SAN FRANCISCO, CA., DEC. 10, 2013 — It seems like simple arithmetic: warmer climate + drier forests = wildfire.
While that’s generally true, the relationship between the globe’s rising temperatures and wildfires, particularly in the American West, is complex and conditions have varied from region to region. Researchers are only now beginning to paint a coherent picture of what’s happening and what’s in store.
On Tuesday at the American Geophysical Union meeting in San Francisco, there were numerous talks by scientists studying climate change and its impact on wildfire — in particular in the American West. Academic posters were featured as well as sessions with multiple speakers. This post features just a few highlights. For more details, check out the AGU schedule for Tuesday online.
Changing storm paths: The North Pacific Jet (NPJ), a high altitude narrow path of strong winds over the North Pacific Ocean, is an important player in how much snowpack accumulates in the Sierra Nevada in California. Continuing climate change is expected to slow down the NPJ so that it follows a more north-south trajectory over North America — diverting Pacific storms north of the Sierra and then south over the Rockies and central North America. The connection to wildfire? Researchers have documented separately the connection between dwindling snowpack and increased wildfire activity. Exactly how continued warming will change the path of the NPJ is controversial, but an analysis of oak tree rings in Central California presented by Valerie Trouet of Arizona State University on Tuesday suggests that from 1990-2010, the NPJ was more consistently in the north-south orientation than at any other similar period in the last 600 years.
The wildfire picture in California is vastly different from north to south: Northern and Southern California may as well be completely different states when it comes to the frequency and severity of wildfires. Despite the attention that the Rim Fire near Yosemite received this year, it’s Southern California that’s seeing major wildfires increasing in frequency, said Jon E Keeley, U.S. Geological Survey, Western Ecological Reserve. The southern half of the state is seeing the most area burned in forest service lands, as well as a rise in area burned in Cal Fire lands managed by the state. Part of the reason is that wildfires in the northern half of the state are occurring at higher elevations, which are historically low-level fires. Lower elevation fires, which predominate in Southern California, more commonly lead to catastrophic crown fires.
Santa Ana winds and wildfire in California: Anyone who lives in Southern California knows that vicious winds that carry hot dry air from inland toward the coast every fall season can fuel wildfires. But these Santa Ana winds are not historically the biggest contributor to wildfires in the region, said Crystal Kolden of the University of Idaho. Her work found that more than half of Southern California wildfires in recent years have not occurred when Santa Ana winds were blowing. Meanwhile, there appears to be no correlation between Santa Ana-related fires and those not caused by Santa Ana fires — which suggests that different forces are driving them. As researchers continue to study how climate change will influence the occurrence and intensity of wildfires in this part of the country, they’ll want to separate Santa Ana-driven fires from others that occur at other times during the year, Kolden said.
Trees primed to burn: As the climate continues to warm, there will not always be a straight-line connection between higher temperatures and wildfires. Trees stressed by other consequences of warming — drought and insect infestations, for example — are changing the character of forests.
“There’s a physical and biological context for what’s going on here,” said Phil van Mantgem, of the USGS’s Redwood Field Station, in Arcata, Calif. “If we have chronic stress already and add fire, you’re going to get more than what you have currently.”