Over the last two decades (compared to 1970 to 1986), the number of major wildfires in the US West increased by a factor of 4, and the area of forest burned increased by a factor of 6. Canada has also seen an increase. Much of this increase was an abrupt change in the mid-1980s.
Political fights have centered on early Forest Service practices of controlling fires, and whether logging companies should thin the forests. A report in the August 18 Science, based on data from western fires, finds climate change to be the largest factor. This is especially true in the northern Rockies, which is seeing the largest increase.
This change has been dramatic. The number of days each year when fires burn (time from first discovered to last controlled) has increased by 78 days. The average large fire now burns 37.1 days, compared to 7.5 days before. The largest increase is in snow-dominated forests at elevations of about 2.1 km (1.3 miles).
Snowmelt provides 3/4 of western streamflow. There is low fire danger during periods of snowmelt and for about one month afterward. Unfortunately, snowpacks are gone one to four weeks earlier than 50 years ago, and streamflows peak earlier.
Data show that years with early snowmelt and a longer dry summer period have five times as many wildfires as years with late snowmelt. Forests that were once protected by late snowpacks, from elevations between 1.7 and 2.7 km (1 to 1.7 mile) are now seeing fires.
Warmer summer temperatures and reduced winter precipitation are associated with early melting of snowpack and so with increased numbers of large fires. In some forests (ponderosa pine), there is a smaller association with moist conditions before the hot summer, increasing the amount of combustible material available. Land use (extensive livestock grazing and effective fire suppression) in some areas, particularly California, may contribute to the increases in large fires.
Fires are much harder to put out. The 1988 Yellowstone Park fire lasted more than 3 months, burning more than 1.5 million acres. $120 million and 25,000 firefighters weren’t able to eradicate it. Finally the snows began in mid-September.
More than 95% of burned acreage comes from less than 5% of wildfires. A large wildfire can cost $20 million/day, and governmental agencies have spent $1.7 billion during fire season in recent years. Many years, damages in the West exceed $1 billion. Damages to natural resources are sometimes extreme and irreversible. And wildfires contribute to climate change.
Wildfires worldwide add an estimated 3.5 billion metric tonnes of carbon to the atmosphere each year. [This is about double the US contribution.] Where increases in wildfire are partly caused by land use, (very expensive) intervention can reduce the numbers and severity. In most of the West, this won’t work.
