and rate of increase in the size of the wildfires during the Big Burn was the occurrence of anomalously high wind speeds. Figure 7 (bottom two panels) shows the pattern of anomalous vector mean winds at 750 mb (~2,300 m—7,500 ft more typical of the mountainous terrain that experienced the most widespread burning) during the same time interval in August when the wildfires flared up (Pyne 2008). The Fig. 6. Map illustrating the anomalous surface relative humidity field (%) for Jul–Aug 1910 (Compo et al. daily weather maps from this period in August 1910 2006). (Data source: HRA.) clearly show the passage of a disturbance and frontal system through the region. Figure 8 illustrates the strong sea level pressure gradient present over western exceeding 10°C were common during the 1910 spring North America for 21 August 1910. It is inferred that months. The generally warm and dry 6-month period preexisting dry soils and woody fuels from the long preceding the wildfire is consistent with drought string of exceptionally warm months, with relatively conditions present in the Northwest (Idaho, Oregon, low humidity and stronger-than-normal near-surface and Washington) at the end of the summer of 1910 winds during the month of the fires were critical (Fig. 5). Interestingly, a sharp moisture gradient factors leading to the exceptional nature of the Big across the northern Rockies is also indicated, as Burn forest fires. moist conditions were prevalent in Montana at Longer-term perspectives of climate and fire con- that time. However, humidity was generally below ditions across the western United States are provided normal during the summer months in the region of by tree-ring reconstructions. Summer and other the Northwest affected by the fires. This is evident seasonal drought reconstructions based on extensive in the July–August 1910 surface relative humidity networks of tree-ring width chronologies have been anomaly field in the HRA dataset (Fig. 6), indicating very useful in recent years in identifying and map- drier-than-normal conditions in the West in general. ping westwide drought years and decades (Cook et al. A similar map computed for the months of July and 2004). Fire history reconstructions from networks of August of 1910 at the 850-mb surface (close to the thousands of tree-ring dated, fire-scarred trees in this average ground elevation for Idaho, map not shown) region also show a regional synchrony of extensive

1364 | SEPTEMBER 2013 Unauthenticated | Downloaded 10/04/21 08:13 AM UTC Fig. 7. (top left) Mean relative humidity; (top right) corresponding anomaly field (%) at 800 mb; (bottom left) mean vector wind; and (bottom right) anomaly field (m s–1) at 750 mb for the period 20–22 Aug 1910 (Compo et al. 2006). (Data source: HRA.)

fire years and westwide drought years (e.g., Kitzberger et al. 2007; Williams et al. 2012). Recent compilations of the largest fire scar data network yet assembled (T. W. Swetnam et al. 2011, unpublished manuscript) from 1,248 sites in western North America confirm this pattern. The most extensive fire year in the past 250 years in this network was 1748, which corresponds with westwide drought, as indicated in the North American Drought Atlas (http://iridl.ldeo.columbia .edu/SOURCES/.LDEO/.TRL/.NADA2004/.pdsi- atlas.html) reconstructions (Fig. 9). Further study

Fig. 8. Sea level pressure map for 21 Aug 1910. A strong pressure gradient of ~20 hPa across the Northwest is evident (Compo et al. 2006). (Data source: HRA.)

AMERICAN METEOROLOGICAL SOCIETY SEPTEMBER 2013 | 1365 Unauthenticated | Downloaded 10/04/21 08:13 AM UTC Rockies (Fig. 11) and to extreme forest fire conditions in the northern Rockies. As noted above, by an interesting coincidence, during the writing of this article an extreme monthly anomaly for the month of March, strikingly similar in magnitude to that of March 1910, occurred in 2012 (Fig. 12, top panel; cf. Fig. 3). Substantially above- normal temperatures continued through the rest of the spring months of 2012 and major forest fires broke out with high loss of property and some lives

