Wildfires & Changing Vegetation
Wildfires have increased in size and frequency in the Western United States in recent decades. Rising temperatures due to human-caused climate change is a significant factor. This trend is expected to continue into the future as rising temperatures make conditions ideal for larger, more destructive wildfires in the Pacific Northwest. Wildfires are expected to contribute to major ecological changes, helping to shift the composition of the Pacific Northwest’s forests.
Over half (55%) of the increase in fuel aridity conditions (the ability of vegetation to burn given the right ignition source) in recent years (1979–2015) is due to warming resulting from human-caused (anthropogenic) climate change in the Western United States (Abatzoglou and Williams 2016).
Declines in spring mountain snowpack, summer soil moisture, and fuel moisture across the mountain ranges of the Western United States are projected to increase the fire potential in many forests. The greatest declines in summer soil and fuel moisture are projected for the Cascade Mountains, making it one of the most at-risk areas in the Western United States for increasing fire activity under climate change (Gergel et al. 2017).
Climate change is expected to increase the prevalence of very large fires— defined as the top 5–10% of fires, or fires that burn more than 5,000 hectares (about 19 square miles). The largest increases are expected in the Intermountain West (Barbero et al. 2015).
Area burned each year is expected to increase as the Pacific Northwest warms under climate change, tripling from roughly 0.5% in the 20th century to 1.5 % for the late 21st century in the region west of the Cascade Mountains’ crest. This estimate was made using the high emissions scenario (RCP 8.5) and does not include the influence of fire suppression (Sheehan et al. 2015).
In the region west of the Cascade Mountains’ crest, the fire return interval—or average time between fires—is projected to decrease from roughly 80 years averaged over the 20th century to between 47 and 27 years averaged over the 21st century under the high emissions scenario (RCP 8.5) and no fire suppression (Sheehan et al. 2015).
Tree types now common to parts of California are expected to migrate north, potentially transforming many Pacific Northwest forests from conifer-dominant to mixed conifer forests. This migration will be aided in some cases by fire. However, conifers will likely continue to be the predominant tree type in the region throughout the 21st century (Sheehan et al. 2015).
• Abatzoglou, John T., and Timothy J. Brown. “A comparison of statistical downscaling methods suited for wildfire applications.”
International Journal of Climatology 32, no. 5 (2012): 772-780.
• Abatzoglou, John T., and A. Park Williams. “Impact
of Anthropogenic Climate Change on Wildfire across Western US Forests.”
Proceedings of the National Academy of Sciences 113, no. 42 (2016): 11770-11775.
• Barbero, Renaud, John T. Abatzoglou, Narasimhan Larkin, Crystal A. Kolden, and B. J. Stocks. “Climate Change Presents Increased Potential for Very Large Fires in
the Contiguous United States.”
International Journal of Wildland Fire 24, no. 7 (2015): 892-899.
• Gergel, Diana R., Bart Nijssen, John T. Abatzoglou, Dennis P. Lettenmaier, and Matt R. Stumbaugh. “Effects of Climate Change on Snowpack and Fire Potential in the Western USA.”
Climatic Change 141, no. 2 (2017): 287- 299.
• Sheehan, Timothy J., Dominique Bachelet, and Ken Ferschweiler. “Projected Major Fire and Vegetation Changes in the Pacific Northwest of the Conterminous United States under Selected CMIP5 Climate Futures.”
Ecological Modeling 317 (2015): 16-29.