Warming climate likely to dramatically increase Yellowstone fires by mid-century

A study published online the week of July 25 in the Proceedings of the National Academy of Sciences shows that climate change could increase the frequency of large fires in the Greater Yellowstone Ecosystem to a point that sparks dramatic shifts in the forest vegetation, from conifer-dominated mature forests to younger stands and more open vegetation.

"Large, severe fires are normal for this ecosystem. It has burned this way about every few hundred years for thousands of years," explains study author Monica Turner, the Eugene P. Odum Professor of Ecology at the University of Wisconsin-Madison and a landscape ecologist who has worked in the Greater Yellowstone area for more than 20 years. "But if the current relationship between climate and large fires holds true, a warming climate will drive more frequent large fires in the Greater Yellowstone Ecosystem in the future."

Wildfires in this ecosystem are climate-driven and are primed by hotter, drier conditions, such as those predicted by numerous global climate models.

Already fire ecologists have noticed increased fire frequency in the west, associated with temperature increases of less than two degrees Fahrenheit and early spring snowmelt in the mountains.

For the new study, the researchers analyzed large wildfires (greater than 500 acres) and climate data in the northern Rocky Mountains from 1972 to 1999, then used these observed relationships with global climate models to project how expected climate change will impact fires during the 21st century.

"What surprised us about our results was the speed and scale of the projected changes in fire in Greater Yellowstone," says lead author Anthony Westerling, a professor of environmental engineering and geography at the University of California, Merced. "We expected fire to increase with increased temperatures, but we did not expect it to increase so much or so quickly. We were also surprised by how consistent the changes were across different climate projections."

They found that fires larger than 500 acres will likely be an annual occurrence by 2050, with fire rotation -- the time span over which the entire landscape burns -- reduced from a historic range of 100 to 300 years to less than 30 years. Interestingly, the predicted new fire regime closely resembles patterns typical of other landscapes, such as the ponderosa pine forests of the southwest.

"More frequent fires will not be catastrophic to the area -- Yellowstone will not be destroyed -- but they will undoubtedly lead to major shifts in the vegetation," says Turner. "It is critical to keep monitoring these forests and study how they respond to future fires."

For example, the iconic lodgepole pines that dominate much of the current landscape may not have time to recover between big fires, especially if hot, dry summers make it difficult for tree seedlings to germinate and grow following future fires. Some forests could shift toward fast-growing aspen and Douglas fir, or even shrubs and grassland. Such changes would also affect the region's wildlife, hydrology, carbon storage, and aesthetics.

Westerling, an expert on climate-fire interactions, cautions that the models used in the study will not work once the increase in fires creates a fundamental change in the ecosystem. As the landscape changes, the relationships between climate and fire will change as well.

"The biggest challenge for us is to understand what can happen when the ecosystem is transformed," he says. "Our projections also depend on the climate models we are using -- for example, if projections for winter snow pack or summer rainfall were to increase significantly, that would change our results."

With more frequent fires, available fuels will also dwindle and eventually become important than climate in limiting fires. At that point, existing models will break down, leaving future outcomes even more unpredictable, Turner says -- large fires could even become less severe in the future, making it an important topic for continued study.

"Our research after the immense 1988 fires revealed surprises and tremendous resilience in Yellowstone's ecosystems, and Yellowstone is likely to surprise us again in the future," Turner says. "It is an incredibly valuable natural laboratory for studying how natural ecosystems adapt to changing environmental conditions."