THE WILDFIRES of 1910 Climatology of an Extreme Early Twentieth-Century Event and Comparison with More Recent Extremes
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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 exten- sive wildfire and drought synchro- ny 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 pre- cipitation, 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 rela- tive 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.