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Global warming not a factor in wildfires
Are the massive fires burning across Southern California a product of global warming?
Scientists said it would be difficult to make that case, given the dangerous mix of drought and wind that has plagued the region for centuries or more.
But they said the extreme conditions that stoked the wildfires could become more common as the world warms.
Research suggests that rising temperatures are already increasing fire damage in many parts of the West.
In a study published last year in the journal Science, researchers looking at Western federal forests found nearly seven times more land burned from 1987 to 2003 than in the previous 17 years.
The analysis mainly attributed this to a 1.5-degree rise in average spring and summer temperatures. With spring arriving earlier and snow melting faster, the forests dried out sooner, extending the average fire season by more than two months.
The study, however, found Southern California was different from the rest of the West, with no increase in the frequency of fire as temperatures rose.
"In Southern California, it's hot and dry much of the year," said Anthony Westerling, a climate scientist at UC Merced and the study's lead author. In other words, Southern California was already perfect for fire.
"That is a fire-prone environment regardless of whether we are in a climate-change scenario," said Tom Wordell, a wildfire analyst at the National Interagency Fire Center in Boise, Idaho. "I don't want to be callous, because many people are homeless and suffering, but if you live in a snake pit, you're going to get bit."
But eventually global warming could make Southern California's occasional droughts more persistent, exacerbating the fire danger.
Conditions as dry as the Dust Bowl of the 1930s could prevail in the Southwest by the middle of this century, according to a study published this year in Science.
The researchers based their conclusion on climate models showing how warmer temperatures would expand the reach of a powerful atmospheric circulation pattern known as the Hadley cell. Changes to the cell would dry air through the subtropics, including a swath from Colorado to California.
The study suggested that the transformation may already be underway. The Southwestern United States has been in drought since 2000, although tree-ring records show there have been far drier periods during the last millennium.
Scientists said more persistent drought would inevitably lead to more fires, as long as intermittent periods of moisture allowed vegetation to grow as fodder for flames.
In Southern California, hillsides were ripe for fire because big rains two years ago allowed vegetation to flourish, then severe drought during the last year dried it out.
The future of the Santa Ana winds is harder to predict.
Norman Miller, a climate scientist at Lawrence Berkeley National Laboratories, published an analysis last year in Geophysical Research Letters predicting that rising temperatures, fueled by greenhouse gas emissions, would eventually push peak Santa Ana winds from mid-October to late November.
That could, over decades, make fires worse by giving the landscape more time to dry out. Global warming, he said, could intensify wind flow by increasing the difference between inland and coastal temperatures.
The Santa Ana winds form when high-pressure air in the Great Basin, between the Rocky Mountains and the Sierra Nevada, rushes toward the low pressure of the coastlands. As the air descends, it heats up, gaining speed as it is forced through narrow canyons.
The winds, which typically gust to 45 mph, were recorded at more than 80 mph several times this week -- strong but inside the range of normal variability.