Wildfires that raged in Southern California this week and forced more than half a million people from their homes spread so rapidly in part because the landscape was parched by a hot, dry summer—conditions that may become more of a norm for the Southwest, thanks to global warming.
But can these wildfires be attributed to a changing climate?
This very issue was brought to light Tuesday when Senate Majority Leader Harry Reid told reporters, “One reason why we have the fires in California is global warming,” according to The Hill.com, a political news Web site, though Reid later said many factors contributed to the wildfires.
Like hurricanes and other extreme events that could possibly be influenced by global warming, it is impossible to connect any one wildfire to climate change. But scientists say that in a warming world, the likelihood of wildfires like the ones tearing across Southern California is definitely higher.
"I think the only thing we can say is that it is likely that the probability of fire initiation [a fire starting] will be higher during [hot] and dry periods of time," said Guy Brasseur of the National Center for Atmospheric Research in Colorado.
Climate models run by scientists have shown a clear drying trend in the subtropics, which includes the American Southwest, Mexico, the Mediterranean, Australia and parts of Asia, over the course of this century.
"If they get drier, the likelihood for fires will be higher," Brasseur said.
The hot, dry conditions that dominated the Southwest this summer were a small taste of what could become the prevailing conditions in the future, and they helped provide the fuel for the fires to start.
"When you have a hot summer and a dry summer, boom, you have a lot of fires," Brasseur told LiveScience.
But temperature and humidity aren't the only factors affecting the likelihood of wildfires occurring. The frequency of the occurrence of thunderstorms is also important because lightning triggers most wildfires—"so if lightning increases, we could have more fires," Brasseur said.
The wildfires burning now in California have also shown how important winds are to the ability of the fire to spread. The ferocious and chaotic Santa Ana winds in California make predicting which way the fires will move very difficult, and so they are a challenge for firefighters to control. Just how these winds will be affected by climate change is a big unknown, Brasseur said.
California's population explosion is another key factor in terms of the threat to humans, because, "the more people you have, the more fires you might also have by people," Brasseur said.
While wildfires are a huge threat to the homes of people who live in areas prone to these disasters, they are also a threat in terms of the pollution they emit.
Wildfires create intense emissions of carbon dioxide, carbon monoxide, smoke, particles, aerosols and other chemical compounds. Carbon monoxide in turn contributes to the formation of ground-level ozone, a pollutant. These pollutants can damage the lungs of firefighters and fleeing residents who breathe them in.
The fires also affect the local ecosystem—forest fires have always been a natural phenomenon that helped clear out underbrush, but the changes that humans have wrought to the landscape have changed that dynamic.
"[Forest fires are] good because [they] rejuvenate the forest in a way, so it's part of the natural system," Brasseur said. "The problem is that, of course, humans have affected the forest to a certain extent in certain places. The resistance of the forest might not be the same as it used to be against fire."
The spate of forest fires in recent years has called forest management practices into question. Many years ago, forest rangers would clear out underbrush with proscribed, or controlled, burns to deprive forest fires of their main fuel source. That approach is similar to what would happen naturally, when frequent lightning-sparked fires would burn underbrush but not be intense enough to destroy entire forests.
But in recent years, the practice was to let forest grow and to stamp out fires before they got out of control, which set up a situation in which "the forest may be more vulnerable to fires than it used to be," Brasseur said.
Wie Min Hao of the United States Forest Service said that in addition to encouraging residents in fire-prone areas to be more careful, the Forest Service is moving back to using proscribed burns, but he said these can only be performed under limited forest and weather conditions when the fires won't accidentally spread out of control.
"You don't get too many chances," he said.