Fig. 9. Western North American fire scar chronology network includes 1,248 in New Mexico, Colorado, sites, where typically 10 or more trees were sampled and dated at each site, and Utah. For compari- providing exact fire dates and percentages of trees scarred each year over son, the 500-mb anom- the past 400 years (T. W. Swetnam et al. 2011, unpublished manuscript). aly for March–May 2012 Year 1748 is the most active fire year, with >200 sites recording fire in that (Fig. 12, bottom panel) and year. 1910 was a relatively light fire year in this reconstruction because fire the associated PDSI values suppression and livestock grazing were already reducing surface fire extent. for May 2012 (Fig. 13, top panel) are presented. of these spatial and temporal paleofire and climate The anomalous warm March 2012 set numerous reconstructions (including new, gridded temperature records across the United States, and edged out 1910 reconstructions; e.g., Wahl and Smerdon 2012) may as the warmest year since the start of state records help identify interannual to decadal patterns useful in 1895 by about 0.5°F. Considering the warming of for improving our understanding of the fire climatol- about 1°F over the last 100 years for the contiguous ogy of western North America. United States, it is likely that March 1910 represents a larger deviation from the prevailing mean than DISCUSSION AND CURRENT CONTEXT. March 2012. March 2012 was followed by much There are two reasonably close analogs to the anoma- above-average temperatures in April and May, with lous spring and summer of 1910 that are notable. One occurred in 1988 and was associated with extreme summer drought in the Midwest United States and extensive fires in the Yellowstone National Park (YNP) region in the northern Rockies (Balling et al. 1992). The 1988 fire was one of the largest wildfire events in the recorded history of YNP. As in most major wildfire episodes, smaller individual fires quickly grew out of control with increasing winds aided by severe drought and combined into one large conflagration, which burned for several months. A total of 793,880 acres (3,213 km2), or 36% of the park, were affected by the wildfires (Schullery 1989). Figure 10 illustrates the anomalous anticyclonic conditions prevailing from late spring through summer of 1988. The anomalous ridging centered in midcontinent led to extreme drought from the upper Midwest to the northern Fig. 10. As in Fig. 3, but for the period of May–Aug 1988.

1366 | SEPTEMBER 2013 Unauthenticated | Downloaded 10/04/21 08:13 AM UTC spring of 2012 again beating the previous record set midelevation drier forest types (such as ponderosa in 1910 (Fig. 4). As documented here, there is con- pine forests, dry mixed conifer forests, and pine sa- siderable resemblance in the anomalous upper-level vannas) are associated with a combination of factors. circulation and surface patterns between the spring These may include disruption of frequent surface and summer of 1910 and 2012; also the fast start and fire regimes that previously existed in some low- large extent of the wildfire season in 2012 appear and midelevation forest types due to firefighting by to be more than the equal of the 1910 Big Blowup, government agencies and extensive livestock grazing though thankfully without the large loss of life. The around the turn of nineteenth to twentieth centuries summer months of 2012 (as we write this in late (Arno 1980; Steele et al. 1986; Keane et al. 1990; Allen August), however, are playing out differently from et al. 2002; Heyerdahl et al. 2008). Higher-elevation 1910, with continued and even increasing extreme forests (such as mesic lodgepole pine and spruce–fir drought conditions in the central Rockies region (and stands) have probably not been altered by twentieth- extending through the Midwest; see Fig. 13, bottom century fire suppression effects because these forests panel). Meanwhile, the Pacific Northwest is one of the generally did not sustain surface fires, and burned few areas in the country to have reached midsummer only at 100–150-plus-year intervals in the past with near-normal moisture. (Schoennagle et al. 2004). Wildfire activity in montane forests of the western United States has greatly increased in recent years (since circa 1986), as measured by the number of large events (more than a fourfold increase in the number of fires >400 ha in size) and increasing length of the fire season (more than 75 days’ increase; Westerling et al. 2006; Swetnam and Betancourt 1998). Fire severity (i.e., proportion of burned area causing tree mortality) has also increased in some subregions and vegetation types (Holden et al. 2007; Williams et al. 2010; Dillon et al. 2011). Moreover, the largest wildfires in state histories (>100 years) have occurred in Arizona, New Mexico, and Colorado in just the past two years (2011 and 2012). In some cases, these record-breaking wildfires have exceeded the previous largest documented wildfires (before 1980) by an order of magnitude. It is likely that increasing areas burned and higher severity fires in some low- and

Fig. 12. (top) As in Fig. 3, but for Mar 2012. (bottom) As in (top), but for the nominal spring season (Mar– Fig. 11. PDSI values for Jul–Aug 1988. May) of 2012.

