Climate Change: Fire

A prescribed fire in Yosemite Valley in 2012 was set to help restore historical forest density and undergrowth after over a century of fire suppression. Prescribed burns are an important tool in preparing for more frequent, larger, and longer fires.

For thousands of years, humans have shared Yosemite’s landscapes with fire. Yosemite’s first people harnessed its power to clear and cultivate land; European American settlers quenched flames and suppressed blazes in an effort to protect the same land. Today, fire managers seek to restore its role as a natural ecosystem process while reducing threats to human health, safety, and property. But the future is uncertain: across the western US, earlier snowmelt and higher temperatures have driven larger, longer, and more severe fires than those seen historically. Over the coming years, these changes may bring landscape-level vegetation shifts, increasing carbon emissions from smoke, and a new chapter in the complicated relationship between humans and fire.

A Departure from History

Summer through fall is fire season in the Sierra Nevada. Yosemite’s plants, animals, and ecosystems depend on these regular cycles of renewal: wildfire thins undergrowth, returns nutrients to the soil, and triggers some plants to flower, release seeds, and sprout. However, when European American settlers arrived in the 1850s, they began fighting, suppressing, and extinguishing all fires. By the time mindsets began to change in the 1960s, Yosemite’s forests had amassed over a century of fuel buildup and dense growth. Fire managers today use prescribed fire, managed wildfire, and other tools to restore forest ecosystems and reduce the fuel load in preparation for future fires.

 

Fire activity in Yosemite increased significantly in the mid-1980s. Similar trends were seen across the western US, including in areas with no history of fire suppression.

Human actions are also shaping wildfire patterns in less intentional ways. Warmer spring temperatures and earlier snowmelt linked to global climate change can lead to earlier, longer dry seasons. Meanwhile, hotter summers increase evapotranspiration (evaporation from the ground and from plant leaves), making vegetation and fuel even dryer and more flammable. Researchers connect these large-scale shifts with a jump in wildfire activity in the mid-1980s. Across the western US, fires burned four times as frequently, six times the total area, and lasted five times as long during 1986–2003 compared to 1970–1986. The most dramatic changes were seen in areas such as the northern Rockies, which have little history of fire suppression to explain the shift. A study looking back to 1916 found that climate was the main factor determining the extent of burned area across the western US.

The complex interaction between humans and fire makes it difficult to pinpoint exact sources of change in Yosemite. Dense forests and fuel buildups left by 120 years of fire suppression continue to contribute to recent large fires in the park. At the same time, Yosemite has seen declining low- and mid-elevation snowpack, while summer temperatures have risen more than 4°F over the past century. There is little evidence that the change in fire management policy itself has been responsible for the increase in large fires; nearly all have been fought with the goal of full suppression.

What changes are we seeing?

• Multiplying burn area: In Yosemite, the average area burned each year during 1984–2005 was over four times higher than it was between 1972 (after modern fire management policy was adopted) and 1984. This jump parallels a marked increase in wildfire activity across the western US in the mid-1980s.
• Low snowpack means high fire: Low-snowpack years in Yosemite can see over six times as many lightning-ignited fires as years with high snowpack. Dry years also increase fuel flammability: three times as many lightning strikes lead to fires during years with low snowpack. These patterns are likely contributing to the jump in fire activity: snowpack across the Sierra Nevada is declining at elevations under 8,500 feet, and models predict total Sierra Nevada snowpack to drop 64% by 2100.
• Lengthening fire season: Fire season throughout the western US lengthened by around 78 days between 1970–1986 and 1987–2003.

 

Low snowpack can lead to an earlier, longer dry season. This reduces the moisture of fuels and vegetation, while also lengthening the time period during which fires could ignite. (Figures from Lutz et al, 2009.)

What’s next?

• More fires: As spring snowpack decreases, models project a 19% increase in the number of annual fires in Yosemite between the periods 1984–2005 and 2020–2049, leading to a proportionate increase in the annual area burned.
• Landscape vegetation shifts: Fire may provide a catalyst for the transition of natural communities as rising temperatures alter what vegetation a landscape can support. After a fire, low-elevation mixed conifer forests may regrow as oak-chaparral, contributing to the ongoing pattern of rising treeline.

While most people never witness an active fire during their visit to the Yosemite, you are likely to encounter signs of their presence: wildfire smoke during fire season and burn scars along roads and hiking trails. Previously dense, shaded conifer forests can be transformed into sunny stands of charred trunks between one year and the next, resetting the ecological clock of fire succession. However, the ability of an ecosystem to recover naturally is compromised by underlying warming and drought. One study using data from Yosemite and elsewhere in the western US found that chronically climate-stressed trees were more likely to die from wildfire. Even if fire intensity (amount of heat released) remained constant, climate stress increased the fire severity (number of trees killed).

The future of wildfire is driven by a complex combination of climate, human management decisions, and history. One fire ecologist wrote “predicting future fire regimes is not rocket science; it is far more complicated than that.” As we continue to observe and experience wildfires in the coming years and decades, it challenges us to consider the growing role that humans play in shaping natural ecosystem processes.

 

 

References

• Keeley, Jon, and Alexandra Syphard. “Climate Change and Future Fire Regimes: Examples from California.” Geosciences 6, no. 3 (2016): 37. https://doi.org/10.3390/geosciences6030037.

• Littell, Jeremy S., Donald Mckenzie, David L. Peterson, and Anthony L. Westerling. “Climate and Wildfire Area Burned in Western U.S. Ecoprovinces, 1916–2003.” Ecological Applications 19, no. 4 (2009): 1003–21. https://doi.org/10.1890/07-1183.1.

• Lutz, James A., Jan W. van Wagtendonk, Andrea E. Thode, Jay D. Miller, and Jerry F. Franklin. “Climate, Lightning Ignitions, and Fire Severity in Yosemite National Park, California, USA.” International Journal of Wildland Fire 18, no. 7 (2009): 765. https://doi.org/10.1071/wf08117.

• van Mantgem, Phillip J., Jonathan C. B. Nesmith, Marybeth Keifer, Eric E. Knapp, Alan Flint, and Lorriane Flint. “Climatic Stress Increases Forest Fire Severity across the Western United States.” Ecology Letters 16, no. 9 (2013): 1151–56. https://doi.org/10.1111/ele.12151.

• Westerling, A. L. “Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity.” Science 313, no. 5789 (2006): 940–43. https://doi.org/10.1126/science.1128834.

Learn more

• Read more about fire management in Yosemite National Park
• How does smoke impact air quality? What should you expect when you visit?