Wildfire is a significant source of ecosystem disturbance and property damage in British Columbia and climate plays a significant role in determining wildfire severity. Building on the experiences of previous studies, this project produced statistically downscaled projections of future fire weather severity for seven meteorological stations in southeast British Columbia using global climate models (GCM) and the Canadian Fire Weather Index (FWI) system.
Methods
The project team developed a technique in which projections of wildfire severity were made for a number of stations within southeast BC using Expanded Downscaling (EDS), a multivariate technique that accounts for the unique climatology of each station (Bürger 1996). The EDS approach uses multivariate linear regression to connect large-scale predictors which are well-simulated by GCMs (e.g., winds in the mid-troposphere) to local variables at specific stations. EDS places constraints on these regressions by maintaining both the variability and covariance across all stations and variable types. The EDS method provided future daily time-series for four climate variables required as input to the FWI system: temperature, precipitation, relative humidity, and wind speed. This was done at each station and for each combination of GCM and emissions scenario. Output from two GCMs and two different emissions scenarios provided a range of plausible future climate projections for the fire weather indices in the study area.
Results
Before applying the EDS procedure to future climate projections, PCIC assessed its ability to downscale historical observations. While the downscaled temperatures were generally accurate, downscaling of precipitation proved problematic. Moreover, the EDS routine had significant difficulty in downscaling wind speed, consistent with the findings of a previous PCIC project aimed specifically at downscaling surface winds in BC. Consequently, a decision was made to focus on the Monthly Drought Code (MDC), a component of the FWI system which requires only monthly values for temperature and precipitation, avoiding the need to simulate daily values and wind speed. The MDC has been shown to correlate well with the severity of the wildfire season.
Preliminary results show that while temperatures are projected to increase by between 1°C and 4°C for the 2080s (2071-2100), there is no obvious systematic shift in precipitation for southeastern BC. Significant negative and positive changes in MDC were calculated, depending on the station. Consequently, the results do not indicate any significant region-wide changes in fire weather severity as represented by the August MDC (Figure 1). This contrasts with results from previous studies which suggested significant increases in fire weather severity for southeastern BC (Flannigan et al. 2005). In these studies, changes for a specific variable were drawn solely from the raw GCM output for that variable, while the changes produced by the EDS are a function of changes in a variety of predictor GCM fields. This difference may explain a considerable portion of the discrepancy between results. Also, the emissions scenarios used here generally result in cooler and wetter projections relative to those used previously.
Figure 1: Anomalies of August MDC values for the 2050s at all seven Environment Canada stations used in this study. Results are provided for CGCM3 A1B (black) and ECHAM5 A2 (grey). Solid circles indicate statistically significant anomalies at the 5% level (determined by a two-sample t-test). The gray dotted lines indicate the average historical standard deviation of August MDC at each station.
References
Bürger, G, 1996: Expanded Downscaling for Generating Local Weather Scenarios. Climate Research, 7, 111-128.
Flannigan, M.D., Amiro, B.D., Logan, K.A., Stocks, B.J. and Wotton, B.M., 2005: Forest Fires and Climate Change in the 21st Century. Mitigation and Adaptation Strategies for Global Change, 11, 847-859.
Acknowledgements
- Climate Data and Analysis Section of Environment Canada
- Steve Taylor (Canadian Forest Service)
- Daniel Perrakis and Eric Meyer (BC Ministry of Forests, Lands and Natural Resource Operations)