I'm pretty conservative about attributing weird weather and other climate anomalies to global warming: all you can say is that a record-setting hot October, or a string of 70-degree days in January in New York, is consistent with what a greenhouse world would be like. But when scientists go on record with a specific prediction of how climate change will play out, and when it indeed plays out that way, attention must be paid.
Last year, a study in the journal Science found that "large wildfire activity increased suddenly and markedly in the mid-1980s, with higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons." The greatest increases were in forests of the Northern Rockies, but was seen throughout the west The pattern of western fires matched what would be expected not from changes in land use--mostly logging and ranching--but from climate change.
Specifically, a warmer world caused by the accumulation of heat-trapping greenhouse gases produces alternating deluges and droughts. The extra heat causes greater evaporation, but the water vapor remains in the atmosphere longer, or travels farther, before falling--in buckets. The result is alternating wet and dry years. In wet years, vegetation grows like mad. In drought years, that vegetation becomes tinder, exactly what southern California is now experiencing. As the scientists said, "an increased incidence of large, high-severity fires may be due to a combination of extreme droughts and overabundant fuels."
And no, it's not just a matter of media attention or the ubiquity of fire video on YouTube. The scientists found that the frequency of wildfires beginning in the mid-1980s was nearly four times that of 1970 to 1986, "and the total area burned by these fires was more than six and a half times its previous level." It's real, and it's going to continue.
. . . and get worse. Just as this season we call summer is now slipping well past Sept. 21, so the fire "season" is busting out of its former bounds. The average time between the reported first wildfire and the last in any given year increased by 78 days (64%), comparing 1970 to 1986 with 1987 to 2003.
Another factor is snowmelt, which has been dissipating in the west (with dire consequences for water supply, but that's another story). The earlier the snowmelt, the worse the wildfire season, because if the snowpack holds on into late spring or summer it releases its water slowly and gradually, reducing the flammability of vegetation. But if the snow has all melted by early in the season, much of it is lost to runoff rather than retained in the soil, where it would dampen the flammability of vegetation. Also, the warmer the summer--another consequence of climate change--the worse the burn: warmer temps, by increasing the rate of plants' evapotranspiration, make brush more flammable.
"This is exactly what we’ve been projecting to happen, both in short-term fire forecasts for this year and the longer term patterns that can be linked to global climate change," says Ronald Neilson of Oregon State University, who is also a bioclimatologist with the U.S. Forest Service.
He and other scientists are careful to acknowledge that there is no way to prove that the increase in wildfires has been caused by climate change--just as every prudent climatologist makes the same disclaimer about a heat wave or intense hurricane. But their conclusion is worth quoting in full: "virtually all climate-model projections indicate that warmer springs and summers will occur over the region in coming decades. These trends will reinforce the tendency toward early spring snowmelt and longer fire seasons. This will accentuate conditions favorable to the occurrence of large wildfires"---as California is seeing all to well this week.
It may soon have a lot of company. Nelson, who has served on the Intergovernmental Panel on Climate Change, the scientific body that periodically assesses climate change research and that shared this year's Nobel Peace Prize, says that other regions of the U.S. may be experiencing a fire-friendly pattern of precipitation: several wet years followed by several that are drier than normal.
The result can be "heavier vegetation loads popping up and creation of a tremendous fuel load," says Nelson, whose climate models accurately predicted this week's California blazes. "But the warmth and other climatic forces are also going to create periodic droughts. If you get an ignition source during these periods, the fires can just become explosive. . . . In the future, catastrophic fires such as those going on now in California may simply be a normal part of the landscape." (The IPCC report released earlier this year also projected worse wildfires, in chapter 14 of the report by Working Group 2.)
How bad might it get? In 2002, OSU's Nelson and colleagues projected that droughts or heat waves made more frequent by climate change would lead to wildfire more frequent and severe than almost any that have occurred since colonial days.