The Firestorm This Time: Why Los Angeles Is Burning (wired.com)

The Thomas Fire spread through the hills above Ventura, in the northern greater Los Angeles megalopolis, with the speed of a hurricane. Driven by 50 mph Santa Ana winds -- bone-dry katabatic air moving at freeway speeds out of the Mojave desert -- the fire transformed overnight from a 5,000-acre burn in a charming chaparral-lined canyon to an inferno the size of Orlando, Florida, that only stopped spreading because it reached the Pacific. Several readers have shared a Wired report: Tens of thousands of people evacuated their homes in Ventura; 150 buildings burned and thousands more along the hillside and into downtown are threatened. That isn't the only part of Southern California on fire. The hills above Valencia, where Interstate 5 drops down out of the hills into the city, are burning. Same for a hillside of the San Gabriel Mountains, overlooking the San Fernando Valley. And the same, too, near the Mount Wilson Observatory, and on a hillside overlooking Interstate 405 -- the flames in view of the Getty Center and destroying homes in the rich-people neighborhoods of Bel-Air and Holmby Hills. And it's all horribly normal. [...] Before humans, wildfires happened maybe once or twice a century, long enough for fire-adapted plant species like chapparal to build up a bank of seeds that could come back after a burn. Now, with fires more frequent, native plants can't keep up. Exotic weeds take root. Fires don't burn like this in Northern California. That's one of the things that makes the island on the land an island. Most wildfires in the Sierra Nevadas and northern boreal forests are slower, smaller, and more easily put out, relative to the south. Trees buffer the wind and burn less easily than undergrowth. Keeley says northern mountains and forests are "flammability-limited ecosystems," where fires only get big if the climate allows it -- higher temperatures and dryer conditions providing more fuel. Climate change makes fires there more frequent and more severe.

Comments

Here come those Santa Ana winds again

[rmdingler]

There are ideal conditions for wildfires in this region (and others) nearly every year. What's special about this year?

"What we don’t have every single year is an ignition during a wind event. And we’ve had several."

Whether by foolish acts or (pyro)maniacal disposition, people are the blight on this land.

Re:Dontcha need wetter for more fuel?

[dfm3]

TFA does a better job than the summary at explaining, but yes, your observation is consistent. Wetter summers mean more vegetation growth, but it's the weather during a few critical weeks in late fall that determine how severe the fires will be.

It's the small stuff (leaves, brush, and weeds) that burns fast, hot, and explosively given the right conditions. In the fall, when the deciduous species lose their leaves, a wet December means that most of this vegetation falls to the ground and begins to decompose, rendering it more dense and less flammable. When you have a combination of dry weather, warm temperatures, and high wind, combined with ignition (historically caused by lightning, but usually by people these days), leaves tend to stay on the trees longer, or fall to the ground without decomposing, and become perfect fuel.

So yes, you're correct that when this stuff dries out, it becomes a hazard.

For example, last fall was a historically notable fire season in the southern Appalachians. Many parts of the mountains in NC/TN/GA had little to no measurable rainfall for a couple months, so the leaves simply dried up and stayed on the trees rather than changing color and falling to the ground. The deadly fires that swept through Gatlinburg, Tennessee became "canopy" fires - an event more common in California but virtually unheard of in eastern forests. Even one good rainstorm could, in theory, have been sufficient to knock enough leaves off of the trees and compress the leaf litter on the forest floor to render it slightly less flammable.

Re:True

[Gravis Zero]

There were never wildfires before climate change was discovered.

The is real problem is that we're been putting out the wildfires for over a hundred years when burning is part of the natural cycle of life for the ecosystem. As a result there are many millions of dead and dry trees just waiting for a spark. However, climate change is exacerbating the issue by causing more extreme weather (longer droughts and more extreme downpours) which ultimately kill more plants and turn them into fuel for the fire. Climate change definitely isn't the cause of these giant wildfires but it is making it worse.