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Don Paul: On droughts in California, wildfires and climate change

Don Paul

Droughts lead to more fires. This New York Times map showing the extent of the deadliest fire in California history, destroying Paradise, gives some idea of the tremendous scope of this disaster.

Even going back to centuries-old paleoclimatology evidence, droughts have frequently been a part of climate in California and in much of the west. There is nothing new about longer-term droughts in California, even long before mean anthropogenic/human-driven warming accelerated in the last four decades.

We do know, however, that the effects of these droughts have grown more severe during this period of mean warming, as predicted by climate models going back to the 1980s. In late 2014, two scientists from the University of Minnesota and the Woods Hole Oceanographic Institute were able to demonstrate that the most recent drought was the worst in 1,200 years.

Summers have always been the dry season in California and in much of the West and the Southwest, even when long-term droughts weren’t underway. If droughts have been so frequent and, sometimes, lengthy, and summers have been so dry due to climatology, why are droughts more severe now, and how can we tie the severity to mean warming?

Simply put, hotter equals drier. The summer of 2017 was the most extreme for heat near San Francisco, with downtown reaching an all-time high of 106 in early September, San Jose hitting 108, and some locations farther inland making into the 111-116 range.

Air has to be extremely dry to reach those extremes. More heat records have been set in recent decades, meaning there have been more episodes of drastically dry, heated air rushing over the landscape on easterly winds from the desert interior. In Los Angeles, these are known as the Santa Ana winds.

The amount of territory covered by such extreme dry heat can be vast:

This past summer, July was the hottest month statewide in California history. The humidity values accompanying that heat are equally extreme.

On the day the fires spread into Malibu, I viewed a special statement from the Los Angeles National Weather Service extending Fire Weather Warnings into Los Angeles and advising humidity levels east of downtown would be dropping to 3-9 percent that day.

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The ratio of record high to record low temperatures in a more stable climate should be 1:1. That ratio has shifted to at least 2:1, often higher in California.

The tie to mean warming becomes more obvious. Droughts, as I wrote, are not new to California. But the combination of more frequent extreme heat leading to more frequent extremely low humidity is new enough to ramp up the wildfire frequency and, as we see, the tragic scope and coverage of these devastating fires. The harsh drought and dry season environment has become harsher due to the warming climate, and the latter is irrefutable.

The relationship between warming and increased aridity is what mathematicians call exponential. That is, each degree of warming has a greater effect on fire and aridity than the previous degree of warming, according to Park Williams, a research scientist with the Columbia University Lamont-Doherty Earth Observatory. Knowing what we now know about anthropogenic warming, these fires are far worse than they would have been prior to the accelerated warming.

This doesn’t mean that each year will be worse than the preceding year. The frequency and coverage rates are nonlinear, or not following a smooth line. But the mean upward trend is there and is going to worsen. There are troubling sidebars, too. A U.S. Geological Survey study shows trees in drought-stricken forests are weakened and more likely to burn more quickly when fires arrive.

Even when lengthy severe droughts finally break, trouble can lie ahead. California finally received abundant rain and snowfall in winter 2016. In the spring, dormant seeds bloomed and the landscape became flooded with new and beautiful vegetative growth. However, the inevitable California dry season for the summer and early autumn came. Much of the new growth withered and dried as it does every summer, drought year or not. Came the fall, and millions of tons of new fuel for wildfires covered vast areas of the state. Following a ferociously hot July and August, a horrific fire season developed, almost as bad as this year’s season.

Finally, let’s get to raking. I listened to the director of California Fire (a state agency) in the car Monday. He agreed that raking can be of some benefit to individual homeowners in a small risk reduction around their homes for slower-moving fires. As for California forests, there are more than 30 million acres of forested land, more than 20 million of which are federal/national forests. To paraphrase Jerry Lee Lewis, that’s a “whole lot of rakin’ goin’ on!”

As for another bizarre comparison between California and Finland forest management stressors, Los Angeles averages 0.02 inches of rain in July. Finland? A bit more. 

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