FIRE RISK
Fire activity is strongly linked to summer climate, with the largest fires occurring exclusively in warm and dry summers. The most obvious impact of climate change in the west in recent years has been fire. Recent catastrophic fires in California and major wildfires in Oregon highlight the vulnerability of the state to increasing wildfire in a warming climate. The Eagle Creek Fire September 2017 closed I-84, a crucial transportation corridor between western and eastern Oregon. Fire risk is projected to increase across the entire state by midcentury, with the largest increases in the Willamette Valley and eastern Oregon. The associated wildfire smoke creates a health hazard for vulnerable communities, especially outdoor laborers and children, who may be exposed to poor air quality.
SNOW/WATER SUPPLY
Nearly every location in Oregon has seen a decline in spring snowpack, and it will continue to significantly decline through mid-century, especially at lower elevations. Oregon’s mountain snowpack serves myriad economic, ecological, and social functions, and the snowcapped volcanic peaks are part of the state’s cultural identity. Mountain snowpack acts as a natural reservoir which enhances summertime surface and groundwater supply. Meager mountain snowpack creates water scarcity in the state, as evidenced by droughts in 2015 and 2018. Snowpack is crucial for Oregon’s vibrant recreation industry. In 2015, low snowpack resulted in a multimillion dollar loss in ski resort revenues in the Northwest. Recent research shows that the observed declines in snowpack since 1985 were smaller than they would have been without natural climate variability, which is expected to reverse and produce much larger declines.
These changes in snowpack present a dual risk to the state. In winter, increases in average streamflow will be the result of precipitation falling as rain instead of snow and rapid runoff, increasing flood risk in some basins. Summer flows may be reduced by as much as 50% in some basins, presenting challenges to junior water rights holders, hydroelectric power generation, and those not served by reservoir or groundwater storage. Lower flows also impact important commercial and tribal fisheries.
Scatterplot of fire season (July-September) mean precipitation and temperature, with area
burned in each year since 2002 indicated by circles. Climate data are from NOAA’s National Center for Environmental Information statewide data for Oregon for 1895-2018 (the average for this entire period is used to calculate anomalies). Wildland fire data for 2002-2017 are from the National Interagency Fire Center (NIFC). Preliminary 2018 data are included here, but the final figure was not available from NIFC due to the lapse in federal appropriations. Prescribed fires were excluded from the analysis.
FIRE-CLIMATE RISK
Wildfires have received considerable attention over the past two years, due to their devastation (Camp and Carr Fires in California; Substation Fire near The Dalles) or economically or socially valued location (Eagle Creek Fire in the Columbia River Gorge and Chetco Bar Fire in southwestern Oregon). Statistical analysis shows that warm, dry summers are associated with higher area burned (McKenzie et al 2004, Westerling et al., 2006). Large fires increased in the western US from 1984-2011 in a warming climate (Dennison et al., 2014) and human-caused climate change was responsible for the increase in area burned in forests in the western US from 1984-2015 (Abatzoglou and Williams, 2016).
Fire season in Oregon runs roughly from late July to mid-September, though it can start earlier and end later, as was the case in 2018. Fire activity is dependent on many anthropogenic and natural variables, and warmer or drier seasons can create conditions favorable for wildfires. In Figure 8 we define fire season using a rough definition of the months of July-September. The upper left quadrant represents the warmest and driest years in this historical record; the lower right quadrant shows the wettest and coolest years. Years for the climate analysis are only labeled from 2002-2018, the same period of record as the fire data, to reduce clutter. Circles are meant to show the acres burned in each of these years, binned into groups.
Projected change in extreme fire risk days, defined as the number of days when the 100-hour fuel moisture in June-July-August (JJA) is below the 3rd percentile of days in the baseline period. Figure prepared with data on the NW Climate Toolbox, climatetoolbox.org. Data source: MACA.
Weather data can be used to calculate fire risk in various ways. Operational agencies often use the energy release component (ERC) as well as measures of fuel moisture and wind speeds. One measure of the fuel moisture is the ‘100-hr’ fuels moisture, which is the amount moisture within vegetation (the ‘fuel’), averaged over 100 hours. Figure 9 shows the ‘100-hour’ fuel moisture, specifically the number of days per summer when the fuel moisture is below the 3rd percentile (“extreme”). The largest increases in the frequency of extreme fire risk are in the eastern third of Oregon and in the Willamette Valley.
OCAR 4
FOURTH CLIMATE ASSESSMENT REPORT