AMERICAN METEOROLOGICAL SOCIETY SEPTEMBER 2013 | 1367 Unauthenticated | Downloaded 10/04/21 08:13 AM UTC A comparison of the 1910 “blow up” event to modern Our findings generally concur with other studies extreme wildfire events is useful because it also was of seasonal climate–wildfire patterns in the western associated with preceding seasonal climate conditions United States, wherein warmer springs, reduced (i.e., warm springs extending into warm summers), snowpacks, and consequent longer drying periods resulting in wildfires exceeding in size all previous leading into the peak wildfire season (from mid- to known events (and still the largest fires on record in late summer) are especially important factors in Idaho forest areas). The fact that antecedent conditions extensive wildfire outbreaks in montane forests of are important suggests there is potential for developing the central and northern Rockies (Balling et al. 1992; predictive models (e.g., Westerling et al. 2003) at least a Westerling et al. 2006; Morgan et al. 2008; Littell season ahead. Interestingly, 1910 was also a westwide et al. 2009). The drying (and subsequent combus- high fire occurrence year (see Fig. 1; Plummer 1912), tion) occurs across a broad range of scales, from tree suggesting that changes in fuels and forest structure needles and grasses to small branches, whole tree may have been less important than during recent stems (logs and snags), and entire forest canopies and events, which are taking place following a century of fire watersheds. Moisture content of dead fuels is par- suppression efforts, other land use practices, and the ticularly important in fire ignition and initial spread extensive spread of invasive grass species in some areas. rates; however, live fuel moistures (e.g., tree needle moisture content) may be a more important factor in some “crown fire” type conflagrations, where very high-intensity burning occurs as a consequence of increased volatility of these fuels. Reduced live fuel moistures can be important in promoting crown fire behavior and are an optional variable in some models (e.g., Scott and Reinhardt 2001), but the precise (and changing) relationships between live fuel moisture and crown fire behavior are mostly theory based (rather than observed and calibrated). Presumably this is because of the dif- ficulties in measuring live fuel moisture content and related fire intensities (and spread rates, etc.) at the requisite scales and in different forest types. Spectacular burning of combustible gases emitted from burning forest canopies are commonly visible in intense conflagrations as briefly burning vertical or tilted shafts of flame, or longer-lasting “fire whirls,” reaching heights of 100 m or more above canopies (Forthofer and Goodrick 2011). These observations indicate that combustible gases can be emitted in large quantities and ignited as living tree canopies are heated and burned, thereby extending flaming fronts (especially when wind driven) and contributing to very rapid spread rates. We hypothesize that the extreme warming and drying conditions over periods of months and seasons (as in 1910 and 2012), which cause lower live fuel moistures at leaf to landscape scales, in turn lead to greater volatility (and hence combustibility) of fuels at all scales. This is not a particularly new idea (e.g., Simard and Donoghue 1987), but we emphasize it here as potentially a factor of greater importance than previously recognized (or modeled) in triggering extraordinary wildfire extent. Another factor of common importance in driving large wildfire events, both in 1910 and during many Fig. 13. PDSI for (top) May and (bottom) Jul 2012. recent very large (>50,000 ha) wildfires, are surface

1368 | SEPTEMBER 2013 Unauthenticated | Downloaded 10/04/21 08:13 AM UTC and near-surface winds. The first-person accounts of wildfire hazard and behavior in montane forest areas the 1910 fires include many lurid descriptions of high of western North America. winds blowing fire and large burning embers far in advance of the burning front. Likewise, winds were a ACKNOWLEDGMENTS. The authors thank three key factor during the largest southwestern wildfires anonymous reviewers and the editor for their helpful com- in recent years, with maximum burn rates of more ments. Partial support for TWS’s time was provided by the than 20,000 ha in less than 24 hours (e.g., Rodeo– U.S. interagency Joint Fire Sciences Program. We thank Chediski Fire 2002, Wallow Fire 2011, Las Conchas E. Bigio, M. Hall, E. Vasquez, and D. Falk at The University Fire 2011). These very rapid fire runs resulted in total of Arizona for help in compiling the North American fire or near-total tree canopy mortality in long, linear scar chronology network shown in Fig. 9. We also thank strips, aligned with prevailing winds and extending the dozens of data contributors to this North American for 15 km or more (T. W. Swetnam et al. 2011, fire scar network, which will be described and analyzed in unpublished manuscript). An important distinction detail in forthcoming multiauthored papers. here is that “wind driven” fire runs may be associated with synoptic weather patterns (e.g., passage of frontal systems, or “jet” wind currents at the surface REFERENCES or near surface; e.g., Crimmins 2006; Wirth 2011) Allen, C. D., and Coauthors, 2002: Ecological restoration versus “plume dominated” fires with runs caused by of southwestern ponderosa pine ecosystems: A broad local, down-drafting (and horizontal) winds from perspective. Ecol. 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